PREFACE Current generation portable computers instruments utilize back
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PREFACE Current generation portable computers instruments utilize backlit LCDs (Liquid Crystal Displays). These displays have also appeared applications ranging from medical equipment automobiles, pumps retail terminals. Cold Cathode Fluorescent Lamps (CCFLs) provide highest available efficiency backlighting display. These lamps require high voltage operate, mandating efficient high voltage DC/AC converter. addition good efficiency, converter should deliver lamp drive sine wave form. This desirable minimize emissions. Such emissions cause interference with other devices, well degrading overall operating efficiency. sine wave excitation also provides optimal current-to-light conversion lamp. circuit should permit lamp control from zero full brightness with hysteresis "pop-on," must also regulate lamp intensity power supply variations. small size battery-powered operation associated with equipped apparatus mandate component count high efficiency these circuits. Size constraints place severe limitations circuit architecture long battery life usually priority. Laptop handheld portable computers offer excellent example. CCFL power supply responsible almost battery drain. Additionally, these components, including board hardware, usually must within enclosure with height restriction 0.25 inches. practical, efficient backlight design classic study compromise transduced electronic system. Every aspect design interrelated, physical embodiment integral part electrical circuit. choice location lamp, wires, display housing other items have major effect electrical characteristics. greatest care every detail required achieve practical high efficiency backlight. Getting lamp light just beginning! First generation backlights were crude, with poor performance almost areas. (Linear Technology Corporation) introduced feedback stabilization optimized lamp driving configurations three successive generations technology. effort culminated dedicated backlight driving. This fourth publication reviews recent work components measurement techniques applicable backlighting. Theoretical considerations presented with practical suggestions, remedies circuits. always, welcome reader comments, questions requests consultation.
CCFL backlight application circuits contained this Application Note covered U.S. patent number 5408162 other patents pending.
registered trademarks Linear Technology Corporation.
AN65-1
Application Note
TABLE CONTENTS
INTRODUCTION. AN65-4 PERSPECTIVES DISPLAY EFFICIENCY AN65-5 Cold Cathode Fluorescent Lamps (CCFLs) AN65-5 CCFL Load Characteristics AN65-7 Display Layout Losses AN65-8 Considerations Multilamp Designs. AN65-31 CCFL Power Supply Circuits AN65-32 Power CCFL Power Supplies. AN65-37 High Power CCFL Power Suppy AN65-39 "Floating" Lamp Circuits. AN65-40 IC-Based Floating Drive Circuits AN65-43 High Power Floating Lamp Circuit AN65-46 Selection Criteria CCFL Circuits. AN65-46 Summary Circuits. AN65-49 General Optimization Measurement Considerations AN65-52 Electrical Efficiency Optimization Measurement. AN65-53 Electrical Efficiency Measurement AN65-55 Feedback Loop Stability Issues AN65-55 REFERENCES. AN65-59 APPENDIX AN65-60 "HOT" CATHODE FLUORESCENT LAMPS AN65-60 APPENDIX AN65-60 MECHANICAL DESIGN CONSIDERATIONS LIQUID CRYSTAL DISPLAYS AN65-60 Introduction AN65-60 Flatness Rigidity Bezel AN65-61 Avoiding Heat Buildup Display AN65-61 Placement Display Components AN65-62 Protecting Face Display AN65-62 APPENDIX AN65-63 ACHIEVING MEANINGFUL EFFICIENCY MEASUREMENTS AN65-63 Current Probe Circuitry. AN65-64 Current Calibrator AN65-67 Voltage Probes Grounded Lamp Circuits AN65-70 Voltage Probes Floating Lamp Circuits AN65-72 Differential Probe Calibrator AN65-76 Voltmeters AN65-82 Calorimetric Correlation Electrical Efficiency Measurements AN65-84 APPENDIX AN65-87 PHOTOMETRIC MEASUREMENTS AN65-87
AN65-2
Application Note
APPENDIX AN65-92 OPEN LAMP/OVERLOAD PROTECTION. AN65-92 Overload Protection AN65-93 APPENDIX AN65-94 INTENSITY CONTROL SHUTDOWN METHODS AN65-94 About Potentiometers AN65-96 Precision Generator AN65-98 APPENDIX AN65-99 LAYOUT, COMPONENT EMISSIONS CONSIDERATIONS AN65-99 Circuit Segmenting AN65-99 High Voltage Layout AN65-99 Discrete Component Selection AN65-106 Basic Operation Converter AN65-107 Requisite Transistor Characteristics AN65-108 Additional Discrete Component Considerations. AN65-110 Emissions AN65-110 APPENDIX AN65-110 ®1172 OPERATION FROM HIGH VOLTAGE INPUTS AN65-110 APPENDIX AN65-111 ADDITIONAL CIRCUITS AN65-111 Desktop Computer CCFL Power Supply AN65-111 Dual Transformer CCFL Power Supply AN65-112 HeNe Laser Power Supply AM65-113 APPENDIX AN65-114 CONTRAST CIRCUITS AN65-114 Dual Output Bias Voltage Generator. AN65-115 LT118X Series Contrast Supplies AN65-116 APPENDIX AN65-119 ROYER, WHAT DESIGN? AN65-119 APPENDIX AN65-120 EARS GOGHS/Some Not-So-Great Ideas AN65-120 Not-So-Great Backlight Circuits AN65-120 Not-So-Great Primary Side Sensing Ideas AN65-122
AN65-3
Application Note
INTRODUCTION This scribing marks fourth publication many years concerning illumination.1 extraordinary user response previous efforts resulted continuing backlight development effort company. This level interest, along with significant performance advances since last publication, justifies further discussion backlighting. Development attractive solutions illumination necessitated longest sustained application engineering effort date. single circuit 1991 publication (Measurement Control Circuit Collection, Application Note June 1991) resulted four years continuous investigation, summarized three successive, dedicated publications. impetus this bustle been overwhelming continuously ascending reader response. Practical, high performance backlighting solutions needed wide range applications. optical, transductive electronic aspects combine (conspire?) present extraordinarily challenging problem. backlight problem's interdisciplinary nature, along with highly interactive effects, provides exquisitely subtle engineering exercise. Backlights present most complex interdependencies author ever encountered. academic interest this challenge course, wellpatinaed with capitalistic intent. Substantial comfort arrives with certainty that audience similarly acculturated. This publication includes pertinent information from previous efforts addition updated sections large body material. partial repetition small penalty compared benefits text flow, completeness time efficient communication. Older material been altered, abridged augmented appropriate, while simultaneously introducing findings. Previous work emphasized obtaining verifying high efficiency. This characteristic still quite desirable, other backlight requirements have become evident. These include voltage operation, improved system interface, minimization display-induced losses, circuitry compaction better measurement/optimization techniques. These advances have been enabled development instrumentation. Finally, this preamble must appreciate text's arrangement review various personnel customers. They transmuted psychotic uproar manuscript into this finessed presentation. Hopefully, readers will join author applause.
Note Previous publications annotated References
AN65-4
Application Note
PERSPECTIVES DISPLAY EFFICIENCY displays currently available require power sources, backlight supply contrast supply. display backlight single largest power consumer typical portable apparatus, accounting almost battery drain with display maximum intensity. such, every effort must expended maximize backlight efficiency. Study energy management should consider problem from interdisciplinary viewpoint. backlight presents cascaded energy attenuator battery (Figure Battery energy lost electrical-toelectrical conversion high voltage drive CCFL. This section energy attenuator most efficient; conversion efficiencies exceeding possible. CCFL, although most efficient electrical-to-light converter available today, losses exceeding 80%. Additionally, optical transmission efficiency present displays under monochrome with color types much lower.
ELECTRICAL ELECTRICAL CONVERSION HIGH VOLTAGE CONVERTER EFFICIENT ELECTRICAL LIGHT CONVERSION COLD CATHODE FLUORESCENT LAMP (CCFL) EFFICIENT LIGHT LIGHT CONVERSION DISPLAY DIFFUSER EFFICIENT
areas. particular, form drive applied lamp quite critical. waveshape supplied lamp influences current-to-light conversion efficiency. Thus, dissimilar waveforms containing equivalent power produce different amounts lamp light output. This implies that more electrically efficient inverter with nonoptimal output waveshape could produce less light than "less efficient" inverter with more appropriate waveform. Experiment reveals this true. such, distinction between electrical photometric efficiency necessary requires attention. Another practical area where improvement possible transmission inverter drive lamp. high frequency waveform subject losses parasitic capacitances wiring display. Controlling parasitic capacitances manner which lamp drive applied yield significant efficiency improvement. Practical methods addressing both aforementioned areas contained subsequent sections this publication. Cold Cathode Fluorescent Lamps (CCFLs) discussion CCFL power supplies must consider lamp characteristics. These lamps complex transducers, with many variables affecting their ability convert electrical current light. Factors influencing conversion efficiency include lamp's current, temperature, drive waveform characteristics, length, width, constituents proximity nearby conductors. These other factors interdependent, resulting complex overall response. Figures through show some typical characteristics. review these curves hints difficulty predicting lamp behavior operating conditions vary. lamp's current, temperature warm-up time clearly critical emission, although electrical efficiency necessarily correspond best optical efficiency point. Because this, both electrical photometric evaluation circuit often required. possible, example, construct CCFL circuit with electrical efficiency which produces less
Note "Call attention problems" constitutes pleasant euphemism complaining. This publication's section displays presents such complaints visual form along with suggested remedies.
BATTERY
PARASITIC CAPACITANCE PATHS ABSORB SOME DC/AC CONVERTER OUTPUT
Figure Backlit Display Presents Cascaded Energy Attenuator Battery. DC/AC Conversion Significantly More Efficient Than Energy Conversions Lamp Display
very high DC/AC conversion efficiency highlights some significant issues. Anything that improves energy transfer other "attenuator" areas will have greater impact than further electrical efficiency improvements. Additional improvements electrical efficiency, while certainly desirable, reaching point diminishing returns. Clearly, overall backlight efficiency gains must come from lamp display improvements. There very little electrical workers improve lamp display efficiency besides call attention problems (see following sections lamps displays).2 Improvements are, however, possible related
AN65-5
Application Note
light output than approach with electrical efficiency. (See Appendix Ears Goghs Some Not-So-Good Ideas.") Similarly, performance very well matched lamp/circuit combination severely degraded lossy display enclosure excessive high voltage wire lengths. Display enclosures with much conducting material near lamp have huge losses capacitive coupling. poorly designed display enclosure easily degrade efficiency 20%. High voltage wire runs typically cause loss inch wire.
RATED MAXIMUM OPERATING POINT
TAMBIENT 25°C 0.1°C
PERCENT FINAL EMISSION
TIME (SEC)
Figure Emissivity On-Time Typical Lamp Free Air. Lamp Must Arrive Temperature Before Emission Stabilizes
25°C
INTENSITY (CD/M2)
LAMP RUNNING VOLTAGE (VRMS)
25°C
TUBE CURRENT (mA)
Figure Emissivity Typical Lamp. Curve Flattens Badly Above
LAMP CURRENT (mARMS)
RELATIVE LIGHT OUTPUT
Figure Lamp Current Voltage Operating Region. Note Large Temperature Coefficient
LAMP TYPICAL ENCLOSURE TEMPERATURE 25°C
1000
TUBE VOLTAGE (VRMS)
NORMALIZED 25°C
25°C
AMBIENT TEMPERATURE (°C)
Figure Ambient Temperature Effects Emissivity Typical Lamp. Lamp Enclosure Must Come Thermal Steady State Before Measurements Made
TUBE LENGTH (mm)
Figure Running Voltage Lamp Length Temperatures. Start-Up Voltages Usually 200% Higher Over Temperature
AN65-6
Application Note
PERCENT RELATIVE EMISSION
ILAMP
CCFL Load Characteristics These lamps difficult load drive, particularly switching regulator. They have "negative resistance" characteristic; starting voltage significantly higher than operating voltage. Typically, start voltage about 1000V, although higher lower voltage lamps common. Operating voltage usually 300V 500V, although other lamps require different potentials. lamps will operate from migration effects within lamp will quickly damage such, waveform must content should present. Figure shows driven lamp's characteristics curve tracer. negative resistance-induced "snap-back" apparent. Figure another lamp, acting against curve tracer's drive, produces oscillation. These tenden-
FREQUENCY (kHz)
Figure Lamp Emission Drive Frequency with Lamp Free Space. Change Measurable from 20kHz 130kHz, Indicating Lamp Insensitivity Frequency
PERCENT RELATIVE EMISSION
ILAMP 2mA/DIV
FREQUENCY (kHz)
200V/DIV
Figure Figure Lamp Shows Significant Emission Drive Frequency Degradation When Mounted Display. Cause Frequency-Dependent Loss Display's Parasitic Capacitance Paths
(9a)
optimum drive frequency determined display wiring losses, lamp characteristics. Figure shows lamp emissivity essentially flat over wide frequency range. Figure shows results with same lamp mounted typical display. apparent emissivity fall-off high frequencies caused reduced lamp current parasitic capacitance-induced losses. frequency increases, display's parasitic capacitance diverts progressively more energy, lowering lamp current emission. This effect sometimes misinterpreted, leading mistaken conclusion that lamp emissivity degrades with increasing frequency.
