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Application Note October 2006 Load Transient Response Testing Voltage Regulators
Practical Considerations Testing Evaluating Results Williams INTRODUCTION Semiconductor memory, card readers, microprocessors, disc drives, piezoelectric devices digitally based systems furnish transient loads that voltage regulator must service. Ideally, regulator output invariant during load transient. practice, some variation encountered becomes problematic allowable operating voltage tolerances exceeded. This mandates testing regulator associated support components verify desired performance under transient loading conditions. Various methods employable generate transient loads, allowing observation regulator response. Basic Load Transient Generator Figure diagrams conceptual load transient generator. regulator under test drives switched resistive loads, which variable. switched current output voltage monitored, permitting comparison nominally stable output voltage versus load current under static dynamic conditions. switched current either off; there controllable linear region. Figure practical implementation load transient generator. voltage regulator under test augmented capacitors which provide energy reservoir, similar mechanical flywheel, transient response. size, composition location these capacitors, particularly COUT, pronounced effect transient response overall regulator stability.1 Circuit operation straightforward. input pulse triggers LTC1693 driver switch generating transient load current
Note Appendix "Capacitor Parasitic Effects Load Transient Response" Appendix "Output Capacitors Stability" extended discussion.
+EREGULATOR REGULATOR INPUT SUPPLY +EREGULATOR REGULATOR INPUT SUPPLY REGULATOR UNDER TEST CURRENT MONITOR LOAD RSWITCHED LOAD LOAD SWITCH ISWITCHED EREGULATOR RSWITCHED LOAD
AN104
REGULATOR UNDER TEST COUT
AC-COUPLED OSCILLOSCOPE
LOAD RLOAD TEKTRONIX P-6042 CURRENT PROBE EQUIVALENT MINIMIZE INDUCTANCE ISWITCHED EREGULATOR RSWITCHED LOAD
AN104
VOLTAGE MONITOR
10µF PULSE INPUT
LTC1693-1
IRLZ24
Figure Conceptual Regulator Load Tester Includes Switched Loads Voltage/Current Monitors. Resistor Values Switched Load Currents. Switched Current Either Off; There Controllable Linear Region
Figure Practical Regulator Load Tester. Driver Switch RLOAD. Oscilloscope Monitors Current Probe Output Regulator Response
Lare registered trademarks Linear Technology Corporation. other trademarks property their respective owners.
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AN104-1
Application Note
regulator. oscilloscope monitors instantaneous load voltage and, "clip-on" wideband probe, current. circuit's load transient generating capabilities evaluated Figure substituting extraordinarily impedance power source regulator. combination high capacity power supply, impedance connections generous bypassing maintains impedance across frequency. Figure shows Figure responding LTC1693-1 driver (Trace cleanly switching 15ns (Trace Such speed useful simulating many loads restricted versatility. Although fast, circuit cannot emulate loads between minimum maximum currents.
HEWLETT-PACKARD 6012A POWER SUPPLY MINIMIZE INDUCTANCE
+EREGULATOR REGULATOR INPUT SUPPLY REGULATOR UNDER TEST CURRENT MONITOR CONTROL INPUT VOLTAGE MONITOR
EINPUT RCURRENT SENSE
CONTROL AMPLIFIER
CURRENT SENSE RESISTOR
AN104
Figure Conceptual Closed Loop Load Tester. Controls Q1's Source Voltage, Setting Regulator Output Current. Q1's Drain Current Waveshape Identical Input, Allowing Linear Control Load Current. Voltage Current Monitors Figure
VOLTAGE MONITOR AC-COUPLED OSCILLOSCOPE
300ns
2200µF* EACH
LOAD TESTER (FIGURE
PULSE
LOAD (OPTIONAL)
TEKTRONIX P-6042 CURRENT PROBE EQUIVALENT *SANYO OSCON
AN104
Figure Substituting Well Bypassed, Impedance Power Supply Regulator Allows Determining Load Tester's Response Time
operating point. Q1's current assumes value dependant control input voltage current sense resistor over very wide bandwidth. Note that once biases Q1's conductance threshold, small variations A1's output result large current changes Q1's channel. such, large output excursions required from small signal bandwidth fundamental speed limitation. Within this restriction, Q1's current waveform identically shaped A1's control input voltage, allowing linear control load current. This versatile capability permits wide variety simulated loads. Based Circuit Figure practical incarnation based closed loop load transient generator, includes bias waveform inputs. must drive Q1's high capacitance gate high frequency, necessitating high peak output currents attention feedback loop compensation. 60MHz current feedback amplifier, output current capacity exceeding Maintaining stability waveform fidelity high frequency while driving Q1's gate capacitance necessitates settable gate drive peaking components, damper network, feedback trimming loop peaking adjustments. trim, also required, made first. With input applied, trim "1mV adjust" Q1's source. trims made utilizing Figure arrangement. Similar Figure this "brick wall" regulated source provides minimal ripple when step loaded load transient generator. Apply inputs shown trim gate drive, feedback loop peaking adjustments cleanest, square cornered response oscilloscope's current probe equipped channel.
