LT®1939 current mode step-down DC/DC converter with internal 2.3A swit
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LT1939 Monolithic Step-Down Regulator Plus Linear Regulator/Controller DESCRIPTION
LT®1939 current mode step-down DC/DC converter with internal 2.3A switch. wide input range makes LT1939 suitable regulating power from wide variety sources, including automotive batteries, industrial supplies unregulated wall adapters. Resistor-programmable 250kHz 2.2MHz frequency range synchronization capability enable optimization between efficiency external component size. Cycleby-cycle current limit, frequency foldback thermal shutdown provide protection against shorted output. soft-start feature controls ramp rate output voltage, eliminating input current surge during start-up, also provides output tracking. LT1939 contains internal transistor with feedback control which configured linear regulator linear regulator controller. LT1939's current shutdown mode (<12A) enables easy power management battery-powered systems.
Lare registered trademarks Linear Technology Corporation. other trademarks property their respective owners.
Wide Input Range: Short-Circuit Protected Over Full Input Range Output Current Capability Adjustable/Synchronizable Fixed Frequency Operation from 250kHz 2.2MHz Soft-Start/Tracking Capability Output Adjustable Down 0.8V Adjustable Linear Regulator/Driver with 13mA Output Capability Power Good Comparator with Complementary Outputs Shutdown Current: Thermally Enhanced Package
APPLICATIONS
Automotive Battery Regulation Industrial Control Wall Transformer Regulation Distributed Power Regulation
TYPICAL APPLICATION
Dual Step-Down Converters
BAT54 2.2F LT1939 SHDN 0.47F RT/SYNC LDRV 40.2k 8.06k
1939 TA01a
Switching Converter Efficiency
VOUT1 EFFICIENCY IOUT2 FREQUENCY 800kHz LOAD CURRENT
1939 TA01b
Output Voltage Ripple
0.47F 6.8H B240A 42.2k
VOUT1 COUPLED 2mV/DIV
8.06k
VOUT2 3.3V COUPLED 2mV/DIV
53.6k 330pF
24.9k
VOUT2 3.3V
500ns/DIV
1939 TA01c
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LT1939 ABSOLUTE MAXIMUM RATINGS
(Note
CONFIGURATION
VIEW SHDN RT/SYNC LDRV
VIN, Operating 25V/-0.3V .VIN 45V/-0.3V Above SW.25V LDRV, SHDN .15V LFB, RT/SYNC .2.5V Operating Junction Temperature Range (Notes LT1939EDD -40°C 125°C LT1939IDD -40°C 125°C Storage Temperature Range. -65°C 150°C
PACKAGE 12-LEAD (3mm 3mm) PLASTIC 45°C/W, JC(PAD) 10°C/W EXPOSED (PIN GND, MUST SOLDERED
ORDER INFORMATION
LEAD FREE FINISH LT1939EDD#PBF LT1939IDD#PBF TAPE REEL LT1939EDD#TRPBF LT1939IDD#TRPBF PART MARKING* LDJZ LDJZ PACKAGE DESCRIPTION 12-Lead (3mm 3mm) Plastic 12-Lead (3mm 3mm) Plastic TEMPERATURE RANGE -40°C 125°C -40°C 125°C
Consult Marketing parts specified with wider operating temperature ranges. *The temperature grade identified label shipping container. Consult Marketing information non-standard lead based finish parts. more information lead free part marking, http://www.linear.com/leadfree/ more information tape reel specifications,
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. VVIN 15V, VRT/SYNC unless otherwise specified.