2mA/DIV
200V/DIV
(9b) Figure Negative Resistance Characteristic CCFL Lamps. "Snap-Back" Readily Apparent, Causing Oscillation These Characteristics Complicate Power Supply Design
AN65-7
Application Note
cies, combined with frequency compensation problems associated with switching regulators, cause severe loop instabilities, particularly start-up. Once lamp operating region assumes linear load characteristic, easing stability criteria. Lamp operating frequencies typically 20kHz 100kHz sine-like waveform preferred. sine drive's harmonic content minimizes emissions, which could cause interference efficiency degradation.3 further benefit continuous sine drive crest factor controlled rise times, which easily handled CCFL. CCFL's current-to-light output efficiency lifetime degrades with fast rise, high crest factor drive waveforms.4 Display Layout Losses physical layout lamp, leads, display housing other high voltage components integral parts circuit. Placing lamp into display introduces pronounced electrical loading effects which must considered. Poor layout easily degrade efficiency higher layout-induced losses have been observed. Producing optimal layout requires attention losses occur. Figure begins study examining potential parasitic paths between transformer's output lamp. Parasitic capacitance ground from point between power supply output lamp creates path undesired current flow. Similarly, stray coupling from point along lamp's length ground induces parasitic current flow.
STRAY CAPACITANCE
parasitic current flow wasted, causing circuit produce more energy maintain desired current flow lamp. high voltage path from transformer display housing should short possible minimize losses. good rule thumb assume efficiency loss inch high voltage lead. board traces, ground power planes should relieved least 1/4" high voltage area. This only prevents losses eliminates arcing paths. Parasitic losses associated with lamp placement within display housing require attention. High voltage wire length within housing must minimized, particularly displays using metal construction. Ensure that high voltage applied shortest wire(s) display. This require disassembling display verify wire length layout. Another loss source reflective foil commonly used around lamps direct light into actual LCD. Some foil materials absorb considerably more field energy than others, creating loss. Finally, displays supplied metal enclosures tend lossy. metal absorbs significant energy path ground unavoidable. Direct grounding metal enclosed displays further increases losses. Some display manufacturers have addressed this issue relieving metal lamp area with other materials. Losses introduced
Note Many characteristics CCFLs shared so-called "Hot" cathode fluorescent lamps. Appendix "Hot" Cathode Fluorescent Lamps. Note Appendix Ears Goghs-- Some Not-So-Great Ideas."
LEAD WIRE OUTPUT CAPACITOR TYPICALLY 15pF 47pF
LEAD WIRE CCFL LAMP
DISPLAY HOUSING AND/OR REFLECTIVE FOIL LAMP
FROM DRIVE CIRCUITRY
STRAY CAPACITANCE
Figure Loss Paths Stray Capacitance Practical Installation. Minimizing These Paths Essential Good Efficiency
AN65-8
Application Note
display substantial vary widely with different displays. These losses only degrade overall efficiency, complicate meaningful determination lamp current. Figure shows effects distributed parasitic capacitance loss paths lamp current. display housing reflective foil-induced loss paths provide continuous conduit loss current flow. This results continuously varying value "lamp current" along lamp's length. cases where lamp near ground, current fall-off greatest lamp's high voltage regions. Although parasitic capacitance usually uniformly distributed, effect becomes greater voltage scales These effects illustrate designing around lamp specifications such frustrating exercise. Display vendors typically call lamp operating parameters based information received from lamp manufacturer. Lamp vendors often determine operating characteristics completely different enclosure, none all. This uncertainties complicates design effort. only viable solution determine lamp performance with display interest. This only practical maximize performance ensure against overdriving lamp, which wastes power shortens lamp life. general, display introduces parasitics which degrade performance. Latter portions this text discuss some compensatory techniques, deleterious effects display parasitics dominate practical backlight design. There some benefits lossy displays. advantage display parasitics that they effectively lower lamp breakdown voltage. parasitic shunt capacitance along tube's length forms distributed electrode, effectively shortening breakdown path, lowering lamp's turnon voltage. This accounts fact that many display mounted lamps start lower voltages than "naked" lamp breakdown voltage specification suggests. This effect aids temperature start-up (see Figures second potential advantage distributed parasitic lamp capacitance enhancement current operation. some cases extended dimming range possible because parasitics provide more evenly distributed field along lamp's length. This tends maintain illumination along lamp's entire length operating currents, allowing luminosity operation.
LAMP WIRING-INDUCED PARASITIC LOSS PATHS 5.35mA
BALLAST CAPACITOR 27pF 5.4mA
5.15mA DISPLAY HOUSING/ REFLECTIVE FOIL-INDUCED PARASITIC LOSS PATHS
5.1mA
5.5mA
PRIMARY DRIVE CIRCUITRY
HIGH VOLTAGE TRANSFORMER SECONDARY
Figure Distributed Parasitic Capacitances Practical Situation Cause Continuous Downward Shift Measured "Lamp Current." This Case 0.5mA Lost Parasitic Paths. Most Loss Occurs High Voltage Regions
AN65-9
Application Note
lessons here clear. thorough characterization lamp/display losses crucial understanding trade-offs obtaining best possible performance. highest efficiency system" backlights have been produced careful attention these issues. some cases entire display enclosure re-engineered lower losses. display loss issue, central backlight design, merits detailed attention. following briefly commented photographs (Figures through illustrate variety display situations. Hopefully, this visual tour will alert display users manufacturers problems involved, promoting appropriate action both.
Figure Ideal Display Display. Drive Electronics Connected "Naked" Lamp Simulates Zero Loss Display. Note Nylon Stand-Offs. Results Obtained Have Relationship Practical Display Driving
AN65-10
Application Note
AN65-11
Figure Measuring Lamp Wire Display Frame Capacitance. Technique Gives Lead Wire-to-Frame Loss Information Lamp-to-Foil-or-Frame Loss Data. Lamp Must Energized Before Parasitics Measurable
AN65-12
Application Note
Figure Loss Display Metal Lamp Region. Reflective Foil Floats from Ground Absorption. Display Loss About 1.5%
Application Note
AN65-13
Figure Another Loss Display Similar Characteristics Figure Running Long Wire Return Across Lamp Length Increases Loss about Spacing Wire Away from Lamp Would Loss Half
AN65-14
Application Note
Figure Custom Designed, Extremely Loss Display. Metal Eliminated Lamp Area (Lower Portion Photo). Good Compromise Between Mechanical Strength Loss Control
Application Note
AN65-15
Figure Figure 16's Reverse Side. Metal Relieved Lamp Area, Maintaining Losses. Excellent, Practical Display
AN65-16
Application Note
Figure Plastic "Cocoon" Cuts Losses. Metallic Foil Absorptive Floats from Grounded Display Frame. Good Compromise with about Loss
Application Note
AN65-17
Figure Plastic "Outrigger" Isolates Lamp from Metal Display Frame Loss Path
AN65-18
Application Note
Figure Plastic Isolates Lamp from Metal Frame This Display's Rear View
Application Note
AN65-19
Figure Figure 20's Display Front View Continues Plastic Isolation Treatment Reflective Foil (over Lamp) Contacts Metal Frame. Massive Losses This Path Cause Overall Loss. Trimming Foil from Metal Cuts Loss
AN65-20
Application Note
Figure Another "Outrigged" Plastic Enclosure Suffers Foil Contacting Display's Frame Metal. Relieving Foil from Metal Cuts Losses from Poor Wire Routing (Lower Right) Causes Loss
Application Note
Figure Isolation Slits (Center Right Left) Metal Reflector Prevent Losses Grounded Metal Frame (Upper Right Left). Overall Losses About
AN65-21
AN65-22
Application Note
Figure Close-Up Figure 23's Isolation Slit Construction. Secondary Benefit Control Reflector-to-Lamp Distance, Minimizing Capacitance
Application Note
AN65-23
Figure Metal Cover over Lamp Causes Loss. Replacing Cover Securing Screws with Nylon Types Floats Cover from Ground, Dropping Loss Replacing Cover with Plastic Improves Loss Only Improvement!
AN65-24
Application Note
Figure Huge Metal Area over Lamp Causes Loss. Replacing Metal Lamp Area with Plastic Cuts Loss
Application Note
AN65-25
Figure Metallic Foil over Lamp (Upper Center) Dumps Absorbed Energy Metal Rear Cover. Loss Results
AN65-26
Application Note
Figure Losses Display's Nonconductive Frame (Black Plastic) Thrown Away Lossy Reflective Foil Contacting Massive Metal Rear Cover. Loss Results
Application Note
AN65-27
Figure Similar Situation Figure Large Metal Rear Cover Contacts Lossy Foil (Not Visible), Causing Huge Losses
AN65-28
Application Note
Figure Grounded Metallic Optical Reflector Automotive Lamp Introduces Loss. Optical Gain over Nonmetallic Reflector Justify Large Electrical Loss
Application Note
AN65-29
Figure Metallic Heater Lamp this Automotive Application Eases Temperature Starting Causes Loss
AN65-30
Application Note
Figure Similar Figure Metallic Cold Start Heaters Automotive Application Induce Loss
Application Note
Considerations Multilamp Designs Multiple-lamp designs recommended lamp intensity matching important. Maintaining emission matching over time, temperature production variations quite difficult. some restricted cases multilamp displays viable option, single lamp with good diffuser optics almost always better approach. Information dual-lamp displays presented here reference purposes only.5 Systems using lamps have some unique layout problems. Almost dual-lamp displays color units. lower light transmission characteristics color displays necessitates more light. such, display manufacturers sometimes lamps produce more light. wiring layout these dual-lamp color displays affects efficiency illumination balance lamps. Figure shows "x-ray" view typical display. This symmetrical arrangement presents equal parasitic losses. lamps well-matched, circuit's current output splits evenly equal illumination occurs. Figure 34's display arrangement less friendly. asymmetrical wiring forces unequal losses lamps receive imbalanced current. Even with identical lamps, illumination balanced. This condition partially correctable skewing values because drives greater parasitic capacitance, should larger than This tends equalize currents, promoting equal lamp drive. important realize that this compensation does nothing recapture lost energy efficiency still compromised. There substitute minimizing loss paths. Similarly, change lamp characteristics (e.g., aging) cause imbalanced illumination recur. general, imbalanced illumination causes fewer problems than might supposed high intensity levels. Unequal illumination much more noticeable lower levels. worst case dimmer lamp only partially illuminate. This phenomenon, sometimes called "Thermometering," discussed detail text section, "Floating Drive Circuits."
Note text's tone intended convey distaste multilamp displays. They very soul heartache.