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5V/DIV
0.5A/DIV
HORIZ 100ns/DIV
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Figure Figure Circuit Responds Driver Output (Trace Switching Load (Trace 15ns
Closed Loop Load Transient Generators Figure conceptual closed loop load transient generator linearly controls Q1's gate voltage instantaneous transient current desired point, allowing simulation nearly load profile. Feedback from Q1's source control amplifier closes loop around stabilizing
AN104-2
Application Note
+EREGULATOR BIAS
LT1006
REGULATOR INPUT SUPPLY
REGULATOR UNDER TEST COUT
VOLTAGE MONITOR AC-COUPLED OSCILLOSCOPE
100k 8.16k* 866* WAVEFORM INPUT 100* GATE DRIVE PEAKING IRLZ24 120k +1mV ADJUST LOOP PEAKING 68pF
AN104
TEKTRONIX P-6042 CURRENT PROBE EQUIVALENT MINIMIZE INDUCTANCE
LT1210
0.01µF
10µF CERAMIC FEEDBACK
0.1** FAIR-RITE #2743001112 METAL FILM RESISTOR VISHAY WSL2512.5%
Figure Detailed Closed Loop Load Tester. Level Pulse Inputs Feed Current Sinking Regulator Load. Q1's Gain Allows Small Output Swing, Permitting Wide Bandwidth. Damper Network, Feedback Peaking Trims Optimize Edge Response
HEWLETT-PACKARD 6012A POWER SUPPLY MINIMIZE INDUCTANCE
VOLTAGE MONITOR AC-COUPLED OSCILLOSCOPE
250ns 500ns 0.5V, (0.5A) 100kHz PULSE CLOSED LOOP LOAD TESTER (FIGURES BIAS (100mA)
2200µF* EACH
TEKTRONIX P-6042 CURRENT PROBE EQUIVILENT *SANYO OSCON
AN104
Figure Closed Loop Load Tester Response Time Determined Figure "Brick Wall" Input Provides Impedance Source
Bipolar Transistor Based Circuit Figure considerably simplifies previous circuit's loop dynamics eliminates trims. major trade-off speed reduction. circuit similar Figure except that bipolar transistor. bipolar's greatly reduced input capacitance allows drive more benign load. This permits lower output current amplifier eliminates dynamic trims required accommodate Figure gate capacitance. sole trim "1mV adjust" which accomplished described before2. Aside from speed reduction bipolar transistor also introduces output current error base current.
Note This trim eliminated some sacrifice circuit complexity. Appendix Trimless Closed Loop Transient Load Tester".
added prevent excessive base current when regulator supply present. diode prevents reverse base bias under circumstances. Closed Loop Circuit Performance Figures show wideband circuits' operation. based circuit (Figure only requires 50mV swing (Trace enforce Trace flat-topped current pulse with 50ns edges through Figure details bipolar transistor based circuit's performance. Trace taken Q1's base, rises less than 100mV causing Trace clean current conduction through This circuit's 100ns edges, about slower than more complex based version, still fast enough most practical transient load testing.