PARAMETER SHDN Threshold SHDN Source Current SHDN Current Hysterisis Minimum Input Voltage (Note Supply Shutdown Current Supply Quiescent Current Voltage Bias Current Error Amplifier Error Amplifier Source Current Error Amplifier Sink Current Error Amplifier High Clamp Error Amplifier Switching Threshold Source Current VSHDN 0.9V 0.6V 1.6V, 0.8V, ±10A 0.6V, 0.6V 0.6V VSHDN 0.4V, 0.9V 2.25
ELECTRICAL CHARACTERISTICS
CONDITIONS
3.25 0.816 0.824 3.75
UNITS
1.25
0.784 0.776
2.75
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LT1939 ELECTRICAL CHARACTERISTICS
PARAMETER Sink Current Sink Current (Note Threshold Offset (VSS VFB) PG/PG Leakage Threshold Hysteresis Sink Current Sink Current RT/SYNC Reference Voltage Switching Frequency CONDITIONS Cycle SHDN 0.4V 0.9V/0.7V, VPG/VPG 0.4V 0.4V 0.4V, 0.7V 0.4V, 0.9V VFB1/2 0.9V, RRT/SYNC RRT/SYNC 90.9k RRT/SYNC 90.9k RRT/SYNC 0.7V, RRT/SYNC 90.9k 0.7V, RRT/SYNC 90.9k VBST 18V, 0.7V VBST 18V, 0.7V VBST 18V, 0.7V 0.7V VLDRV 1.2V VVIN 25V, VLDRV 0.8V, VLDRV VLFB VLFB 0.8V, VLDRV ILDRV VLDRV
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. VVIN 15V, VRT/SYNC unless otherwise specified.
0.685 0.75 0.784 0.776 0.816 0.824 0.708 0.850 0.730 1100 0.975 2500 UNITS
SYNC Frequency Range Minimum Switch Time Minimum Switch Time Switch Leakage Current Switch Saturation Voltage Switch Peak Current Boost Current Minimum Boost Voltage (Note Voltage Line/Load Regulation Offset (VSS VLFB) Bias Current LDRV Dropout LDRV Maximum Current
Note Stresses beyond those listed under Absolute Maximum Ratings cause permanent damage device. Exposure Absolute Maximum Rating condition extended periods affect device reliability lifetime. Note2: LT1939EDD guaranteed meet performance specifications from 125°C junction temperature. Specifications over -40°C 125°C operating junction temperature range assured design, characterization correlation with statistical process controls. LT1939IDD guaranteed over full -40°C 125°C operating junction temperature range. Note Minimum input voltage defined voltage where internal bias lines regulated that reference voltage oscillator remain constant. Actual minimum input voltage maintain regulated output will depend upon output voltage load current. Applications Information.
Note internal power-on reset (POR) latch positive transition SHDN through threshold. output latch activates current source which typically sinks 600A, discharging capacitor. latch reset when driven below soft-start threshold SHDN taken below threshold. Note This minimum voltage across boost capacitor needed guarantee full saturation internal power switch. Note This includes overtemperature protection that intended protect device during momentary overload conditions. Junction temperature will exceed maximum operating junction temperature when overtemperature protection active. Continuous operation above specified maximum operating junction temperature impair device reliability.
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LT1939 TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Voltage Temperature
0.820 0.815 0.810 VOLTAGE VOLTAGE 0.805 0.800 0.795 0.790 0.785 0.780 TEMPERATURE (°C)
1939
RT/SYNC Voltage Temperature
1.10 1.08 1.06 1.04 1.02 1.00 0.98 0.96 0.94 0.92 0.90 TEMPERATURE (°C)
1939
Shutdown Threshold Minimum Input Voltage Temperature
MINIMUM INPUT VOLTAGE
RRT/SYNC 90.9k VOLTAGE RRT/SYNC
SHUTDOWN THRESHOLD
TEMPERATURE (°C)
1939
Shutdown Currents Temperature
VSHDN 0.7V VSHDN 0.9V CURRENT 15.0
Shutdown Quiescent Current Temperature
TRANSCONDUCTANCE (mhos)
Error Amplifier Temperature
12.5 10.0
CURRENT
TEMPERATURE (°C)
1939
TEMPERATURE (°C)
1939
TEMPERATURE (°C)
1939
Soft-Start Source Current Temperature
CURRENT VOLTAGE (mV) TEMPERATURE (°C)
1939
Soft-Start Feedback Offset Temperature
0.95 0.90 VOLTAGE 0.85 0.80 0.75 0.70 0.65 0.60 0.55 TEMPERATURE (°C)
1939
Switching Threshold Temperature
0.50
TEMPERATURE (°C)
1939
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LT1939 TYPICAL PERFORMANCE CHARACTERISTICS
Power Good Thresholds Temperature
0.75 0.74 0.73 0.72 VOLTAGE 0.71 0.70 0.69 0.68 0.67 0.66 0.65 TEMPERATURE (°C)
1939
Power Good Sink Currents Temperature
1000 FREQUENCY (kHz) TEMPERATURE (°C)
1939
Frequency Temperature
RRT/SYNC 90.9k
RISING EDGE CURRENT
FALLING EDGE
TEMPERATURE (°C)
1939
Peak Switch Current Temperature
TEMPERATURE (°C)
1939
LDRV Short-Circuit Current Temperature
DUTY CYCLE CURRENT (mA) TEMPERATURE (°C)
1939
External Sync Duty Cycle Range External Sync Frequency
MINIMUM DUTY CYCLE MAXIMUM DUTY CYCLE
CURRENT