DISPLAY HOUSING
CCFL LAMP
TRANSFORMER SECONDARY
CSTRAY FROM TRANSFORMER SECONDARY SCREEN
MATCHED CSTRAY
CCFL LAMP
Figure Loss Paths "Best Case" Dual-Lamp Display. Symmetry Promotes Balanced Illumination, Lamp Limitations Dominate Achievable Results
AN65-31
Application Note
DISPLAY HOUSING
CCFL LAMP
TRANSFORMER SECONDARY
SCREEN
CSTRAY
FROM TRANSFORMER SECONDARY
CCFL LAMP MISMATCHED CSTRAY
Figure Asymmetric Losses Dual-Lamp Display. Skewing Values Compensates Imbalanced Loss Paths Wasted Energy
CCFL Power Supply Circuits Choosing approach general purpose CCFL power supply difficult. variety disparate considerations make determining "best" approach thoughtful exercise. Above all, architecture must extraordinarily flexible. sheer number diversity applications demands this. considerations take many degrees freedom. Power supply voltages range from with output power from minuscule 50W. load highly nonlinear varies over operating conditions. backlight often located some distance from primary power source, meaning supply must tolerate substantial supply impedances. Similarly, must corrupt supply with noise, introduce appreciable into system environment. Component count should supply must physically quite small space usually extremely limited. Additionally, circuit must relatively layout-insensitive because varying board shape requirements. Interface shutdown dimming control should accommodate either digital analog inputs, including voltage, current, resistive, serial bit-stream addressing. Finally, lamp current should predictable stable with changes time, temperature supply voltage. current-fed, feedback-controlled resonant Royer converter meets these requirements.6 This approach, because extreme flexibility, favorable compromise. operates over wide supply ranges scales well over broad output power range. Current taken from supply almost continuously, making circuit tolerate supply impedance. This characteristic also means that circuit operation does corrupt power supply lines. There problem component count low. small, relatively insensitive layout easy interface Lastly, lamp current stable predictable over operating conditions.
Note Appendices detailed discussion architecture selection Royer configuration.
AN65-32
Application Note
Figure practical CCFL power supply circuit based above discussion. Efficiency with input voltage range 6.5V 20V. This efficiency figure degraded about ®1172 powered from same supply main circuit terminal. Lamp intensity continuously smoothly variable from zero full intensity. When power applied LT1172 switching regulator's Feedback below device's internal 1.2V reference, causing full duty cycle modulation (Trace Figure 36). conducts current (Trace which flows from L1's center tap,
27pF +VIN LAMP 10µF
20V/DIV 0.4A/DIV 20V/DIV 20V/DIV 1000V/DIV 5V/DIV
4µs/DIV THRU 20µs/DIV TRIGGERS FULLY INDEPENDENT
1N4148
Figure Waveforms Cold Cathode Fluorescent Lamp Power Supply. Note Independent Triggering Traces through
through transistors, into L2's current deposited switched fashion ground regulator's action. transistors comprise current driven Royer class converter7 which oscillates frequency primarily L1's characteristics (including load) 0.068µF capacitor. LT1172 driven sets magnitude Q1/Q2 tail current, hence L1's drive level. 1N5818 diode maintains L2's current flow when LT1172 off. LT1172's 100kHz clock rate asynchronous with respect push/pull converter's (60kHz) rate, accounting Trace waveform thickening. 0.068µF capacitor combines with L1's characteristics produce sine wave voltage drive collectors (Traces respectively). furnishes voltage step-up about 1400VP-P appears secondary (Trace Current flows through 27pF capacitor into lamp. negative waveform cycles, lamp's current steered ground Positive waveform cycles directed ground referred 562/50k potentiometer chain. positive half-sine appearing across resistors (Trace represents lamp current. This signal filtered 10k/0.1µF pair presented LT1172's Feedback pin. This connection closes control loop which regulates lamp current. capacitor LT1172's provides stable loop compensation. loop forces LT1172 switch
0.068µF
+VIN 6.5V (SEE APPENDIX HIGHER INPUT VOLTAGES) (SEE TEXT) 1N5818
1N4148
CONNECT LT1172 LOWEST VOLTAGE AVAILABLE (VMIN LT1172
300µH 562* INTENSITY ADJUST
0.1µF
C1=MUST LOSS CAPACITOR. METALIZED POLYCARB WIMA MKP-20 (GERMAN) PANASONIC ECH-U RECOMMENDED L1=SUMIDA 6345-020 COILTRONICS CTX110092-1 (PIN NUMBERS SHOWN COILTRONICS UNIT) L2=COILTRONICS CTX300-4 Q2=ZETEX ZTX849, ZDT1048 ROHM 2SC5001 *=1% FILM RESISTOR SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876, SUMIDA (708) 956-0666
Figure Efficiency Cold Cathode Fluorescent Lamp Power Supply
Note Appendix "Who Royer What Design?" also Reference
AN65-33
Application Note
mode modulate L2's average current whatever value required maintain constant current lamp. constant current's value, hence lamp intensity, varied with potentiometer. constant current drive allows full 100% intensity control with lamp dead zones "pop-on" intensities.8 Additionally, lamp life enhanced because current cannot increase lamp ages. circuit's 0.1% line regulation notably better than some other approaches. This tight regulation prevents lamp intensity variation when abrupt line changes occur. This typically happens when battery-powered apparatus connected AC-powered charger. circuit's excellent line regulation derives from fact that L1's drive waveform never changes shape input voltage varies. This characteristic permits simple 0.1µF produce consistent response. averaging characteristic serious error compared true conversion, error constant "disappears" shunt's value. This circuit similar previously described9 efficiency higher. efficiency improvement primarily transistor's higher gain lower saturation voltage. base drive resistor's value (nominally should selected provide full saturation without inducing base overdrive beta starvation. procedure doing this described following section, "General Optimization Measurement Considerations." Figure 37's circuit similar, uses transformer with lower copper core losses increase efficiency 91%. trade-off slightly larger transformer size. Additionally, higher frequency switching regulator offers slightly lower current, aiding efficiency. L1's smaller value, result higher frequency operation, permits slightly reduced copper loss. transformer options listed allow efficiency optimization over supply range interest. Value shifts base drive resistor reflect different transformer characteristics. This circuit also features shutdown pulse width controlled dimming input. Appendix "Intensity Control Shutdown Methods," details operation these features. Figure directly derived from Figure produces 10mA output drive color LCDs efficiency.
27pF 4.5V LAMP 0.1µF 1N4148
10µF
2.7V 5.5V
1N4148
2.2µF
33µH 1N5818 562*
DIMMING
LT1372/LT1377 1N4148 OPTIONAL REMOTE DIMMING WIMA MKP-20, PANASONIC ECH-U COILCRAFT DT3316-333 ZETEX ZTX849, ZDT1048 ROHM 2SC5001 COILTRONICS 02-12614-1 CTX110600-1 (SEE TEXT) FILM RESISTOR SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876 COILCRAFT (708) 639-6400 0.1µF
Figure Efficient CCFL Supply Loads Features Shutdown Dimming Inputs. Higher Frequency Switching Regulator Reduces L1's Size While Requiring Less Current
slight efficiency improvement comes from reduction regulator "housekeeping" current percentage total current drain. Value changes components result higher power operation. most significant change involves driving lamps. Accommodating lamps involves separate ballast capacitors circuit operation similar. Dual-lamp designs reflect slightly different loading back through transformer's primary. usuNote Controlling nonlinear load's current, instead voltage, permits applying this circuit technique wide variety nominally evil loads. Appendix "Additional Circuits." Note "Illumination Circuity Liquid Crystal Displays," Linear Technology Corporation, Application Note August 1992 "Techniques Efficient Illumination," Linear Technology Corporation, Application Note August 1993.
AN65-34
Application Note
ally ends 10pF 47pF range. Note that appear with their lamp loads parallel across transformer's secondary. such, C2's value often smaller than single-lamp circuit using same type lamp. Ideally, transformer's secondary current splits evenly between C2-lamp branches, with total load current being regulated. practice, differences between differences lamps lamp wiring layout preclude perfect current split. Practically, these differences small lamps appear emit equal amount light high intensity. Layout lamp matching influence C2's value. Some techniques dealing with these issues appear text section, "Considerations Multilamp Designs." previously stated, dual-lamp designs distinctly recommended, particularly balanced illumination over wide dimming ranges required.
LAMP 27pF LAMP 27pF 10mA 1N4148
Figure uses dedicated CCFL LT1183, enhance circuit performance. Royer-based high voltage converter portion recognizable from previous circuits, with 200kHz LT1183 performing switching regulator/feedback function. This also features open lamp protection circuitry, simplified frequency compensation, separate regulator providing contrast other features.10 contrast supply driven LT1183 with associated discrete components completing function. CCFL contrast outputs adjusted with potentiometers.
Note Open lamp protection often desirable added previous circuits cost some discrete components. Appendix "Open Lamp/Overload Protection." Frequency compensation issues covered text section "Feedback Loop Stability Issues." Appendix discussion contrast supplies.
0.1µF (SEE TEXT)
2.7V 5.5V
1N4148
1N5818 33µH LT1372/LT1377 300* WIMA MKP2 PANASONIC ECH-U COILTRONICS CTX150-4 ZETEX ZTX849, ZDT1048 ROHM 2SC5001 COILTRONICS CTX210605 SUMIDA EPS-207 (PIN NUMBERS SHOWN COILTRONICS UNIT) FILM RESISTOR SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876, SUMIDA (708) 956-0666 DIMMING INPUT (SEE TEXT)
0.1µF
Figure Efficient CCFL Supply 10mA Loads Features Shutdown Dimming Inputs. DualLamp Designs, Typical Early Color Displays, Recommended
AN65-35
Application Note
LAMP 27pF 2.2µF
1000pF 220k
2.2µF
0.068µF 100k BAT-85 100µH (PWM) 1kHz 38.3k R5,38.3k, SHUTDOWN C8,0.68µF CCFL PGND ICCFL LT1183 CCFL AGND SHDN PGND ROYER CCFL BULB 1N5818
2.2µF 1N5934A 1N914
VARYING V(CONTRAST) VOLTAGE FROM GIVES VARIABLE NEGATIVE CONTRAST FROM -10V -30V
1N914
22µF NEGCON
2.2µF
(CONTRAST) R10, 10k,
2.2µF
4.99k,
0.1µF 40.2k
MUST LOSS CAPACITOR C1=WIMA MKP-20 PANASONIC ECH-U L1=COILTRONICS CTX210605 L2=COILTRONICS CTX100-4 L3=COILTRONICS CTX02-12403 Q2=ZETEX ZTX849, ZDT1048 ROHM 2SC5001 SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876
0.01µF
1N4148
Figure Dedicated Backlight Includes Switching Regulator, Open Lamp Protection Contrast Supply. 200kHz Operation Minimizes Size. Shutdown Control Inputs Simplified
AN65-36
Application Note
Power CCFL Power Supplies Many applications require relatively power CCFL backlighting. Figure 40's variation, optimized voltage inputs, produces output. Circuit operation similar previous examples. fundamental difference L1's higher turns ratio, which accommodates reduced available drive voltage. circuit values given typical, although some variation occurs with various lamps layouts. Figure 41's design, so-called "dim backlight," optimized very current lamp operation. circuit meant input voltages, typically with maximum lamp current. This circuit maintains control down lamp currents 1µA, very light! intended applications where longest possible
27pF +VIN LAMP 10µF 1N4148
battery life desired. Primary supply drain ranges from hundreds microamperes 100mA with lamp currents microamps 1mA. shutdown circuit pulls only 100µA. Maintaining high efficiency lamp currents requires modifying basic design. Achieving high efficiency operating current requires lowering quiescent power drain. this previously employed pulse width modulator-based devices replaced with LT1173. LT1173 Burst Modeoperation regulator. When this device's Feedback delivers burst output current pulses, putting energy into transformer restoring feedback point. regulator maintains control appropriately modulating burst duty cycle. ground referred diode prevents substrate turn-on excessive ring-off.
Burst Mode trademark Linear Technology Corporation.