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Application Note
+EREGULATOR REGULATOR INPUT SUPPLY LT1006 VN2222L TEKTRONIX P-6042 CURRENT PROBE EQUIVALENT REGULATOR UNDER TEST COUT VOLTAGE MONITOR AC-COUPLED OSCILLOSCOPE
BIAS
100k
8.16k* 866* WAVEFORM INPUT 100*
LT1206 D44H2 MUR11O
MINIMIZE INDUCTANCE
MINIMIZE CAPACITANCE 120k
0.1**
+1mV ADJUST
AN104
METAL FILM RESISTOR VISHAY WSL2512.5%
Figure Figure Implemented with Bipolar Transistor. Q1's Reduced Input Capacitance Simplifies Loop Dynamics, Eliminating Compensation Components Trims. Trade Speed Reduction Base Current Induced Error
0.05V/DIV AC-COUPLED 2.5VDC
0.05V/DIV AC-COUPLED 0.6VDC
0.5A/DIV AC-COUPLED 0.1ADC HORIZ 50ns/DIV
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0.5A/DIV AC-COUPLED 0.1ADC
HORIZ 100ns/DIV
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Figure Figure Closed Loop Load Tester Step Response Current Trace Quick Clean, Showing 50ns Edges Flat Top. A1's Output (Trace Swings Only 50mV, Allowing Wideband Operation. Trace Presentation Slightly Delayed Voltage Current Probe Time Skew
Figure Figure Bipolar Output Load Tester Response Slower than Version, Circuit Less Complex Eliminates Compensation Trims. Trace A1's Output, Trace Q1's Collector Current
Load Transient Testing previously discussed circuits permit rapid thorough voltage regulator load transient testing. Figure uses Figure circuit evaluate LT1963A linear regulator. Figure shows regulator response (Trace Trace asymmetrically edged input pulse. ramped leading edge, within LT1963A's bandwidth, results Trace smooth 10mVP-P excursion. fast trailing edge, well outside LT1963A passband, causes Trace abrupt disruption. COUT cannot supply enough current maintain output level 75mVP-P spike results before regulator resumes control. Figure 500mA peak-to-peak 500kHz noise load, emulating multitude incoherent
loads, feeds regulator Trace This within regulator bandwidth only 6mVP-P disturbance appears Trace regulator output. Figure maintains same conditions, except that noise bandwidth increased 5MHz. Regulator bandwidth exceeded, resulting over 50mVP-P error, increase. Figure shows what happens when 0.2A biased, swept DC-5MHz, 0.35A load presented regulator. regulator's rising output impedance versus frequency results ascending error frequency scales. This information allows determination regulator output impedance versus frequency.
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Application Note
3.3V
BIAS
LT1006
10µF
LT1963A 3.3V
SENSE 10µF
VOLTAGE MONITOR AC-COUPLED OSCILLOSCOPE
100k 8.16k* 866* WAVEFORM INPUT MINIMIZE CAPACITANCE 120k +1mV ADJUST LOOP PEAKING 68pF
AN104
GATE DRIVE PEAKING IRLZ24 10µF CERAMIC FEEDBACK
TEKTRONIX P-6042 CURRENT PROBE EQUIVALENT MINIMIZE INDUCTANCE
100* LT1210
0.01µF
0.1** FAIR-RITE #2743001112 METAL FILM RESISTOR VISHAY WSL2512.5%
Figure Closed Loop Load Tester Shown with LT1963A Regulator. Load Testing Variety Current Load Waveshapes Possible
0.5A/DIV AC-COUPLED 0.3ADC LEVEL 0.02V/DIV AC-COUPLED 3.3VDC
0.5A/DIV 0.1ADC LEVEL
0.02V/DIV AC-COUPLED 3.3VDC
HORIZ 10µs/DIV
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HORIZ 2ms/DIV
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Figure Figure Responds (Trace Assymetrically Edged Pulse Input (Trace Ramped Leading Edge, Within LT1963A Bandwidth, Results Trace Smooth 10mVP-P Excursion. Fast Trailing Edge, Outside LT1963A Bandwidth, Causes Trace Abrupt 75mVP-P Disruption. Traces Latter Portion Intensified Photographic Clarity
Figure 500mAP-P, 500kHz Noise Load (Trace Within Regulator Bandpass, Produces Only Artifacts Trace Regulator Output
0.5A/DIV 0.1ADC LEVEL 0.02V/DIV AC-COUPLED 3.3VDC
0.02V/DIV AC-COUPLED 3.3VDC
HORIZ 2ms/DIV
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HORIZ 500KHz/DIV
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Figure Same Conditions Figure Except Noise Bandwidth Increased 5MHz. Regulator Bandwidth Exceeded, Resulting 50mVP-P Output Error
Figure Swept 5MHz, 0.35A Load 0.2ADC) Results Above Regulator Response. Regulator Output Impedance Rises with Frequency, Causing Corresponding Ascending Output Error
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Application Note
Capacitor's Role Regulator Response regulator employs capacitors input (CIN) output (COUT) augment high frequency response. capacitor's dielectric, value location greatly influence regulator characteristics must quite carefully considered.3 COUT dominates regulator's dynamic response; much less critical, long does discharge below regulator's dropout point. Figure shows typical regulator circuit emphasizes COUT parasitics. Parasitic inductance resistance limit capacitor effectiveness frequency. capacitor's dielectric value significantly influence load step response. "hidden" parasitic, impedance build-up regulator output trace runs, also influences regulation characteristics, although effects minimized remote sensing (shown) distributed capacitive bypassing. Figure shows Figure 16's circuit responding (Trace 0.5A load step biased 0.1A (Trace with