2250 1750 1250 SYNCHRONIZATION FREQUENCY (kHz)
19939
Minimum Switching Times
TIME (ns) TEMPERATURE (°C)
1939
Frequency RRT/SYNC
2500 SWITCH SATURATION VOLTAGE (mV) 2250 2000 FREQUENCY (kHz) 1750 1500 1250 1000 RRT/SYNC
1939
Switch Saturation Voltage Switch Current
-50°C 150°C SWITCH CURRENT
1939
MINIMUM TIME
MINIMUM TIME
25°C
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LT1939 TYPICAL PERFORMANCE CHARACTERISTICS
Boost Current Switch Current
BOOST CURRENT (mA) SWITCH CURRENT
1939
Minimum Boost Voltages Temperature
BOOST VOLTAGE MINIMUM BOOST SWITCH SATURATION INPUT VOLTAGE TEMPERATURE (°C)
1939
Minimum Input Voltage
150°C -50°C
VOUT1
VOUT1 3.3V
25°C
1MHz 3.3H LOAD CURRENT
1939
LDRV Dropout Voltage Temperature
1.50 1.45 1.40 1.35 VOLTAGE 1.30 1.25 1.20 1.15 1.10 1.05 1.00 TEMPERATURE (°C)
1939
Switcher Dropout Operation
OUTPUT VOLTAGE VOUT1 FREQUENCY (kHz) VOUT1 3.3V IOUT1 2500 2250 2000 1750 1500 1250 1000 INPUT VOLTAGE
1939
Inductor Value Maximum Load Current (VOUT 3.3V, IRIPPLE 250mA)
1.5H
ILDRV
2.2H
3.3H 4.7H 6.8H
INPUT VOLTAGE
1939
FUNCTIONS
(Pin powers internal control circuitry monitored undervoltage comparator. also connected collectors internal power switch linear output NPN. high dI/dt edges must decoupled ground close device. SHDN (Pin SHDN used shut down LT1939 reduce quiescent current typical value 12A. accurate 0.76V threshold input current hysteresis used undervoltage lockout, preventing regulator from operating until input voltage reached predetermined level. Force SHDN above threshold float normal operation. (Pin used control slew rate output both switching linear regulators. single capacitor from ground determines regulators' ramp rate. soft-start details Applications Information section.
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LT1939 FUNCTIONS
(Pin power good open-collector output that sinks current when falls below nominal regulating voltage. above output state remains true, although during SHDN, undervoltage lockout, thermal shutdown, current sink capability reduced. (Pin output error amplifier input peak switch current comparator. normally used frequency compensation, also used current clamp control loop override. error amplifier drives above maximum switch current level, voltage clamp activates. This indicates that output overloaded current pulled from reducing regulation point. RT/SYNC (Pin This RT/SYNC provides modes setting constant switch frequency. Connecting resistor from RT/SYNC ground will RT/SYNC typical value resultant switching frequency will resistor value. minimum value maximum value 200k switching frequency 2.5MHz 250kHz respectively. Driving RT/SYNC with external clock signal will synchronize switch applied frequency. Synchronization occurs rising edge clock signal after clock signal detected. Each rising clock edge initiates oscillator ramp reset. gain control loop servos oscillator charging current maintain constant oscillator amplitude. Hence, slope compensation remains unchanged. clock signal removed, oscillator reverts resistor mode reapplies bias RT/SYNC after synchronization detection circuitry times out. clock source impedance should such that current RT/SYNC resistor mode generates frequency roughly equivalent synchronization frequency. Floating holding RT/SYNC above 1.1V will damage device, will halt oscillation. (Pin power good open-collector output that sinks current when rises above nominal regulating voltage. (Pin negative input switcher error amplifier. output switches regulate this 0.8V with respect exposed ground pad. Bias current flows pin. (Pin negative input linear error amplifier. LDRV servo's regulate this 0.8V with respect exposed ground pad. Bias current flows pin. LDRV (Pin 10): LDRV emitter internal that configured output linear regulator drive external high current regulator. Current flows LDRV when voltage below 0.8V. LDRV typical maximum current capability 13mA. (Pin 11): provides higher than base drive power ensure switch drop. comparator imposes minimum time voltage drops low. Forcing time allows boost capacitor recharge. (Pin 12): emitter on-chip power NPN. switch off, inductor will drive this below ground with high dV/dt. external catch diode ground, close respective decoupling capacitor's ground, must used prevent this from excessive negative voltages. Exposed (Pin 13): GND. Exposed only ground connection device. Exposed should soldered large copper area reduce thermal resistance. also serves small-signal ground. ideal operation small-signal ground paths should connect single point, avoiding high current ground returns.