0.1µF
15pF
LAMP 10µF 1N4148 1N4148
+VIN
+VIN 3.6V 5.5V LT1172 1N5818
1N4148
0.01µF
700* 50µH INTENSITY ADJUST
+VIN ILIM LT1173
0.1µF
1N5818 82µH 3.3k
INTENSITY ADJUST 1N5818 1N4148 SHUTDOWN 0.01µF
MUST LOSS CAPACITOR. METALIZED POLYCARB WIMA MKP-20 (GERMAN) PANASONIC ECH-U RECOMMENDED COILTRONICS CTX110654-1 COILTRONICS CTX50-4 ZETEX ZTX849, ZDT1048 ROHM 2SC5001 FILM RESISTOR SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876, SUMIDA (708) 956-0666
MUST LOSS CAPACITOR. METALIZED POLYCARB WIMA FKP2, MKP-20 (GERMAN) PANASONIC ECH-U RECOMMENDED SUMIDA 6345-020 COILTRONICS CTX110092-1 NUMBERS SHOWN COILTRONICS UNIT TOKO 262LYF-0091K (408) 432-8251 ZETEX ZTX849, ZDT1048 ROHM 2SC5001 SUBSTITUTE COMPONENTS
Figure Design Intended Voltage Operation. L1's Modified Turns Ratio Allows Operation Down 3.6V
Figure Power CCFL Power Supply. Circuit Controls Lamp Current over Range
AN65-37
Application Note
During periods regulator essentially shut down. This type operation limits available output power, cuts quiescent current losses. contrast, other circuit's pulse width modulator type regulators maintain "housekeeping" current between cycles. This results more available output power higher quiescent currents. Figure shows operating waveforms. When regulator comes (Trace Figure delivers bursts output current L1/Q1/Q2 high voltage converter. converter responds with bursts ringing resonant frequency.11 circuit's loop operation similar previous designs except that T1's drive waveform varies with supply. Because this, line regulation suffers circuit recommended wide ranging inputs. Some lamps display nonuniform light emission very excitation currents. text section, "Floating Lamp Circuits." CCFL power supply that addresses previous circuit's line regulation problems operates from detailed Figure This circuit, contributed Steve Pietkiewicz LTC, drive small CCFL over 100µA range.
5V/DIV
5V/DIV
50µs/DIV
Figure Waveforms Power CCFL Power Supply. LT1173 Burst Type Regulator (Trace Periodically Excites Resonant High Voltage Converter Collector Trace
Note discontinous energy delivery loop causes substantial jitter burst repetition rate, although high voltage section maintains resonance. Unfortunately, circuit operation "chop" mode region most oscilloscopes, precluding detailed display. "Alternate" mode operation causes waveform phasing errors, producing inaccurate display. such, waveform observation requires special techniques. Figure taken with dual-beam instrument (Tektronix 556) with both beams slaved time base. Single sweep triggering eliminated jitter artifacts. Most oscilloscopes, whether analog digital, will have trouble reproducing this display.
1N5817
22pF
0.068µF CCFL
0.1µF SHDN SENSE LT1301
SELECT
47µH 2N3904
10µF
ILIM PGND
7.5K
1N4148
SHUTDOWN
L1=COILCRAFT D03316-473 Q2=ZETEX ZTX849, ZDT1048 ROHM 2SC5001 T1=COILTRONICS CTX110654-1 0.068 µF=WIMA MKP-20 PANASONIC ECH-U
5VDCIN INTENSITY ADJUST 100µA BULB CURRENT
Figure Power Cold Cathode Fluorescent Lamp Supply Optimized Voltage Inputs Small Lamps
AN65-38
Application Note
circuit uses LT1301 micropower DC/DC converter conjunction with current driven Royer class converter comprised When power intensity adjust voltage applied, LT1301's ILIM driven slightly positive, causing maximum switching current through IC's internal Switch (SW). Current flows from T1's center tap, through transistors, into L1's current deposited switched fashion ground regulator's action. Circuit efficiency ranges from full load, depending line voltage. Current mode operation combined with Royer's consistent waveshape input results excellent line rejection. circuit none line rejection problems attributable hysteretic voltage control loops typically found voltage micropower DC/DC converters. This especially desirable characteristic CCFL control, where lamp intensity must remain constant with shifts line voltage. Royer converter oscillates frequency primarily T1's characteristics (including load) 0.068µF capacitor. LT1301 driven sets magnitude Q1/Q2 tail current, hence T1's drive level. 1N5817 diode maintains L1's current flow when LT1301's switch off. 0.068µF capacitor combines with T1's characteristics produce sine wave voltage drive collectors. furnishes voltage step-up about 1400VP-P appears secondary. Alternating current flows through 22pF capacitor into lamp. positive half-cycles lamp's current steered ground negative half-cycles lamp's current flows through Q3's collector filtered LT1301's ILIM acts summing point with about 25µA bias current flowing into LT1301 regulates L1's current equalize Q3's average collector current, representing lamp current, R1's current, represented /R1. smooths current flow When zero, ILIM pin's bias current forces about 100µA bulb current. High Power CCFL Power Supply mentioned, CCFL circuit approach presented here scales quite nicely over wide range output power. Most circuits 0.5W region application's small size battery-driven nature. Automotive, aircraft, desktop computer other displays often require much higher power. Figure 44's arrangement scaled-up version text's CCFL circuits. This design, similar ones employed automotive use, drives CCFL. There virtually configuration changes, although most component power ratings have increased. transistors handle higher currents, other power components higher capacity. Efficiency about 80%. Additional high power circuits appear Appendix "Additional Circuits."
47pF
LAMP
1N4148 0.47µF
22µF
MUR405 150µH INTENSITY CONTROL 1N4148 LT1170 2.2µF
1N4148
2.2µF
L1=COILTRONICS CTX02-11128 L2=COILTRONICS CTX150-3-52 Q2=ZETEX ZTX849, ZDT1048 ROHM 2SC5001 0.47 µF=WIMA 0.15 TYPE MKP-20 COILTRONICS (407) 241-7876
Figure CCFL Supply Scaled Version Lower Power Circuits
AN65-39
Application Note
"Floating" Lamp Circuits circuits presented this point drive lamp singleended fashion. Similarly, Figure shows lamp electrode receiving drive with other terminal essentially ground. This causes significant loss parasitic paths associated with lamp's driven end. This because large voltage swing this region. parasitic paths near lamp's grounded undergo relatively little swing, contributing small energy loss. Unfortunately, lost energy heavily voltage-dependent CV2) energy loss excessive driven parasitics large. Figure minimizes losses altering drive scheme. this case lamp driven from both ends instead grounding end. This "floating" lamp arrangement requires only half voltage swing each lamp instead full swing end. This introduces more loss parasitic paths previously associated with
PARASITIC LOSS PATH PARASITIC LOSS PATH
grounded end. most cases these increased losses favorably offset reduced swing because loss term associated with voltage amplitude. advantage gained varies considerably with display type, although reduction lost energy common. some displays loss reduction good, occasionally improvement negligible. Heavily asymmetric wiring within display sometimes make floating drive more lossy than grounded drive. such cases testing both modes necessary determine which type drive most efficient. second advantage floating operation extended illumination range. "Grounded" lamps operating relatively currents display "thermometer effect," that light intensity nonuniformly distributed along lamp length.
PARASITIC LOSS PATH
FULL AMPLITUDE HIGH VOLTAGE BALLAST CAPACITOR
CCFL LAMP PARASITIC LOSS PATHS ESSENTIALLY
DIODES INTERNAL LT118X-BASED CIRCUITS FEEDBACK NODE
ENERGY LOST PARASITIC PATHS
Figure Ground Referred Lamp Drive Large Energy Loss High Voltage Regions Full Amplitude Swing
PARASITIC LOSS PATH
PARASITIC LOSS PATH
PARASITIC LOSS PATH
PARASITIC LOSS PATH
AMPLITUDE HIGH VOLTAGE BALLAST CAPACITOR
CCFL LAMP PARASITIC LOSS PATHS
AMPLITUDE HIGH VOLTAGE
ENERGY LOST PARASITIC PATHS
Figure "Floating" Lamp Allows Reduced, Bipolar Drive, Cutting Losses Parasitic Capacitance Paths. Formerly Grounded Lamp End's Paths Absorb More Energy Than Before, Overall Loss Lower Equation's Term
AN65-40
Application Note
Figure shows that although lamp current density uniform, associated field imbalanced. field's intensity, combined with imbalance, means that there enough energy maintain uniform phosphor glow beyond some point. Lamps displaying thermometer effect emit most their light near driven electrode, with rapid emission fall-off distance from electrode increases. Placing conductor along lamp's length largely alleviates "thermometering." trade-off decreased efficiency energy leakage.12 worth noting that various lamp types have different degrees susceptibility thermometer effect. Some displays require extended illumination range. "Thermometering" usually limits lowest practical illumination level. acceptable minimize "thermometering" eliminate large field imbalance. floating drive used reduce energy loss also provides minimize "thermometering." Figure reviews
Note very simple experiment quite nicely demonstrates effects energy leakage. Grasping lamp voltage (low field intensity) with thumb forefinger produces almost change circuit input current. Sliding thumb/forefinger combination towards high voltage (higher field intensity) lamp produces progressively greater input currents. Don't touch high voltage lead receive electrical shock. Repeat: touch high voltage lead receive electrical shock.
BAT-85 LAMP 100k
FIELD STRENGTH INCREASES WITH INCREASING DISTANCE FROM GROUNDED LAMP HIGH VOLTAGE LAMP FEEDBACK
ESSENTIALLY GROUNDED
Figure Field Strength Distance Ground Referred Lamp. Field Imbalance Promotes Uneven Illumination Drive Levels
0.1, WIRE SHUNT
27pF
10µF 162*
10µF
2N7002
0.1µF
LT1077 (SEE TEXT)
TP0610
4.99k*
2.2µF C1=WIMA MKP-20 PANASONIC ECH-U L1=COILTRONICS CTX150-4 Q2=ZETEX ZTX849, ZDT1048 ROHM 2SC5001 T1=SUMIDA EPS-207 *=1% FILM RESISTOR SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876, SUMIDA (708) 956-0666
1N5818 LT1172
1N4148
0.03µF
DIMMING INPUT (SEE TEXT)
Figure Practical "Floating" Lamp Drive Circuit. Senses Royer Input Current with Providing Resultant Feedback Information Switching Regulator. Circuit Reduces Lost Energy Parasitics
AN65-41
Application Note
circuit originally introduced previous publication.13 circuit's most significant aspect that lamp fully floating--there galvanic connection ground previous designs. This allows deliver symmetric, differential drive lamp. Such balanced drive eliminates field imbalance, reducing thermometering lamp currents. This approach precludes feedback connection floating output. Maintaining closedloop control necessitates deriving feedback signal from some other point. theory, lamp current proportions T1's L1's drive level some form sensing this used provide feedback. practice, parasitics make practical implementation difficult.14 Figure derives feedback signal measuring Royer converter current feeding this information back LT1172. Royer's drive requirement closely proportions lamp current under conditions. senses this current across shunt biases closing
LAMP 27pF 2.2µF
local feedback loop. Q3's drain voltage presents amplified, single-ended version shunt voltage feedback point, closing main loop. A1's power supply bootstrapped T1's boosted swing BAT-85 diode, permitting sense across supply-fed shunt resistor. Internal characteristics ensure start-up substitution this device recommended.15 lamp current tightly controlled before 0.5% regulation over wide supply ranges possible. dimming this circuit controlled 1kHz signal. Note heavy filtering (33k/1µF) outside feedback loop. This allows fast time constant, minimizing turn-on overshoot.16
Note Reference Note Appendix Ears Goghs-- Some Not-So-Great Ideas," details. Note Reference then don't didn't warn you. Note text section, "Feedback Loop Stability Issues."
1000pF 220k
2.2µF
0.068µF 100k BAT-85 100µH CCFL PGND ICCFL CCFL BULB 1N5818
LT1184F CCFL ROYER AGND SHDN 15.4k
SHUTDOWN
2.2µF
ALUMINUM ELECTROLYTIC RECOMMENDED C3B. PREVENTS TURN-ON SURGE CURRENT DAMAGE LT1184F HIGH SIDE SENSE RESISTOR. MUST LOSS CAPACITOR. C1=WIMA MKP-20 PANASONIC ECH-U L1=COILTRONICS CTX210605 L2=COILTRONICS CTX100-4 Q2=ZETEX ZTX849, ZDT1048 ROHM 2SC5001 SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876
Figure LT1184F Version Figure 48's Floating Lamp Circuit Offers Similar Performance with Fewer Components. Open Bulb Protection Shutdown Included
AN65-42
Application Note
other respects operation similar previous circuits. This circuit typically permits lamp operate with less energy loss over 40:1 intensity range without "thermometering." normal feedback connection usually limited 10:1 range. IC-Based Floating Drive Circuits Figure compacts Figure into component count, floating drive circuit. LT1184F contains funcUP LAMP 27pF 2.2µF
tions except Royer-based high voltage converter. circuit also "open lamp" protection 1.23V reference biasing dimming potentiometer. Figure adds bipolar contrast supply output Figure LT1182 allows setting contrast supply polarity simply grounding appropriate output terminal. CCFL portion similar previous circuit, although intensity controlled with varying input.