10µF
COUT 10µF. loss capacitors employed result Trace well controlled output. Figure greatly expands horizontal time scale investigate high frequency behavior. Regulator output deviation (Trace smooth, with abrupt discontinuities. Figure runs same test Figure using output capacitor claimed "equivalent" employed Figure 10µs/division things seem very similar, Figure indicates problems. This photo, taken same higher sweep speed Figure reveals "equivalent" capacitor have amplitude error versus Figure higher frequency content
0.5A/DIV AC-COUPLED 0.1ADC
0.1V/DIV AC-COUPLED 3.3VDC
HORIZ 100ns/DIV
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LT1963A 3.3V
RTRACE SENSE
LTRACE 3.3V
Figure Expanding Horizontal Scale Shows Trace Smooth Regulator Output Response. Mismatched Current Voltage Probe Delays Account Slight Time Skewing
COUT (WITH PARASITICS)
0.5A/DIV AC-COUPLED 0.1ADC
AN104
0.1V/DIV AC-COUPLED 3.3VDC
Figure COUT Dominates Regulator's Dynamic Response; Much Less Critical. Parasitic Inductance Resistance Limit Capacitor Effectiveness Frequency. Capacitor Value Dielectric Significantly Influence Load Step Response. Excessive Trace Impedance Also Factor
0.5A/DIV AC-COUPLED 0.1ADC
HORIZ 10µs/DIV
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Figure "Equivalent" 10µF COUT Capacitor's Performance Appears Similar Figure 17's Type 10µs/DIV
0.5A/DIV AC-COUPLED 0.1ADC
0.1V/DIV AC-COUPLED 3.3VDC
0.1V/DIV AC-COUPLED 3.3VDC HORIZ 10µs/DIV
AN104
Figure Stepped 0.5A Load Figure 16's Circuit (Trace with COUT 10µF Results Trace Regulator Output. Loss Capacitors Promote Controlled Output Excursions
Note Appendices extended discussion these concerns.
HORIZ 100ns/DIV
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Figure Horizontal Scale Expansion Reveals "Equivalent" Capacitor Produces Amplitude Error Figure Mismatched Probe Delays Cause Time Skewing Between Traces
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Application Note
resonances.4 Figure substitutes very lossy 10µF unit COUT. This capacitor allows 400mV excursion (note Trace vertical scale change), Figure 18's amount. Conversely, Figure increases COUT loss 33µF type, decreasing Trace output response transient versus Figure Figure 23's further increase, loss 330µF capacitor, keeps transients inside 20mV; lower than Figure 18's 10µF value.
0.5A/DIV AC-COUPLED 0.1ADC
lesson from preceding study clear. Capacitor value dielectric quality have pronounced effect transient load response. before specifying! Load Transient Risetime versus Regulator Response closed loop load transient generator also allows investigating load transient risetime regulation high speed. Figure shows Figure 16's circuit (CIN COUT 10µF) responding 0.5A, 100ns risetime step 0.1A load (Trace Response decay (Trace peaks 75mV with some following aberrations. Decreasing Trace load step risetime (Figure almost doubles Trace response error, with attendant enlarged following aberrations. This indicates increased regulator error higher frequency. regulators present increasing error with frequency, some more than others. slow load transient unfairly make poor regulator look good. Transient load testing that does indicate some response outside regulator bandwidth suspect.
0.2A/DIV AC-COUPLED 0.1ADC 0.05V/DIV AC-COUPLED 3.3VDC
0.2V/DIV AC-COUPLED 3.3VDC
HORIZ 100ns/DIV
AN104
Figure Excessively Lossy 10µF COUT Allows 400mV Excursion Figure 18's Amount. Time Skewing Between Traces Derives from Probe Mismatch
0.5A/DIV AC-COUPLED 0.1ADC 0.1V/DIV AC-COUPLED 3.3VDC
HORIZ 10µs/DIV
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HORIZ 100ns/DIV
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Figure Increasing COUT with Loss 33µF Unit Reduces Output Response Transient Over Figure
Figure Regulator Output Response (Trace 100ns. Risetime Current Step (Trace COUT 10µF. Response Decay Peaks 75mV
0.5A/DIV AC-COUPLED 0.1ADC 0.1V/DIV AC-COUPLED 3.3VDC
0.2A/DIV AC-COUPLED 0.1ADC
0.05V/DIV AC-COUPLED 3.3VDC HORIZ 10µs/DIV
AN104 AN104
HORIZ 100ns/DIV
Figure Loss 330µF Capacitor Keeps Output Response Transients Inside 20mV Lower than Figure 17's 10µF
Figure Faster Risetime Current Step (Trace Increases Response Decay Peak (Trace 140mV, Indicating Increased Regulation Loss Frequency
Note Always specifiy components according observed performance, never salesman's claims.