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LT1939
BLOCK DIAGRAM
SHDN
DRIVER CIRCUITRY
0.76V
RT/SYNC OSCILLATOR
POWER RESET THERMAL OVERLOAD
2.75A
0.7V
100mV
Figure LT1939 Block Diagram
2.5A 0.8V SLOPE COMPENSATION INTERNAL REGULATOR REFERENCES
LDRV VOUT2
115mV
VOUT1
0.8V 100mV
1939
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LT1939 OPERATION
LT1939 constant frequency, current mode buck converter with internal 2.3A switch plus linear regulator with 13mA output capability. Control both outputs achieved with common SHDN pin, internal regulator, oscillator, undervoltage detect, soft-start, thermal shutdown power-on reset. SHDN taken below 0.8V threshold, LT1939 will placed quiescent current mode. this mode LT1939 typically draws from pin. When SHDN floated driven above 0.76V, internal bias circuits turn generating internal regulated voltage, 0.8(VFB) 1V(RT/SYNC) references, signal which sets soft-start latch. RT/SYNC reaches regulation point, internal oscillator will start generating clock signal frequency determined resistor from RT/SYNC ground. Alternatively, synchronization signal detected LT1939 RT/SYNC pin, clock signal will generated incoming frequency rising edge synchronization pulse. addition, internal slope compensation will automatically adjusted prevent subharmonic oscillation during synchronization. LT1939 constant frequency, current mode stepdown converter. Current mode regulators controlled internal clock feedback loops that control duty cycle power switch. addition normal error amplifier, there current sense amplifier that monitors switch current cycle-by-cycle basis. This technique means that error amplifier commands current delivered output rather than voltage. voltage system will have phase shift resonant frequency inductor output capacitor, then abrupt 180° shift will occur. current system will have phase shift much lower frequency, will have additional shift until well beyond resonant frequency. This makes much easier frequency compensate feedback loop also gives much quicker transient response. During power signal sets soft-start latch, which discharges ensure proper start-up operation. When voltage drops below 100mV, driven disabling switching softstart latch reset. Once latch reset soft-start capacitor starts charge with typical value 2.75A. voltage rises above 100mV pin, will driven high error amplifier. When voltage exceeds 0.8V, clock set-pulse sets driver flip-flop which turns internal power switch. This causes current from VIN, through switch, inductor internal sense resistor, increase. When voltage drop across internal sense resistor exceeds predetermined level voltage pin, flip-flop reset internal switch turned off. Once switch turned inductor will drive voltage until external Schottky diode starts conduct, decreasing current inductor. cycle repeated with start each clock cycle. However, internal sense resistor voltage exceeds predetermined level start clock cycle, flip-flop will resulting further decrease inductor current. Since output current controlled voltage, output regulation achieved error amplifier continually adjusting voltage. error amplifier transconductance amplifier that compares voltage either voltage minus 100mV internally regulated 800mV, whichever lowest. Compensation loop easily achieved with simple capacitor series resistor/capacitor from ground. Since driven constant current source, single capacitor soft-start will generate controlled linear ramp output voltage. current demanded output exceeds maximum current dictated clamp, will discharged, lowering regulation point until output voltage supported maximum current. When overload removed, output will soft-start from overload regulation point. undervoltage detection thermal shutdown will soft-start latch, resulting complete soft-start sequence. switch driver operates from either voltage. external diode capacitor used generate
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LT1939 OPERATION
drive voltage higher than saturate output maintain high efficiency. addition switching regulator, LT1939 contains linear regulator with 0.8V reference, 13mA current capability. reference will track same manner switching regulator. linear output also configured drive external provide linear regulator with higher current capability. power good comparator with 30mV hysteresis trips when both above 0.8V reference. output open collector that when output regulation allowing resistor pull desired voltage. output opencollector that when output regulation providing either drive output disconnect transistor inverted power good logic.