1000pF 220k
2.2µF
EITHER NEGCON POSCON MUST GROUNDED. GROUNDING NEGCON GIVES VARIABLE POSITIVE CONTRAST FROM 30V. GROUNDING POSCON GIVES VARIABLE NEGATIVE CONTRAST FROM -10V -30V. 22µF POSCON
0.068µF 100k BAT85 100µH (PWM) 1kHz 46.4k 2.2µF 43.2k, SHUTDOWN 0.15µF CCFL PGND ICCFL LT1182 CCFL AGND SHDN PGND ROYER CCFL BULB 1N5818
2.2µF 1N5934A 1N914
NEGCON 1N914 8.45k 1.21k 0.01µF
0.01µF (CONTRAST)
2.2µF
ALUMINUM ELECTROLYTIC RECOMMENDED C3B. PREVENTS TURN-ON SURGE CURRENT DAMAGE LT1182 HIGH SIDE SENSE RESISTOR. MUST LOSS CAPACITOR. C1=WIMA MKP-20 PANASONIC ECH-U L1=COILTRONICS CTX210605 L2=COILTRONICS CTX100-4 L3=COILTRONICS CTX02-12403 Q2=ZETEX ZTX849, ZDT1048 ROHM 2SC5001 SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876
4.99k
Figure LT1182 Bipolar Output Contrast Supply Addition Floating Lamp Drive
AN65-43
Application Note
Figure 51's circuit similar, although contrast supply included. LT1186 implements floating lamp drive similar Figure This contains internal converter which addressed accumulating
BAT-85
stream serial protocol. Figure shows typical arrangement using 80C31 type microcontroller. Figure gives complete software listing which written Tommy LTC.
LAMP 27pF 2.2µF 2.2µF
SHUTDOWN FROM
CCFL PGND ICCFL
CCFL BULB
1000pF
220k
LT1186 CCFL ROYER AGND SHDN IOUT DOUT
3.3V 2.2µF
0.068µF 100k
100µH
1N5818
ALUMINUM ELECTROLYTIC RECOMMENDED C3B. PREVENTS TURN-ON SURGE CURRENT DAMAGE LT1186 HIGH SIDE SENSE RESISTOR. MUST LOSS CAPACITOR. C1=WIMA MKP-20 PANASONIC ECH-U L1=COILTRONICS CTX210605 L2=COILTRONICS CTX100-4 Q2=ZETEX ZTX849, ZDT1048 ROHM 2SC5001 SUBSTITUTE COMPONENTS COILTRONICS (407) 241-7876
Figure LT1186 Permits Serial Stream Data Addressing Floating Lamp Current
ROYER CONVERTER
LAMP
LT1186 DOUT RS232 FROM
P1.4 P1.3 P1.0 P1.1
80C31
Figure Typical Processor Interface Figure
AN65-44
Application Note
LT1186 algorithm written assembly code file named LT1186A.ASM function call from MAIN fuction below. Note: user inputs integer from keyboard LT1186 adjusts IOUT programming current control operating lamp current brightness display. #include <stdio.h> #include <reg51.h> #include <absacc.h> extern char lt1186(char); external assembly function lt1186a.asm*/ sbit Clock 0x93; main() number LstCode; Clock TMOD 0x20; Establish serial communication 1200 baud 0xE8; SCON 0x52; TCON 0x69; while(1) Endless loop printf("\nEnter code from 255:"); scanf("%d",&number); if((0>number)|(number>255)) number printf("The number exceeds range. again!"); else LstCode lt1186(number); printf("Previous %u",(LstCode&0xFF)); previous number with 0xFF turn sign extension number following assembly program named LT1186A.ASM receives word from main program, lt1186 lt1186(). Assembly interface headers, declarations memory allocations listed before actual assembly code. Port p1.4 Port p1.3 Port p1.1 Dout Port p1.0 Figure Complete Software Listing Figure 52's Processor Interface
AN65-45
Application Note
NAME LT1186_ CCFL PUBLIC lt1186, ?lt1186?BYTE ?PR?ADC_INTERFACE?LT1186_CCFL SEGMENT CODE ?DT?ADC_INTERFACE?LT1186_CCFL SEGMENT DATA RSEG ?DT?ADC_INTERFACE?LT1186_CCFL ?lt1186?BYTE: RSEG ?PR?ADC_INTERFACE?LT1186_CCFL DOUT p1.4 p1.3 p1.1 P1.0 r7,?lt1186?BYTE #01h #08h DIN, DOUT loop ;set high initialize LT1186 ;move input number(Din) from keyboard ;setup port p1.0 becomes input goes low, enable ;move accumulator ;load counter counts ;clear carry before rotating ;rotate left bit(MSB) into carry ;move carry port ;Clk goes high LT1186 latch ;read Dout into carry ;rotate left Dout into accumulator ;clear clock shift next Dout ;next data loop ;move previous code character return ;bring high disable
lt1186: setb loop: setb djnz setb
Note: When goes low, previous code appears Dout.
Figure (continued). Complete Software Listing Figure 52's Processor Interface
High Power Floating Lamp Circuit High power floating lamp circuits require more current than LT118X series deliver. such cases function built from discrete components ICs. Figure shows CCFL circuit used automotive application. This 4-lamp circuit uses LT1269 currentfed Royer converter provide high power. Lamp current sensed current transformer associated components form synchronous rectifier T2's level output. provides gain closes loop back
LT1269's feedback terminal. T2's isolated sensing permits advantages floating operation with LT1269 providing high power capability. This circuit about efficiency output, wide dimming range 0.1% line regulation. Selection Criteria CCFL Circuits Selecting which CCFL circuit specific application involves numerous trade-offs. variety issues determine which circuit "best" approach.
AN65-46
Application Note
27pF LAMP 27pF LAMP 27pF LAMP 27pF LAMP 3.01k*
LM392
10k*
22µF 1N755
ZTX849
ZTX849
MUR405
LM392 4.7k* 1000pF 15k*
10k* 0.1µF 680k*
100k* CD4066
LT1269
BAT-85 INTENSITY
22µF
WIMA 0.15µF MKP-20, UNITS COILTRONICS, CTX150-3-52 COILTRONICS, CTX02-11128-2 PULSE ENGINEERING, PE-51688 METAL FILM RESISTOR
2.2µF
AN65
Figure High Power, Multilamp Display Using Floating Drive Approach. Power Requirement Necessitates LT1269 Regulator Discrete Component Approach. Floating Feedback Path Current Transformer
minimum, user should consider following guidelines before committing approach. Related discussion following topics covered appropriate text sections.
Display Characteristics
display characteristics (including wiring losses) should well-understood. Typically, display manufacturers list lamp requirements. These specifications often obtained from lamp vendor, usually tests free air, with significant parasitic loss paths. This means that actual required power, start running voltages significantly differ from data sheet specifications. only
certain display characteristics measure them. measured display energy loss determine floating grounded circuit applicable. loss displays (relatively rare) usually provide better overall efficiency with grounded drive. losses become worse (unfortunately, relatively common) floating drive becomes better choice. Efficiency measurements required both modes determine best choice. (See "General Optimization Measurement Considerations.")
Operating Voltage Range
operating voltage range includes minimum maximum voltages circuit must operate from.
AN65-47
Application Note
battery-driven apparatus supply range easily 3:1, sometimes greater. Best backlight performance usually obtained range. general, potentials below require some efficiency trade-offs moderate (1.5W power levels. Some systems reduce backlight power when running from battery, this have pronounced effect design. Even seemingly small (e.g., 20%) reductions power make painful trade-offs unnecessary. particular, high turns ratio transformers required support voltage operation full lamp output. They work well somewhat less efficiently than lower ratio types higher peak currents characteristic their operation. Current trends battery technology encourage system operation voltages, necessitating extreme care transformer selection Royer circuit design.
Supply Current Profile
backlight often physically located "forward" system. Impedances cables, switches, traces connectors build significant levels. This means that CCFL circuit should draw operating power continuously, rather than requiring discrete, high current "chunks" from lossy supply line. Royer-based architectures nearly ideal this regard, pulling current smoothly over time requiring special bypassing, supply impedance layout treatment. Similarly, Royer type circuits cause significant disturbances supply line, preventing noise injection back into supply.
Lamp Current Certainty
ability predict lamp current full intensity important maintain lamp life. Excessive overcurrent greatly shortens lamp life, while yielding little luminosity benefit (see Figure Grounded circuits excellent this category with usually achieved. Floating circuits typically range. Tight current tolerances benefit unit/unit display luminosity because lamp emission display attenuation variations approach ±20% vary over life.
Auxiliary Operating Voltages
Auxiliary, logic supply voltages should used available) CCFL "housekeeping" currents, such "VIN" pins. This saves power. Always switching regulators from lowest potential available, usually 3.3V Many systems provide these voltages switched form, making separate shutdown lines unnecessary. Simply turning switching regulator's supply shuts entire backlight circuit down.
Efficiency
CCFL backlight efficiency should considered from perspectives. electrical efficiency ability circuit convert power high voltage deliver load (lamp parasitics) with minimum loss. optical efficiency perhaps more meaningful user. simply ratio display luminosity power into CCFL circuit. electrical optical losses lumped together this measurement produce luminosity power specification. quite significant that electrical optical peak efficiency operating points necessarily coincide. This primarily lamp's emissivity dependence waveshape. optimum waveshape emissivity coincide with circuit's electrical operating peak. fact, quite possible "inefficient" circuits produce more light than "more efficient" versions. only ensure peak efficiency given situation optimize circuit display.
Line Regulation
Grounded lamp circuits, virtue their true global feedback, provide best line regulation. abrupt changes, user notice anything beyond regulation. grounded circuit easily meets this requirement; floating circuit usually will. Slowly changing line inputs causing excursions outside normally problem because they detectable. Rapid line changes, such plugging systems line adapter, require good regulation avoid annoying display flicker.
Power Requirements
CCFL's power requirement, including display wiring losses, should well-defined over conditions, including temperature lamp specification variations. Usually, versions floating lamp circuits restricted output power while grounded circuit power easily scaled.
AN65-48
Application Note
Shutdown
System shutdown almost always requires turning backlight. many cases voltage supply already available switched form. this CCFL circuits shown off, absorbing very little power. switched voltage power available shutdown inputs used, requiring extra control line. necessary physically segment circuit this should considered last resort.17
Contrast Supply Capability
Some LT118X parts provide contrast supply outputs. other circuits not. LT118X's onboard contrast supply usually advantage space sometimes restricted that cannot used. such cases contrast supply must remotely located.
Transient Response
CCFL circuit should turn lamp without attendant overshoot poor control loop settling characteristics. This cause objectionable display flicker, worst case result transformer overstress failure. Properly prepared floating grounded CCFL circuits have good transient response, with LT118X-based types inherently easier optimize.
Emissions
Backlight circuits rarely cause emission problems shielding usually required. Higher power versions (e.g., require attention meet emission requirements. fast rise switching regulator output sometimes causes more than high voltage waveform. shielding used, parasitic effects part inverter load optimization must carried with shield place. Summary Circuits interdependence backlight parameters makes summarizing rating various approaches hazardous exercise. There simply intellectually responsible streamline selection design process optimum results desired. meaningful choice must outcome laboratory-based experimentation. There just many interdependent variables surprises systematic, theoretically based selection. Pure analytics pretty; working circuits come from bench. Some generalizations having limited usefulness are, however, possible. Figures attempt summarize salient characteristics part type (however cautiously) considered beginning point.18 Figure summarizes characteristics circuits. Figure focuses features LT118X series parts.
Note Appendix "Layout, Component Emissions Considerations." Note Readers detecting author ambivalence about inclusion Figures hallucinating.
Dimming Control
method dimming should considered early design. circuits shown controlled potentiometers, voltages currents, pulse width modulation serial data protocol. dimming scheme with high accuracy maximum current prevents excessive lamp drive should employed.
Open Lamp Protection
CCFL circuits deliver current source output. lamp broken disconnected, compliance voltage limited transformer turns ratio input voltage. Excessive voltages cause arcing resultant damage. Typically, transformers withstand this condition open lamp protection ensures against failures. This feature built into LT118X series; must added other circuits.