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Application Note
Practical Example Intel Embedded Memory Voltage Regulator good example importance voltage regulator load step performance furnished Intel embedded memory. This memory requires 1.8V supply, typically regulated down from +3V. Although current requirements relatively modest, supply tolerances tight. Figure 26's error budget shows only 0.1V allowable excursion from 1.8V, including dynamic errors. LTC1844-1.8 regulator 1.75% initial tolerance (31.5mV), leaving only 68.5mV dynamic error allowance. Figure test circuit. Memory control line movement causes 50mA load transients, necessitating attention capacitor selection.5 regulator close power source optional. not, good grade capacitor CIN. COUT loss type. other respects circuit appears deceptively routine. load transient generator provides Figure 28's output load test step
Intel Embedded Memory Voltage Regulator Error Budget
PARAMETER Intel Specified Supply Limits LTC1844 Regulator Initial Accuracy Dynamic Error Allowance LIMITS 1.8V 0.1V ±1.75% (±31.5mV) ±68.5mV
50mA/DIV AC-COUPLED 1mADC 0.05V/DIV AC-COUPLED 1.8VDC
(Trace A).6 Trace regulator response shows just 30mV peaks, better than required. Increasing COUT 10µF, Figure reduces peak output error 12mV, almost better than specification. However, poor grade 10µF 1µF, that matter) capacitor produces Figure 30's unwelcome surprise. Severe peaking error both edges occurs (Trace latter portion been intensified photograph clarity) with 100mV observable negative going edge. This well outside error budget would cause unreliable memory operation.
50mA/DIV AC-COUPLED 1mADC 0.05V/DIV AC-COUPLED 1.8VDC
HORIZ 50µs/DIV
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Figure 50mA Load Step (Trace Results 30mV Regulator Response Peaks, Better than Error Budget Requirements. COUT Loss
Figure Error Budget Intel Embedded Memory Voltage Regulator. 1.8V Supply Must Remain Within ±0.1V Tolerance, Including Static Dynamic Errors
VCCQ INTEL EMBEDDED MEMORY CONTROL LINES
HORIZ 50µs/DIV
AN104
LTC1844 1.8V
1.8V 0.1V COUT (SEE TEXT)
Figure Increasing COUT 10µF Decreases Regulator Output Peaks 12mV, Almost Better than Required
OPTIONAL INPUT CAPACITOR (SEE TEXT)
OPTIONAL NOISE REDUCTION CAPACITOR (SEE TEXT)
50mA/DIV AC-COUPLED 1mADC
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Figure Embedded Memory Regulator Must Maintain ±0.1V Error Band. Control Line Movement Causes 50mA Load Steps, Necessitating Attention COUT Selection
0.05V/DIV AC-COUPLED 1.8VDC
HORIZ 50µs/DIV
AN104
Note LTC1844-1.8's noise bypass ("BYP") used with optional external capacitor achieve extremely output noise. required this application left unconnected.
Figure Poor Grade 10µF COUT Causes 100mV Regulator Output Peaks (Trace Violating Memory Limits. Traces Latter Portion Intensified Photographic Clarity
Note Figure circuit used this test, with Q1's emitter current shunt changed
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Application Note
REFERENCES LT1584/LT1585/LT1587 Fast Response Regulators Datasheet. Linear Technology Corporation. LT1963A Regulator Datasheet. Linear Technology Corporation. Williams, Jim, "Minimizing Switching Residue Linear Regulator Outputs". Linear Technology Corporation, Application Note 101, July 2005 Shakespeare, William, "The Taming Shrew," 1593-94.
Note. This application note derived from manuscript originally prepared publication magazine.