APPLICATIONS INFORMATION
Choosing Output Voltage output voltage programmed with resistor divider between output pin. Choose resistors according VOUT1 0.8V maximum recommended frequency approximated equation: Frequency (Hz) VOUT1 tON(MIN)
should 10.0k less avoid bias current errors. Reference designators refer Block Diagram Figure Choosing Switching Frequency LT1939 switching frequency resistor Figure RT/SYNC internally regulated Setting resistor sets current RT/SYNC which determines oscillator frequency illustrated Figure switching frequency typically high possible reduce overall solution size. LT1939 employs techniques enhance dropout high frequencies efficiency maximum input voltage decrease switching losses minimum switch times.
where forward voltage drop catch diode Figure voltage drop internal switch, tON(MIN) minimum time switch, maximum load current.
2500 2250 2000 FREQUENCY (kHz) 1750 1500 1250 1000 RRT/SYNC
1939
Figure Frequency RT/SYNC Resistance
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LT1939 APPLICATIONS INFORMATION
following example along with data Table illustrates tradeoffs switch frequency selection. Example. 25V, VOUT1 3.3V, IOUT1 Temperature 85°C tON(MIN) 185ns (85°C from Typical Characteristics graph), 0.6V, 0.4V (85°C) Frequency RT/SYNC 49.9k Frequency 820kHz Input Voltage Range Once switching frequency been determined, input voltage range regulator determined. minimum input voltage determined either LT1939's minimum operating voltage ~2.8V maximum duty cycle. duty cycle fraction time that internal switch during clock cycle. maximum duty cycle determined from clock frequency minimum time from typical characteristics graph. This leads minimum input voltage VIN(MIN) VOUT1 DCMAX 835kHz 185ns where voltage drop internal switch, DCMAX tOFF(MIN) Frequency. Figure shows typical graph minimum input voltage load current 3.3V applications. maximum input voltage determined absolute maximum ratings pins frequency minimum duty cycle. minimum duty cycle defined DCMIN tON(MIN) Frequency Maximum input voltage VIN(MAX)
INPUT VOLTAGE VOUT1 START-UP VOUT1 RUNNING VOUT1 3.3V START-UP VOUT1 3.3V RUNNING LOAD CURRENT
1939
VOUT1 DCMIN
1MHz 3.3H
Figure Minimum Input Voltage Load Current
Table Efficiency Size Comparisons Different RRT/SYNC Values, 3.3V Output
FREQUENCY 2.5MHz 2.0MHz 1.5MHz 1.0MHz 500kHz RT/SYNC 24.9k 40.2k 90.9k EFFICIENCY 73.6 81.5 84.5 87.3 88.9 VIN(MAX) AREA (mm2)
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LT1939 APPLICATIONS INFORMATION
Note that LT1939 will regulate input voltage taken above calculated maximum voltage long maximum ratings pins violated. However operation this region input voltage will exhibit pulse skipping behavior. Example: VOUT1 3.3V, IOUT1 Frequency 1MHz, Temperature 25°C, 0.3V, 0.4V, tON(MIN) 150ns, tOFF(MIN) 110ns DCMAX (110ns)1MHz VIN(MIN) 4.06V 0.89 24.57V 0.15 physically smaller inductor, with lower resulting higher efficiency. current inductor triangle wave with average value equal load current. peak switch current equal output current plus half peak-to peak inductor ripple current. LT1939 limits switch current order protect itself system from overload faults. Therefore, maximum output current that LT1939 will deliver depends current limit, inductor value, switch frequency, input output voltages. inductor chosen based output current requirements, output voltage ripple requirements, size restrictions efficiency goals. When switch off, inductor sees output voltage plus catch diode drop. This gives peak-to-peak ripple current inductor:
DCMIN tON(MIN) Frequency VIN(MAX)
VOUT1
Inductor Selection Maximum Output Current good first choice inductor value (VIN VOUT1) VOUT1
where switching frequency LT1939 value inductor. peak inductor switch current ISW(PK) =ILPK =IOUT1
where frequency With this value maximum load current will ~2A, independent input voltage. inductor's current rating must greater than your maximum load current saturation current should about higher. keep efficiency high, series resistance (DCR) should less than 0.05. applications with duty cycle about 50%, inductor value should chosen obtain inductor ripple current less than peak switch current. course, such simple design guide will always result optimum inductor your application. larger value provides slightly higher maximum load current, will reduce output voltage ripple. your load lower than 1.5A, then decrease value inductor operate with higher ripple current. This allows
maintain output regulation, this peak current must less than LT1939's switch current limit, ILIM. ILIM guaranteed greater than 2.3A over entire duty cycle range. maximum output current function chosen inductor value: IOUT1(MAX) =ILIM =2.3
inductor value chosen that ripple current small, then available output current will near switch current limit. approach choosing inductor start with simple rule given above, look available inductors choose meet cost space goals. Then these equations check that LT1939 will able deliver required output current. Note again that these equations assume that inductor current continuous.