Size
Backlight circuits usually have severe size component count limitations. board must within tightly defined dimensions. LT118X series-based circuits offer lowest component count, although board space usually dominated Royer transformer. extremely tight spaces
AN65-49
Application Note
ISSUES Optical Efficiency LT118X SERIES Grounded output versions display dependent. Floating versions usually better. Grounded output versions-- 90%, depending supply voltage display. Floating output versions slightly lower. grounded versions, floating output types 0.1% 0.3% grounded types, 0.5% floating versions 5.3V 30V, depending output power, temperature range, display, etc. 0.75W typical Continuous--no high current peaks LT117X SERIES Display dependent LT137X SERIES Display dependent
Electrical Efficiency
90%, depending supply voltage display
92%, depending supply voltage display
Lamp Current Certainty Line Regulation Operating Voltage Range
maximum 0.1% 0.3% 4.0V 30V, depending output power, temperature range, display etc. 0.75W typical Continuous--no high current peaks
maximum 0.1% 4.0V 30V, depending output power, temperature range, display, etc. 0.5W typical Continuous--no high current peaks
Power Range Supply Current Profile
Shutdown Control Transient Response-- Overshoot Dimming Control
Yes--logic compatible Excellent--no optimization required Pot., PWM, variable voltage current. LT1186 serial digital input with data storage.
Requires small bipolar transistor Excellent--requires optimization some cases Pot., PWM, variable voltage current
Yes--logic compatible Excellent--requires optimization some cases Pot., PWM, variable voltage current Low, although high power versions require attention layout shielding Requires external small-signal transistor some discretes high supply voltages Small--1MHz magnetics fastest versions
Emissions
Open Lamp Protection
Internal
Requires external small-signal transistor some discretes high supply voltages Small--100kHz magnetics
Size
component count, small overall board footprint. 200kHz magnetics. Various contrast supply options available, including bipolar output
Contrast Supply Capability
Figure Design Issues Typical Part Choice. Chart Makes Simplistic Assumptions Intended Guide Only
AN65-50
Application Note
LT1269/LT1270 Display dependent LT1301 Display dependent LT1173 Display dependent
90%, depending supply voltage display
88%, depending supply voltage display
75%, depending supply voltage display
maximum 0.1% 0.3%
typical 0.1% 0.3%
4.5V 30V, depending output power, temperature range, display, etc. typical Continuous--no high current peaks Requires small bipolar transistor Excellent--requires optimization some cases Pot., PWM, variable voltage current High power mandates attention layout shielding Requires external small-signal transistor some discretes high supply voltages Relatively large high power 100kHz magnetics
practical
practical
0.02W practical Continuous--no high current peaks Yes--logic compatible Excellent--no optimization required Pot., PWM, variable voltage current Real
Essentially about 0.6W Irregular--relatively high current peaking requires attention supply rail impedance Logic compatible shutdown practical Excellent--no optimization required Pot., PWM, variable voltage current Itsy-bitsy
Requires external small-signal tansistor some discretes, supply voltages usually eliminate this consideration Very small--low power magnetics size
None, supply, power operation usually eliminates this issue Small--low power magnetics size
AN65-51
Application Note
LT1182 Floating Lamp Operation Grounded Lamp Operation Contrast Supply Voltage Reference Available Internal Control LT1183 LT1184 LT1184F LT1186
quite complex with number variables determining just where peak optical efficiency occurs. Typically, optical output peaking occurs with fairly clean, harmonic waveform Royer collectors (Figure 57). This usually result relatively large resonating capacitor small ballast capacitor. Conversely, converter's peak electrical efficiency point usually comes just appreciable second harmonic appears Royer collector waveform (Figure 58). peak electrical optical efficiency points almost never coincide optical efficiency often occurs more electrical efficiency peak. Happily, this very messy situation resolved relatively simple functional trim. trimming procedure assumes transformer turns ratio ballast capacitor values commensurate with lowest
Bipolar Unipolar Contrast Contrast Outputs Outputs
Figure Features Various LT118X Backlight Controllers
General Optimization Measurement Considerations Once display/lamp combination been picked, appropriate circuit selected optimized. "Optimization" implies maximizing performance those areas most important particular application. This involve trading characteristics area gain advantage another. circuit types described impose mild penalty this regard because they quite flexible. desirable characteristic something often loosely referred "efficiency." There really types efficiency backlight circuit. optical efficiency measures circuit/display combination transducer. ratio light output electrical power input. This ratio lumps converter's electrical loss with lamp display losses. backlight's electrical efficiency measures converter's electrical input output power without regard optical performance. Obviously, high electrical efficiency required reliable measure desirable. More subtly, ability measure manipulate purely electrical terms offers influence optical efficiency. This because lamp sensitive drive waveform's shape. Best emissivity lifetime usually obtained with crest factor, sinusoidal waveforms. Royer circuit's transformer capacitors selected provide this characteristic given display/lamp combination. Doing this optimizes lamp drive also effects converter's electrical efficiency. This interaction between optimum electrical optical operating points must accounted obtain best optical efficiency. relationship
5V/DIV
5µs/DIV
Figure Typical Royer Collector Waveform Peak Optical Output Point. Relatively Large Resonating Capacitor Degrade Electrical Efficiency
5V/DIV
5µs/DIV
Figure Typical Royer Collector Waveform Peak Electrical Efficiency Point. Relatively High Harmonic Content Degrade Optical Efficiency
AN65-52
Application Note
required circuit operating voltage have been chosen. this factor considered, optical efficiency peak will realized design regulate supply voltages. supply voltage operation mandates high turns ratio larger ballast capacitor values given display loss. display loss high, ballast capacitor value generally must rise offset voltage dividing effects between display's parasitic loss paths. Establish lowest values turns ratio ballast capacitor that maintain regulation minimum supply voltage before performing trim. Achieving peak optical efficiency involves comparing display luminosity input power different resonating capacitor values. given lamp/transformer/ ballast capacitor combination different resonating capacitors produce varying amounts light. Large values tend smooth harmonics, peaking optical output increasing converter circulating losses. Smaller values promote lower circulating currents less light output. Figure shows typical results five capacitor values forced main supply lamp current. Large values produce more light require more supply current. data expressed ratio light output-per-watt input power right-most column. This Nits-per-Watt ratio peaks 0.1µF, indicating best optical efficiency.19 This test must performed stable thermal environment because lamp's emission sensitivity temperature (see Figure Additionally, some arrangement rapidly switching capacitor values desirable. This avoids power interruptions resultant long display warm-up times.
CAPACITOR (µF) MAIN SUPPLY CURRENT SUPPLY CURRENT TOTAL SUPPLY WATTS INTENSITY (NITS)
Electrical Efficiency Optimization Measurement Several points should kept mind when observing operation these circuits. high voltage secondary only monitored with wideband, high voltage probe fully specified this type measurement. vast majority oscilloscope probes will break down fail used this measurement.20 Tektronix probe types P-6007 P-6009 (acceptable some cases) types P6013A P6015 (preferred) probes must used read L1's output. Another consideration involves observing waveforms. switching regulator frequency completely asynchronous from Royer converter's switching. such, most oscilloscopes cannot simultaneously trigger display circuit's waveforms. Figure obtained using dual beam oscilloscope (Tektronix 556).Traces triggered beam while remaining traces triggered other beam. Single beam instruments with alternate sweep trigger switching (e.g., Tektronix 547) also used less versatile restricted four traces. Obtaining verifying high electrical efficiency requires some amount diligence. optimum efficiency values given resonating capacitor ballast capacitor) typical will vary specific types lamps. important realization that term "lamp" includes total load seen transformer's secondary. This load, reflected back primary, sets transformer input impedance. transformer's input impedance forms integral part tank that produces high voltage drive. Because this, circuit efficiency must optimized with
Note Optical measurement units beyond arcane; monument obscuration. Candela/Meter2 basic unit, Candela/Meter2. "Nit" contracted form Latin word "Nitere," meaning emit light sparkle." Note Don't didn't warn you! Note term "efficiency" used here applies electrical efficiency. fact, ultimate concern centers around efficient conversion power supply energy into light. Unfortunately, lamp types show considerable deviation their current-to-light conversion efficiency. Similarly, emitted light given current varies over life history particular lamp. such, this text portion treats "efficiency" electrical basis; ratio power removed from primary supply power delivered lamp. When lamp/display combination been selected, ratio primary supply power lamp emitted light energy measured with photometer. This covered immediately preceding test Appendix
NITS/ WATT
0.15 0.068 0.047 0.033
0.304 0.269 0.259 0.251 0.240
0.014 0.013 0.013 0.013 0.013
3.11 2.75 2.65 2.57 2.46
37.9 40.7 38.1 37.3 35.7
Note: Maintain IMAIN Supply 10.0V ILAMP 5mARMS under conditions.
Figure Typical Data Taken Optical Efficiency Optimization. Note Emissivity Peak (Nits/Watt) 0.1µF Resonating Value, Indicating Best Trade-Off Point Electrical Optical Efficiency. Data Should Retaken Several Ballast Capacitor Values Ensure Maximum Optical Efficiency
AN65-53
Application Note
wiring, display housing physical layout arranged exactly same they will built production. Deviations from this procedure will result lower efficiency than might otherwise possible. practice, "first cut" efficiency optimization with "best guess" lead lengths intended lamp display housing usually produces results within achievable figure. Final values established when physical layout used production been decided sets circuit's resonance point, which varies some extent with lamp's characteristic. ballasts lamp, effectively buffering negative resistance characteristic. Small values provide most load isolation require relatively large transformer output voltage loop closure. Large values minimize transformer output voltage degrade load buffering. values also affect waveform distortion, influencing lamp emissivity optical efficiency (see previous text discussion). Also, C1's "best" value somewhat dependent lamp type used. Both must selected given lamp types. Some interaction occurs, generalized guidelines possible. Typical values 0.01µF 0.15µF. usually ends 10pF 47pF range. must loss capacitor substitution recommended devices recommended. poor quality dielectric easily degrade efficiency 10%. Before capacitor selection Q1/Q2 base drive resistor should value which ensures saturation, e.g., 470. Next, selected trying different values each iterating towards best efficiency. During this procedure ensure that loop closure maintained. Several trials usually produce optimum values. Note that highest efficiencies necessarily associated with most esthetically pleasing waveshapes, particularly output. Finally, base drive resistor's value should optimized. base drive resistor's value (nominally should selected provide full saturation without inducing base overdrive beta starvation. This point established lamp type determining peak collector current full lamp power. base resistor should largest value that ensures saturation worst-case transistor beta. This condition verified varying base drive resistor about ideal value noting small variations input supply current. minimum obtainable current corresponds best beta saturation trade-off. practice, supply current rises slightly either side this point. This "double value" behavior efficiency degradation caused either excessive base drive saturation losses. Other issues influencing efficiency include lamp wire length energy leakage from lamp. high voltage side(s) lamp should have smallest practical lead length. Excessive length results radiative losses which easily reach 3-inch wire. Similarly, metal should contact close proximity lamp. This prevents energy leakage which exceed 10%.22 worth noting that custom designed lamp affords best possible results. jointly tailored lamp/circuit combination permits precise optimization circuit operation, yielding highest efficiency. These considerations should made with knowledge other issues. Appendix "Mechanical Design Considerations Liquid Crystal Displays." This section guest-written Charles Guthrie Sharp Electronics Corporation. Special attention should given circuit board layout since high voltage generated output. output coupling capacitor must carefully located minimize leakage paths circuit board. slot board will further minimize leakage. Such leakage permit current flow outside feedback loop, wasting power. worst case, long term contamination buildup increase leakage inside loop, resulting starved lamp drive destructive arcing. good practice minimization leakage break silk screen line which outlines transformer. This prevents leakage from
Note This footnote annotates similar issues raised Footnote associated text. repetition based necessity emphasis. very simple experiment quite nicely demonstrates effects energy leakage. Grasping lamp voltage (low field intensity) with thumb forefinger produces almost change circuit input current. Sliding thumb/forefinger combination towards high voltage (higher field intensity) lamp produces progressively greater input currents. Don't touch high voltage lead your receive electrical shock. Repeat: touch high voltage lead receive electrical shock.