APPENDIX Capacitor Parasitic Effects Load Transient Response Tony Bonte Large load current changes typical digital systems. load current step contains higher order frequency components that output decoupling network must handle until regulator throttles load current level. Capacitors ideal elements contain parasitic resistance inductance. These parasitic elements dominate change output voltage beginning transient load step change. (equivalent series resistance) output capacitors produces instantaneous step output voltage.(V ESR). (equivalent series inductance) output capacitors produces droop proportional rate change output current I/t). output capacitance produces change output voltage proportional time until regulator respond I/C). These transient effects illustrated Figure capacitors with ESR, ESL, good high frequency characteristics critical meeting output load voltage tolerances. These requirements dictate high quality, surface mount tantalum, ceramic organic electrolyte capacitors. capacitor's location critical transient response performance. Place capacitor close possible regulator pins keep supply line traces planes impedance, bypassing individual loads necessary. regulator remote sensing capability, consider sensing heaviest load point. Strictly speaking, above only time related terms that influence regulator settling. Figure lists different terms, occurring over decades time, that potentially influence regulation. regulator must carefully designed minimize regulator loop thermal error contributions.
EFFECTS AMPLITUDE EFFECTS CAPACITANCE EFFECTS
AN104 FA01
SLOPE,
POINT WHICH REGULATOR TAKES CONTROL TIME
Figure Parasitic Resistance, Inductance Finite Capacitance Combine with Regulator Gain-Bandwidth Limitations Form Load Step Response. Capacitors Equivalent Series Resistance (ESR) Inductance (ESL) Dominate Initial Response; Capacitor Value Regulator Gain-Bandwidth Determine Responses Latter Profile
10ns 100ns CAPACITOR 10µs BULK CAPACITOR RESISTANCE 1000µs 20ms 200ms PACKAGE HEAT SINK
AN104 FA02
BULK (DISTRIBUTED) CAPACITANCE
REGULATOR LOOP
THERMAL REGULATION DIE)
PACKAGE (THERMAL)
Figure Time Constants Potentially Influencing Regulator Settling Time After Load Step Electrical Thermal. Effects Span Over Decades
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Application Note
APPENDIX Output Capacitors Loop Stability Dennis O'Neill Editorial Note: following text, excerpted from LT1963A datasheet, concerns output capacitor's relationship transient response. Although originally prepared LT1963A application, generalizable most regulators presented here reader convenience. voltage regulator feedback circuit. Like feedback circuit, frequency compensation needed make stable. LT1963A, frequency compensation both internal external output capacitor. size output capacitor, type output capacitor, particular output capacitor affect stability. addition stability, output capacitor also affects high frequency transient response. regulator loop finite bandwidth. high frequency transient loads recovery from transient combination output capacitor bandwidth regulator. LT1963A designed easy accept wide variety output capacitors. However, frequency compensation affected output capacitor optimum frequency stability require some ESR, especially with ceramic capacitors. ease use, polytantalum capacitors (POSCAP) good choice both transient response stability regulator. These capacitors have intrinsic that improves stability. Ceramic capacitors have extremely ESR, while they good choice many cases, placing small series resistance element will sometimes achieve optimum stability minimize ringing. cases, minimum 10µF required while maximum allowable place where most helpful with ceramics output voltage. output voltages, below 2.5V, some helps stability when ceramic output capacitors used. Also, some allows smaller capacitor value used. When small signal ringing occurs with ceramics insufficient ESR, adding increasing capacitor value improves stability reduces ringing. Figure gives some recommended values
VOUT 1.2V 1.5V 1.8V 2.5V 3.3V 10µF 22µF 47µF 100µF
Figure Capacitor Minimum