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LT1939 APPLICATIONS INFORMATION
Discontinuous operation occurs when IOUT less than calculated above. Figure illustrates inductance value needed 3.3V output with maximum load capability Referring Figure inductor value between 3.3H 4.7H will sufficient input voltage switch frequency 750kHz. There several graphs Typical Performance Characteristics section this data sheet that show inductor selection function input voltage switch frequency several popular output voltages output ripple currents. Also, inductance result discontinuous mode operation, which okay, further reduces maximum load current. details maximum output current discontinuous mode operation, Linear Technology Application Note Finally, duty cycles greater than (VOUT/VIN 0.5), there minimum inductance required avoid subharmonic oscillations. Application Note more information.
2500 2250 2000 FREQUENCY (kHz) 1750 1500 1250 1000 INPUT VOLTAGE
1939
capacitor required reduce resulting voltage ripple LT1939 force this very high frequency switching current into tight local loop, minimizing EMI. input capacitor must have impedance switching frequency this effectively, must have adequate ripple current rating. conservative value input current given ICIN(RMS) IOUT1 VOUT1 VOUT1)
IOUT1
largest when 2VOUT1 (50% duty cycle). frequency, VOUT ratio, maximum load current requirement LT1939 along with input supply source impedance, determine energy storage requirements input capacitor. Determine worstcase condition input ripple current then size input capacitor such that reduces input voltage ripple acceptable level. Typical values input capacitors from frequencies 2.2F higher frequencies. combination small size impedance (low equivalent series resistance ESR) ceramic capacitors make them preferred choice. results very voltage ripple capacitors handle plenty ripple current. They also comparatively robust used this application their rated voltage. types stable over temperature applied voltage, give dependable service. Other types (Y5V Z5U) have very large temperature voltage coefficients capacitance, they have only small fraction their nominal capacitance your application. While they will still handle ripple current, input voltage ripple become fairly large, ripple current flowing from your input supply from other bypass capacitors your system, opposed being fully sourced from local input capacitor. alternative high value ceramic capacitor lower value along with larger electrolytic capacitor, example ceramic capacitor parallel with tantalum capacitor. electrolytic capacitor, value larger than will required meet ripple current requirements. Because input capacitor likely high
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1.5H
2.2H
3.3H 4.7H 6.8H
Figure Inductor Values Maximum Load Current (VOUT1 3.3V, IRIPPLE
Input Capacitor Selection Bypass input LT1939 circuit with 4.7F higher ceramic capacitor type. lower value less expensive type used there additional bypassing provided bulk electrolytic tantalum capacitors. following paragraphs describe input capacitor considerations more detail. Step-down regulators draw current from input supply pulses with very fast rise fall times. input
LT1939 APPLICATIONS INFORMATION