AN65-54
Application Note
high voltage secondary primary. Another technique minimizing leakage evaluate specify silk screen ability withstand high voltages. Appendix "Layout, Component Emissions Considerations," details high voltage layout practice. Electrical Efficiency Measurement Once these procedures have been followed efficiency measured. Efficiency measured determining lamp current voltage. Measuring current involves utilization wideband, high accuracy clip-on current probe having true (thermally based) readout. commercially manufactured current probe will meet accuracy bandwidth requirements probe must constructed.23 Lamp voltage measured lamp with wideband, properly compensated high voltage probe.24 Multiplying these results gives power watts, which compared input supply (E)(I) product. practice, lamp's current voltage contain small outof-phase components their error contribution negligible. Both current voltage measurements require wideband true voltmeter. meter must employ thermal type converter--the more common logarithmic computing type-based instruments inappropriate because their bandwidth low. previously recommended high voltage probes designed 1M/10pF-22pF oscilloscope input. voltmeters have input. This difference necessitates impedance matching network between probe voltmeter. Floating lamp circuits require this matching differential measurement, severely complicating instrumentation design. Footnote Feedback Loop Stability Issues circuits shown this point rely closed-loop feedback maintain operating point. linear closedloop systems require some form frequency compensation achieve dynamic stability. Circuits operating with relatively power lamps frequency compensated simply overdamping loop. Text Figures this approach. higher power operation
Figure Delay Terms Feedback Path. Time Constant Dominates Loop Transmission Delay Must Compensated Stable Operation
Note Justification this requirement construction details appear Appendix "Achieving Meaningful Electrical Measurements." Note Measuring floating lamp circuit voltages particularly demanding exercise requiring wideband differential high voltage probe. Probe construction details appear Appendix
associated with color displays requires more attention loop response. transformer produces much higher output voltages, particularly start-up. Poor loop damping allow transformer voltage ratings exceeded, causing arcing failure. such, higher power designs require optimization transient response characteristics. LT118X series parts almost never require optimization because their error amplifier's gain/phase characteristics specially tailored CCFL load characteristics. LT1172, LT1372 other general purpose switching regulators require more attention ensure proper behavior. following discussion, applicable general purpose switching regulators CCFL applications, uses LT1172 example. Figure shows significant contributors loop transmission these circuits. resonant Royer converter delivers information about 50kHz lamp. This information smoothed averaging time constant delivered LT1172's feedback terminal
HIGH VOLTAGE BALLAST CAPACITOR RESONANT ROYER 50kHz WAVE 50kHz CCFL LAMP
LT1172 100kHz FEEDBACK TERMINAL AVERAGING TIME CONSTANT
COMPENSATION CAPACITOR
INTENSITY CONTROL, TYPICALLY 1kHz
AN65-55
Application Note
LT1172 controls Royer converter 100kHz rate, closing control loop. capacitor LT1172 rolls gain, nominally stabilizing loop. This compensation capacitor must roll gain-bandwidth enough value prevent various loop delays from causing oscillation. Which these delays most significant? From stability viewpoint LT1172's output repetition rate Royer's oscillation frequency sampled data systems. Their information delivery rate above averaging time constants delay significant. time constant major contributor loop delay. This time constant must large enough turn half wave rectified waveform into also must large enough average intensity control signal Typically, these intensity control signals come 1kHz rate (see Appendix "Intensity Control Shutdown Methods"). RC's resultant delay dominates loop transmission. must compensated capacitor LT1172. large enough value this capacitor rolls loop gain enough frequency provide stability. loop simply does have enough gain oscillate frequency commensurate with delay.25 This form compensation simple effective. ensures stability over wide range operating conditions. does, however, have poorly damped response system turn-on. turn-on delays feedback, allowing output excursions well above normal operating point. When acquires feedback value loop stabilizes properly. This turn-on overshoot concern well within transformer breakdown ratings. Color displays, running higher power, usually require large initial voltages. loop damping poor, overshoot dangerously high. Figure shows such loop responding turn-on. this case values 4.7µF, with compensation capacitor. Turnon overshoot exceeds 3500V over 10ms! Ring-off takes over 100ms before settling occurs. Additionally, inadequate (too small) ballast capacitor excessively lossy layout force 2000V output once loop settling occurs. This photo taken with transformer rated well below this figure. resultant arcing caused transformer destruction, resulting field failures. typical destroyed transfomer appears Figure
1000V/DIV
20ms/DIV
Figure Destructive High Voltage Overshoot Ring-Off Poor Loop Compensation. Transformer Failure Field Recall Nearly Certain. Loss also Occur
Figure Poor Loop Compensation Caused This Transformer Failure. Occured High Voltage Secondary (Lower Right). Resultant Shorted Turns Caused Overheating
Figure shows same circuit with values reduced 1µF. ballast capacitor layout have also been optimized. Figure shows peak voltage reduced 2.2kV with duration down about (note horizontal scale change). Ring-off also much quicker with lower amplitude excursion. Increased ballast capacitor value wiring layout optimization reduce running voltage 1300V. Figure 64's results even better. Changing compensation capacitor 3k/2µF network introduces leading response into loop, allowing faster acquisition. Now, turn-on excursion slightly lower, greatly reduced duration (again, note horizontal scale change). running voltage remains same.
Note high priests feedback refer this "Dominant Pole Compensation." rest reduced more pedestrian descriptives.
AN65-56
Application Note
Figure shows loss display responding turn-on with compensation capacitor 10k/1µF values. Trace transformer's output while Traces LT1172's VCOMPENSATION Feedback pins, respectively. output overshoots rings badly, peaking about 3000V. This activity reflected overshoots VCOMPENSATION (the LT1172's error amplifier output) Feedback pin. Figure reduced 10k/0.1µF. This substantially reduces loop delay. Overshoot goes down only 800V--a reduction almost factor four. Duration also much shorter.
1000V/DIV
5ms/DIV
Figure Reducing Time Constant Improves Transient Response, Although Peaking, Ring-Off Voltage Still Excessive
2000V/DIV
0.5V/DIV
1V/DIV 1000V/DIV 10ms/DIV
Figure Waveform Lower Loss Layout Display. High Voltage Overshoot (Trace Reflected Compensation Node (Trace Feedback (Trace
2ms/DIV
Figure Additional Optimization Time Constant Compensation Capacitor Reduces Turn-On Transient. Voltage Large, Indicating Possible Lossy Layout Display
2000V/DIV
photos show that changes compensation, ballast value layout result dramatic reductions overshoot amplitude duration. Figure 62's performance almost guarantees field failures while Figures overstress transformer. Even with improvements, more margin possible display losses controlled. Figures were taken with exceptionally lossy display. metal enclosure very close metallic foil wrapped lamp, causing large losses with subsequent high turn-on running voltages. display selected lower losses, performance greatly improved.
0.5V/DIV
1V/DIV 10ms/DIV
Figure Reducing Time Constant Produces Quick, Clean Loop Behavior. Loss Layout Display Result 650VRMS Running Voltage
AN65-57
Application Note
VCOMPENSATION Feedback pins reflect this tighter control. Damping much better, with slight overshoot induced turn-on. Further reduction 10k/ 0.01µF (Figure results even faster loop capture problem appears. Trace lamp turn-on fast overshoot does register photo. VCOMPENSATION (Trace feedback nodes (Trace reflect this with exceptionally fast response. Unfortunately, RC's light filtering causes ripple appear when feedback node settles. such, Figure 66's values probably more realistic this situation. lesson from this exercise clear. higher voltages involved color displays mandate attention transformer outputs. Under running conditions layout display losses cause higher loop compliance voltages, degrading efficiency stressing transformer. turn-on improper compensation causes huge overshoots, resulting possible transformer destruction. Isn't loop layout optimization worth field recall?
2000V/DIV
0.5V/DIV
1V/DIV 10ms/DIV
Figure Very Value Provides Even Faster Response, Ripple Feedback (Trace High. Figure Best Compromise
AN65-58
Application Note
REFERENCES Williams, "Techniques Efficient Illumination," Linear Technology Corporation, Application Note August 1993. Bright, Pittman Royer, "Transistors On-Off Switches Saturable Core Circuits," Electrical Manufacturing, December 1954. Available from Technomic Publishing, Lancaster, Sharp Corporation, "Flat Panel Displays," 1991. Kitchen, Counts, "RMS-to-DC Conversion Guide," Analog Devices, Inc., 1986. Williams, Jim, Monolithic 100MHz RMSDC Conversion," Linear Technology Corporation, Application Note September 1987. Hewlett-Packard, "1968 Instrumentation. Electronic-Analytical-Medical," Voltage Measurement, 197-198, 1968. Hewlett-Packard, "Model 3400RMS Voltmeter Operating Service Manual," 1965. Hewlett-Packard, "Model 3403C True Voltmeter Operating Service Manual," 1973. Ott, W.E., Technique Thermal Measurement," IEEE Journal Solid State Circuits, December 1974. Williams, J.M. Longman, T.L., 25MHz Thermally Based RMS-DC Converter," 1986 IEEE ISSCC Digest Technical Papers. O'Neill, P.M., Monolithic Thermal Converter," H.P. Journal, 1980. Williams, "Thermal Techniques Measurement Control Circuitry," "50MHz Thermal RMS-DC Converter," Linear Technology Corporation, Application Note December 1984. Williams, Huffman, "Some Thoughts DC-DC Converters," Appendix "The ±15V Converter--A Special Case," Linear Technology Corporation, Application Note October 1988. Baxendall, P.J., "Transistor Sine-Wave Oscillators," British Journal IEEE, February 1960, Paper 2978E. Williams, "Temperature Controlling Microdegrees," Massachusetts Institute Technology, Education Research Center, 1971 (out print). Fulton, S.P., "The Thermal Enzyme Probe," Thesis, Massachusetts Institute Technology, 1975. Williams, "Designer's Guide Temperature Measurement," Part EDN, 1977. Williams, "Illumination Circuitry Liquid Crystal Displays," Linear Technology Corporation, Application Note August 1992. Olsen, J.V., High Stability Temperature Controlled Oven," Thesis, Massachusetts Institute Technology, 1974. "The Ultimate Oven," Reports Research, March 1972. McDermott, James, "Test System Controls Temperature Microdegrees," Electronic Design, January 1972. McAbel, Walter, "Probe Measurements," Tektronix, Inc. Concept Series, 1969. Weber, Joe, "Oscilloscope Probe Circuits," Tektronix, Inc. Concept Series, 1969. Tektronix, Inc., "P6015 High Voltage Probe Operating Manual." Williams, Jim, "Measurement Control Circuit Collection," Linear Technology Corporation, Application Note June 1991. Williams, "High Speed Amplifier Techniques," Linear Technology Corporation, Applicaton Note August 1991. Williams, "Practical Circuitry Measurement Control Problems," Linear Technology Corporation, Application Note August 1994. Chadderton, Neil, "Transistor Considerations Backlighting," Zetex plc. Application Note February 1995.
AN65-59
Application Note
APPENDIX "HOT" CATHODE FLUORESCENT LAMPS Many CCFL characteristics shared so-called "Hot" Cathode Fluorescent Lamps (HCFLs). most significant difference that HCFLs contain filaments each lamp (see Figure A1). When filaments powered they emit electrons, lowering lamp's ionization potential. This means significantly lower voltage will start lamp. Typically, filaments turned relatively modest voltage impressed across lamp start-up occurs. Once lamp starts, filament power removed. Although HCFLs reduce high voltage requirement they require filament supply sequencing circuitry. CCFL circuits shown text will start HCFLs without using filaments. practice, this involves simply driving filament connections HCFL ends they were CCFL electrodes.