minimize ringing caused fast, hard current transitions. Figures through show effect transient response regulator. These scope photos show transient response LT1963A three different output voltages with various capacitors various values ESR. output load conditions same traces. cases there load 500mA. load steps first transition steps back 500mA second transition. worst case point 1.2VOUT with 10µF COUT (Figure B2), minimum amount required. While enough eliminate most ringing, value closer provides more optimum response. 2.5V output with 10µF COUT (Figure output rings transitions with still settles within 10mV 20µs after 0.5A load step. Once again small value will provide more optimum response. 5VOUT with 10µF COUT (Figure response well damped with ESR. With COUT 100µF output 1.2V (Figure B5), output rings although amplitude only 20mVP-P. With COUT 100µF takes only provide good damping 1.2V output. Performance 2.5V output with 100µF COUT shows similar characteristics 10µF case (see Figures B7). 2.5VOUT improve transient response. 5VOUT response well damped with ESR. Capacitor types with inherently higher combined with ceramic capacitors achieve both good high frequency bypassing fast settling time. Figure illustrates improvement transient response that seen when parallel combination ceramic
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Application Note
VOUT 1.2V IOUT 500mA WITH 500mA PULSE COUT 10µF RESR 50mV/DIV VOUT 2.5V IOUT 500mA WITH 500mA PULSE COUT 10µF 50mV/DIV
RESR
20µs/DIV
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20µs/DIV
AN104 FB03
Figure
Figure
VOUT IOUT 500mA WITH 500mA PULSE COUT 10µF RESR 50mV/DIV
VOUT 1.2V IOUT 500mA WITH 500mA PULSE COUT 100µF 50mV/DIV
RESR
20µs/DIV
AN104 FB04
50µs/DIV
AN104 FB05
Figure
Figure
VOUT 2.5V IOUT 500mA WITH 500mA PULSE COUT 100µF RESR 50mV/DIV
VOUT IOUT 500mA WITH 500mA PULSE COUT 100µF 50mV/DIV
RESR
50µs/DIV
AN104 FB06
50µs/DIV
AN104 FB07
Figure
Figure
RESR
VOUT 1.2V IOUT 500mA WITH 500mA PULSE COUT 10µF CERAMIC 10µF CERAMIC 22µF/45m POLY 10µF CERAMIC 100µF/35m POLY
50mV/DIV
50µs/DIV
AN104 FB08
Figure
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Application Note
POSCAP capacitors used. output voltage worst case value 1.2V. Trace with 10µF ceramic output capacitor, shows significant ringing with peak amplitude 25mV. Trace 22µF/45m POSCAP added parallel with 10µF ceramic. output well damped settles within 10mV less than 20µs. Trace 100µF/35m POSCAP connected parallel with 10µF ceramic capacitor. this case peak output deviation less than 20mV output settles about 10µs. improved transient response value bulk capacitor (tantalum aluminum electrolytic) should greater than twice value ceramic capacitor. Tantalum Polytantalum Capacitors There variety tantalum capacitor types available, with wide range specifications. Older types have specifications hundreds several Ohms. Some newer types polytantalum with multi-electrodes have maximum specifications general lower specification, larger size higher price. Polytantalum capacitors have better surge capability than older types generally lower ESR. Some types such Sanyo series have specifications range, which provide near optimum transient response. Aluminum Electrolytic Capacitors
CHANGE VALUE
dielectrics good providing high capacitances small package, exhibit strong voltage temperature coefficients shown Figures B10. When used with regulator, 10µF capacitor exhibit effective value over operating temperature range. dielectrics result more stable characteristics more suitable output capacitor. type better stability across temperature, while less expensive available higher values.
CHANGE VALUE -100 BOTH CAPACITORS 16V, 1210 CASE SIZE, 10µF
BIAS VOLTAGE
AN104 FB09
Figure Ceramic Capacitor Bias Characteristics
BOTH CAPACITORS 16V, 1210 CASE SIZE, 10µF TEMPERATURE (°C)
Aluminum electrolytic capacitors also used with LT1963A. These capacitors also used conjunction with ceramic capacitors. These tend cheapest lowest performance type capacitors. Care must used selecting these capacitors some types have which easily exceed maximum value. Ceramic Capacitors Extra consideration must given ceramic capacitors. Ceramic capacitors manufactured with variety dielectrics, each with different behavior over temperature applied voltage. most common dielectrics used Z5U, Y5V, X7R.