surge currents when input source applied, tantalum capacitors should surge rated. manufacturer also recommend operation below rated voltage capacitor. sure place ceramic close possible pins optimal noise immunity. final caution regarding ceramic capacitors input bypassing. ceramic input capacitor combine with stray inductance form resonant tank circuit. power applied quickly (for example, plugging circuit into live power source) this tank ring, doubling input voltage damaging LT1939. solution either clamp input voltage dampen tank circuit adding lossy capacitor parallel with ceramic capacitor. details Application Note Output Capacitor Selection Typically step-down regulators easily compensated with output crossover frequency that 1/10 switching frequency. This means that time that output capacitor must supply output load during transient step switching periods. With allowable drop output voltage during step, good starting value output capacitor expressed VOUT1 Example: VOUT1 3.3V, Frequency 1MHz, Load Step VOUT1 1MHz 0.05 Load Step Frequency 0.05 VOUT1 free ceramic capacitors achieve very output ripple small circuit size. Estimate output ripple with following equations: VRIPPLE Frequency COUT1
ceramic capacitors and, VRIPPLE electrolytic (tantalum aluminum) where peak-to-peak ripple current inductor. content this ripple very low, current rating output capacitor usually concern. Another constraint output capacitor that must have greater energy storage than inductor; stored energy inductor transferred output, would like resulting voltage step small compared regulation voltage. overshoot, this requirement becomes: COUT1 VOUT1
Finally, there must enough capacitance good transient performance. last equation gives good starting point. Alternatively, start with designs this data sheet experiment desired performance. This topic covered more thoroughly section loop compensation. high performance (low ESR), small size robustness ceramic capacitors make them preferred type LT1939 applications. However, ceramic capacitors same. mentioned above, many high value capacitors poor dielectrics with high temperature voltage coefficients. particular, types lose large fraction their capacitance with applied voltage temperature extremes. Because loop stability transient response depend value COUT, able tolerate this loss. types. also electrolytic capacitors.
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calculated value only suggested starting value. Increase value transient response needs improvement reduce capacitance size priority. output capacitor filters inductor current generate output with voltage ripple. also stores energy order satisfy transient loads stabilize LT1939's control loop. switching frequency LT1939 determines value output capacitance required. Also, current mode control loop doesn't require presence output capacitor series resistance (ESR). these reasons,
LT1939 APPLICATIONS INFORMATION
ESRs most aluminum electrolytics large deliver output ripple. Tantalum newer, lower organic electrolytic capacitors intended power supply use, suitable manufacturers will specify ESR. choice capacitor value will based required ripple. Because volume capacitor determines ESR, both size value will larger than ceramic capacitor that would give similar ripple performance. benefit that larger capacitance give better transient response large changes load current. Catch Diode diode conducts current only during switch time. Schottky diode limit forward voltage drop increase efficiency. Schottky diode must have peak reverse voltage that equal regulator input voltage sized average forward current normal operation. Average forward current calculated from: ID(AVG) IOUT1 VOUT1) typical value determined peak switch current limit LT1939. This safe short periods time, would prudent check with diode manufacturer continuous operation under these conditions tolerated. Considerations capacitor diode tied generate voltage that higher than input voltage. most cases 0.47F capacitor fast switching diode (such CMDSH-3 FMMD914) will work well. Almost type film ceramic capacitor suitable, should ensure fully recharged during time switch. capacitor value approximated VOUT1 VBST(MIN)
IOUT1(MAX)
where IOUT1(MAX) maximum load current, VBST(MIN) minimum boost voltage fully saturate switch. Figure shows four ways arrange boost circuit. must more than 2.2V above full efficiency.