START-UP
FILAMENT SUPPLY
FILAMENT
HCFL
FILAMENT
HIGH VOLTAGE SUPPLY
Figure Conceptual Cathode Fluorescent Lamp Power Supply. Heated Filaments Liberate Electrons, Lowering Lamp's Start-Up Voltage Requirement. CCFL Supply Discussed Text Eliminates Filament Supply
APPENDIX MECHANICAL DESIGN CONSIDERATIONS LIQUID CRYSTAL DISPLAYS Charles Guthrie, Sharp Electronics Corporation Introduction more companies begin manufacturing their next generation computers, there need reduce overall size weight units improve their portability. This sparked need more compact designs where various components placed closer proximity, thus making them more susceptible interaction from signal noise heat dissipation. following summary guidelines placement display components suggestions overcoming difficult design constraints associated with component placement. notebook computers thickness display housing important. design usually requires display pivotal structure that display folded down over keyboard transportation. Also, outline dimensions must minimal that package will remain compact possible. These constraints drive display housing design placement
Reprinted with permission Sharp Electronics Corporation
display components. This discussion surveys each problems facing designer detail offers suggestions overcoming difficulties provide reliable assembly. problems facing pen-based computer designer similar those realized notebook designs. addition, however, pen-based designs require protection face display. pen-based applications, moved across surface display, potential scratching front polarizer. this reason front display must protected. Methods protecting display face while minimizing effects display image given. Additionally, need specify flatness bezel discussed. Suggestions acceptable construction techniques sound design included. Further, display components likely cause problems heat buildup identified methods minimization heat's effects presented.
AN65-60
Application Note
ideas expressed here only solutions various problems have been assessed whether they infringe patents issued applied for. Flatness Rigidity Bezel notebook computer bezel several distinct functions. houses display, inverter backlight, some instances, controls contrast brightness display. bezel usually designed tilt optimum viewing angle display. important understand that bezel must provide mechanism keep display flat, particularly mounting holds. Subtle changes flatness place uneven stress glass which cause variations contrast across display. Slight changes pressure cause significant variation display contrast. Also, extreme, significantly uneven pressures cause display glass fail. Because bezel must functional maintaining flatness display, consideration must made strength bezel. Care must taken provide structural members while minimizing weight unit. This executed using parallel grid, normal edges bezel, angled about edges bezel. angled structure more desirable that provides resistance torquing unit while lifting cover with hand. Again, display sensitive stresses from uneven pressure display housing. Another structure which will provide excellent rigidity, adds more weight computer, "honeycomb" structure. This "honeycomb" structure resists torquing from directions tends provide best protection display. With each these structures easy provide mounting assemblies display. "Blind nuts" molded into housing. mounting done either front rear bezel. Attachment rear provide better rigidity placement mounting hardware. last caution worth noting development bezel. bezel should engineered absorb most shock vibration experienced portable computer. Even though display been carefully designed, notebook computer presents extraordinary shock abuse problems. Avoiding Heat Buildup Display Several display components sources heat problems. Thermal management must taken into account design display bezel. heated display adversely affected; loss contrast uniformity usually results. Cold Cathode Fluorescent Tube (CCFT) itself gives small amount heat relative amount power dissipated glow discharge. Likewise, even though inverters designed extremely efficient, there some heat generated. buildup heat these components will aggravated typically "tight" designs currently being introduced. There little ventilation designed into most display bezels. compound problem, plastics used poor thermal conductors, thus causing heat build which affect display. Some current designs suffer from poor placement inverter and/or poor thermal management techniques. These designs improved even where redesign display housing with improved thermal management impractical. most common mistakes current designs that there been consideration buildup heat from CCFT. Typically, displays notebook applications have only CCFT minimize display power requirements. lamp usually placed along right edge display. Since lamp placed very close display glass, cause temperature rise liquid crystal. important note that variations temperature little cause apparent nonuniformity display contrast. Variations caused slightly higher temperature variations will cause objectionable variations contrast display appearance. further aggravate situation, some designs have inverter placed bottom bezel. This tendency cause same variations contrast, particularly when housing does have heat sinking inverter. This problem manifests itself "blooming" display, just above inverter. This "blooming"
AN65-61
Application Note
looks like washed area where, worst case, characters display fade completely. following section discusses recommended methods overcoming these design problems. Placement Display Components things that done design inverter into base computer with motherboard. some applications this impractical because this requires high voltage leads mounted within hinges connecting display bezel main body. This causes problem with strain relief high voltage leads thus with certification. mistake made most often placing inverter bottom bezel next lower edge display. fact that heat rises, this most overlooked problems notebook designs. Even though inverters very efficient, some energy lost inverter form heat. Because insulating properties plastic materials used bezel construction, heat builds affects display contrast. Designs with inverter bottom improved three ways. inverter relocated away from display, heat sinking materials placed between display inverter, ventilation provided remove heat. mature designs impractical what obvious move inverter side display towards housing. these cases inverter insulated from display with "heat dam." method accomplishing this would piece mica insulator tightly between inverter display. This heat would divert heat around display bezel rise harmlessly housing. Mica recommended this application because thermal electrical insulation properties. last suggestion removing heat provide some ventilation inverter area. This done very carefully prevent exposing high voltage. Ventilation practical solution because resistance liquids dust compromised. best solution designer hardware consider placement inverter side display bezel. existing designs this type effects heat from inverter, even tight housings, been minimal nonexistent. problem which aggravated placement inverter bezel heat dissipated CCFT. designs where inverter placed side display, fading display contrast CCFT heat problem. However, when inverter placed bezel bottom, some designs experience loss contrast aggravated heat from CCFT inverter. cases where inverter must left bottom, CCFT causing loss contrast, problem minimized using aluminum foil heat sink. This does remove heat from display dissipates over entire display area, thus normalizing display contrast. aluminum foil easy install some present designs successfully improved display contrast. Remember that objection contrast variation stems more from nonuniformity than from total loss contrast. Protecting Face Display last considerations design notebook pen-based computers protection display face. front polarizer made mylar base thus susceptible scratching. front protection display, along with providing scratch protection, also provide antiglare surface. There several ways that scratch resistance antiglare surfaces incorporated. glass plastic cover placed over display, thus providing protection. material should placed close display possible minimize possible parallax problems reflections cover material. With antiglare materials further material from front display greater distortion. applications, front antiscratch material best placed contact with front glass display. cover glass material normally needs slightly thicker
AN65-62
Application Note
protect display from distortion when pressure being exerted front. There several methods making input devices. Some front surface cover glass provide input data some field effect printed wiring board back display. When input front display, input device usually glass surface. limit specular reflection this application, front cover glass should bonded display. Care must taken ensure that coefficient thermal expansion matched materials used system. Because difficulties encountered with bonding cover glass, potential destroy display through improper workmanship, consulting expert strongly recommended.
APPENDIX ACHIEVING MEANINGFUL ELECTRICAL MEASUREMENTS Obtaining reliable efficiency data CCFL circuits presents high order difficulty measurement problem. accuracy required high frequency measurements uncomfortably close state-of-the-art. Establishing maintaining accurate wideband measurements textbook example attention measurement technique. combination high frequency, harmonic laden waveforms high voltage makes meaningful results difficult obtain. choice, understanding test instrumentation crucial. Clear thinking needed avoid unpleasant surprises!1 lamp's current voltage waveforms contain energy content over wide frequency range. Most this energy concentrated inverter's fundamental frequency immediate harmonics. However, measurement uncertainty desirable, then energy content 10MHz must accurately captured. Figure spectrum analysis lamp current, shows significant energy 500kHz. Diminished, still significant, content shows
Note worth considering that various constructors text Figure have reported efficiencies ranging from 115%.
Figure Hewlett-Packard HP89410A Spectral Plot Lamp Current Shows Significant Energy 500kHz
AN65-63
Application Note
Figure C2's 6MHz wide plot. This data suggests that monitoring instrumentation must maintain high accuracy over wide bandwidth. Accurate determination operating current important electrical emissivity efficiency computations ensure long lamp life. Additionally, desirable able perform current measurements presence high common mode voltage 1000VRMS). This capability allows investigation quantification display wiring-induced losses, regardless their origins lamp drive circuitry. Current Probe Circuitry Figure C3's circuitry meets discussed requirements. signal-conditions commercially available "clip-on" current probe with precision amplifier provide measurement accuracy 10MHz. "clip-on" probe provides convenience, even presence high common voltages noted. current probe biases operating gain about 3.75. impedance matching required probe's output impedance termination. Additional amplifiers provide distributed gain, maintaining wide bandwidth with overall gain about
Figure Extended HP89410A Spectral Plot Shows Lamp Current Measurable Energy Well into Range. Data Indicates that Lamp Voltage Current Instrumentation Must Have Precision, Wideband Response
TEKTRONIX P6021 CURRENT PROBE 2mA/mV LT1223
LT1223
1.1M LT1004 1.2V
LT1223
SELECT VALUE POLARITY. ±15V TEXT
LT1223
OUTPUT THERMALLY BASED VOLTMETER, e.g., HP3403, 3400, FLUKE 8920A. SCALE FACTOR 10.00mARMS 1.000VRMS 20kHz 10MHz
CALIBRATE
RESISTORS FILM UNLESS NOTED. LAYOUT TECHNIQUES--SEE FOLLOWING PHOTOS TEXT SUPPLIES ±15V, BYPASS EACH AMPLIFIER WITH 0.1µF CERAMIC CAPACITORS. DIODE CLAMP SUPPLIES REVERSE VOLTAGE
Figure Precision "Clip-On" Current Probe CCFL Measurements Maintains Accuracy over 20kHz 10MHz Bandwidth
AN65-64
Application Note
200. individual amplifiers avoid possible crosstalkbased error that could introduced monolithic quad amplifier. selected polarity value trim overall amplifier offset. trimmer sets gain, fixing scale factor. output drives thermally based, wideband voltmeter. practice, circuit built into 2.25" enclosure which directly connected hardware voltmeter. cable used. Figure shows probe/amplifier combination. Figure details layout techniques used amplifier's
Figure Current Probe Amplifier Mated Current Probe Termination
Figure Layout Technique Current Probe Amplifier Required Performance Levels Quoted Text
AN65-65
Application Note
Figure Version Current Probe Amplifier Housing. Current Probe Terminator Left
construction. Figure shows version amplifier, detailing enclosure layout construction. result "clip-on" current probe with accuracy over 20kHz 10MHz bandwidth. This tool proven indispensable rigorously conducted backlight work. Figure shows response probe/amplifier measured Hewlett-Packard HP4195A network analyzer.
Figure Amplitude Frequency Output HP4195A Network Analyzer. Current Probe/Amplifier Maintains (0.1dB) Error Bandwidth from 20kHz 10MHz. Small Aberrations Between 10MHz 20MHz Test Fixture Related
AN65-66
Application Note
Current Calibrator Figure C8's circuit, current calibrator, permits calibration probe/amplifier used periodically check probe accuracy. form Wein bridge oscillator. Oscillator output rectified compared reference A3's output controls closing amplitude stabilization loop. stabilized amplitude terminated into 100, 0.1% resistor provide precise 10.00mA, 60kHz current through series current loop. Trimming performed altering nominal resistor exactly 1.000VRMS across unit. use, this current probe shown 0.2% baseline stability with absolute accuracy over year's time. sole maintenance requirement preserving accuracy keep current probe jaws clean avoid rough abrupt handling probe.2 Figure shows probe/calibrator used with voltmeter. Figure shows current probe use, this case determining display frame parasitic loss.
Note Private communication, Tektronix, Inc.
0.012µF
0.012µF 10.00mA 60kHz CURRENT LOOP OUTPUT 0.1%
LT1122 LT1010
2.4k
5.6k
LT1122
1N4148
2N4338
100k 0.03µF
RESISTORS FILM UNLESS NOTED SUPPLIES ±15V
Figure Current Calibrator Probe Trimming Accuracy Checks. Stabilized Oscillator Forces 10.00mA Through Output Current Loop 60kHz
LT1097 NOMINAL. TRIM 1.000V ACROSS RESISTOR. TEXT 4.7k LT1029
-15V
LT1122
1N4148
AN65-67
AN65-68
Application Note
Figure C9a. Complete Current Probe Test Includes Probe, Amplifier, Calibrator Thermally Based Voltmeter. Accuracy 10MHz
Application Note
AN65-69
Figure C9b. Current Probe Measuring Display Frame Parasitic Current. "Clip-On" Capability Allows Measurement Point Lamp Circuit
Application Note
Voltage Probes Grounded Lamp Circuits high voltage measurement across lamp quite demanding probe. simplest case measuring grounded lamp circuits. waveform fundamental 20kHz 100kHz, with harmonics into region. This activity occurs peak voltages kilovolt range. probe must have high fidelity response under these conditions. Additionally, probe should have input capacitance avoid loading effects which would corrupt measurement. design construction such probe requires significant attention. Figure lists some recommended probes along with their characteristics. stated text,
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