-100
AN104 FB10
Figure B10. Ceramic Capacitor Temperature Characteristics
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Application Note
Voltage temperature coefficients only sources problems. Some ceramic capacitors have piezoelectric response. piezoelectric device generates voltage across terminals mechanical stress, similar piezoelectric accelerometer microphone works. ceramic capacitor stress induced vibrations system thermal transients. "FREE" Resistance with Traces resistance values shown Figure easily made using small section trace series with output capacitor. wide range non-critical makes easy trace. trace width should sized handle ripple current associated with load. output capacitor only sources sinks current microseconds during fast output current transitions. There
0.5oz 1.0oz 2.0oz Width Length Width Length Width Length 0.011" (0.28mm) 0.102" (2.6mm) 0.006" (0.15mm) 0.110" (2.8mm) 0.006" (0.15mm) 0.224" (5.7mm)
current output capacitor. Worst case ripple current will occur output load high frequency (>100kHz) square wave with high peak value fast edges (<1µs). Measured value this case times peak-to-peak current change. Slower edges lower frequency will significantly reduce ripple current capacitor. This resistor should made using inner layers board which well defined. resistivity determined primarily sheet resistance copper laminate with additional plating steps. Figure gives some sizes 0.75A current various copper thicknesses. More detailed information regarding resistors made from traces found Application Note Appendix
0.011" (0.28mm) 0.204" (5.2mm) 0.006" (0.15mm) 0.220" (5.6mm) 0.006" (0.15mm) 0.450" (11.4mm)
0.011" (0.28mm) 0.307" (7.8mm) 0.006" (0.15mm) 0.330" (8.4mm) 0.006" (0.15mm) 0.670" (17mm)
Figure B11. Trace Resistors
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Application Note
APPENDIX Probing Considerations Load Transient Response Measurements Signals interest load transient response studies occur within bandwidth about 25MHz (tRISE 14ns) This modest speed range probing technique requires some care high fidelity measurement. Load current measured with stabilized (Hall Effect based) "clip current probe such Tektronix P-6042 AM503. conductor loop placed probe jaws should encompass smallest possible area minimize introduced parasitic inductance, which degrade measurement. higher speeds, grounding probe case slightly decrease measurement aberrations, this usually small effect. Voltage measurement, typically AC-coupled 10mV 250mV range, best accomplished with Figure C1's arrangement. measured voltage fixtured back terminated cable, which drives oscilloscope blocking capacitor termination.
CONNECTION BOARD COAXIAL IN-LINE BACK TERMINATION (OPTIONAL, TEXT)
back termination strict practice, enforcing true signal path. Practically, attenuation presents problems, usually eliminated with only minor signal degradation 25MHz measurement passband. termination oscilloscope negotiable. Figure shows typical observed load transient with back termination oscilloscope. presentation clean well defined. cable's termination removed, causing distorted leading edge,ill-defined peaking pronounced post-event ringing. Even relatively modest frequencies cable displays unterminated transmission line characteristics, resulting signal distortion. theory, scope probe using probe-tip coaxial connection could replace above such probes usually have bandwidth limitations 10MHz 20MHz. Conversely, probe wideband, oscilloscope vertical sensitivity must accommodate introduced attenuation.
10µF COAXIAL COUPLING CAPACITOR*
REGULATOR UNDER TEST COUT COAXIAL LINE LOAD TRANSIENT GENERATOR IN-LINE TERMINATION
OSCILLOSCOPE
AN104 FC01
VISHAY #430P FIXTURED ENCLOSURE
Figure Coaxial Load Transient Voltage Measurement Path Promotes Observed Signal Fidelity. Back Termination Removed with Minimal Impact 25MHz Signal Path Integrity. Termination Oscilloscope Cannot Deleted
VERT 0.05V/DIV AC-COUPLED
VERT 0.05V/DIV AC-COUPLED
HORIZ 200ns/DIV
AN104 FC02
HORIZ 200ns/DIV
AN104 FC03
Figure Typical High Speed Transient Observed Through Figure C1's Measurement Path. Presentation Clean Well Defined
Figure Figure C2's Transient Measured with Oscilloscope Termination Removed. Waveform Distortion Post-Event Ringing Result
an104f
AN104-14
Application Note
APPENDIX Trimless Closed Loop Transient Load Tester Text Figure circuit attractive because eliminates based design's trims. does, however, retain trim. Figure trades circuit complexity eliminate trim. Operation similar text Figure circuit except that appears. This amplifier replaces trim measuring circuits input, comparing Q1's emitter level controlling A1's positive input stabilize circuit. High frequency signals filtered A1's inputs corrupt A1's stabilizing action. useful consider circuit operation that will balance inputs, hence circuit's input output, regardless A1's input errors. current bias desired point variable reference source directed A2's positive input. This network's resistors arranged minimum load current 10mA, avoiding loop disruption currents near zero.
VN2222L 909* WAVEFORM INPUT 909* BIAS 10k* 100* REGULATOR INPUT SUPPLY VREG
LT1210 D44H2 MUR11O 0.1**
LT1001
100*
MYLAR METAL FILM RESISTOR VISHAY WSL2512.5%
10k*
0.01µF
AN104 FD01
Figure Feedback Controls A1's Errors, Eliminating Text Figure Trim. Filtering Restricts A2's Response Frequency
an104f
Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
AN104-15
Application Note
an104f
AN104-16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408) 432-1900 FAX: (408) 434-0507
1006 PRINTED
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2006
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