LT1939 VOUT1 VBST VBST(MAX) LDRV VOUT1
only reason consider larger diode worstcase condition high input voltage shorted output. With shorted condition, diode current will increase
LT1939 LDRV VBST VOUT1 VBST(MAX) VOUT1
(5a)
(5b)
LT1939
LDRV
VOUT2
LT1939
LDRV VOUT1
VOUT1 VBST VBST(MAX)
VBST VOUT2 VBST(MAX) VOUT2 VOUT2 2.5V
1939
(5c)
(5d)
Figure Considerations
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LT1939 APPLICATIONS INFORMATION
Generally, outputs 3.3V higher standard circuit (Figure best. outputs between 2.8V 3.3V, replace with small Schottky diode such PMEG4005. lower output voltages boost diode tied input (Figure 5b). circuit Figure more efficient because current comes from lower voltage source. Figure shows boost voltage source from linear output that greater than 2.5V (any available sources that greater than 2.5V sufficient). highest efficiency attained choosing lowest boost voltage above 2.5V. must also sure that maximum voltage less than maximum specified Absolute Maximum Ratings section. boost circuit also directly from voltage that higher than input voltage more than 2.5V, Figure diode used prevent damage LT1939 case held while present. circuit eliminates capacitor, efficiency lower dissipation LT1939 higher. Also, absent, LT1939 will still attempt regulate output, will with very efficiency high dissipation because switch will able saturate, dropping 1.5V conduction. minimum input voltage LT1939 application limited minimum operating voltage (<2.8V) maximum duty cycle outlined above. proper start-up, minimum input voltage also limited boost circuit. input voltage ramped slowly, LT1939 turned with when output already regulation, then boost capacitor fully charged. Because boost capacitor charged with energy stored inductor, circuit will rely some minimum load current boost circuit running properly. This minimum load will depend input output voltages arrangement boost circuit. Typical Performance Characteristics section shows plots minimum load current start function input voltage 3.3V outputs. many cases discharged output capacitor will present load switcher which will allow start. plots show worst-case situation where ramping very slowly. Schottky diode lowest start-up voltage. Frequency Compensation LT1939 uses current mode control regulate output. This simplifies loop compensation. particular, LT1939 does require output capacitor stability free ceramic capacitors achieve output ripple small circuit size. Frequency compensation provided components tied pin. Generally capacitor resistor series ground determine loop gain. addition, there lower value capacitor parallel. This capacitor part loop compensation used filter noise switching frequency. Loop compensation determines stability transient performance. Designing compensation network complicated best values depend application particular type output capacitor. practical approach start with circuits this data sheet that similar your application tune compensation network optimize performance. Stability should then checked across operating conditions, including load current, input voltage temperature. LT1375 data sheet contains more thorough discussion loop compensation describes test stability using transient load. Figure shows equivalent circuit LT1939 control loop. error transconductance amplifier with finite output impedance. power section, consisting modulator, power switch, inductor, modeled transconductance amplifier generating output current proportional voltage pin. Note that output capacitor integrates this current, that capacitor (CC) integrates error amplifier output current, resulting poles loop. most cases zero required comes from either output capacitor from resistor series with This simple model works well long value inductor high loop crossover frequency
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LT1939 APPLICATIONS INFORMATION
LT1939 CURRENT MODE POWER STAGE 3mho ERROR 250mhos 0.8V VOUT1
Figure Model Loop Response
much lower than switching frequency. phase lead capacitor (CPL) across feedback divider improve transient response. Synchronization RT/SYNC used synchronize LT1939 external clock source. Driving RT/SYNC resistor with clock source triggers synchronization detection circuitry. Once synchronization detected, rising edge will synchronized rising edge RT/SYNC signal. loop will adjust slope compensation avoid subharmonic oscillation. synchronizing clock signal input LT1939 must have frequency between 250kHz 2.5MHz, duty cycle between 80%, state below 0.5V high state above 1.6V. Synchronization signals outside these parameters will cause erratic switching behavior. RT/SYNC resistor should such that free running frequency ((VRT/SYNC VSYNCLO)/RRT/SYNC) approximately equal synchronization frequency. synchronization signal halted, synchronization detection circuitry will timeout typically which time LT1939 reverts free-running frequency based current through RT/SYNC. RT/SYNC resistor held above 1.6V time, switching will disabled. synchronization signal present during regulator start-up (for example, synchronization circuitry powered from regulator output) RT/SYNC must equivalent resistance ground between
TANTALUM POLYMER
CERAMIC
1939
200k until synchronization circuitry active proper start-up operation. synchronization signal powers undetermined state (VOL, VOH, Hi-Z), connect synchronization clock LT1939 shown Figure circuit shown will isolate synchronization signal when output voltage below regulated output. LT1939 will start-up with switching frequency determined resistor from RT/SYNC ground.
LDRV LT1939 RT/SYNC SYNCHRONIZATION CIRCUITRY
1939
Figure Synchronous Signal Powered from Regulator's Output
synchronization signal powers impedance state (VOL), connect resistor between RT/SYNC synchronizing clock. equivalent resistance seen from RT/SYNC ground will start-up frequency. synchronization signal powers high impedance state (Hi-Z), connect resistor from RT/SYNC ground. equivalent resistance seen from RT/SYNC ground will start-up frequency. synchronization signal changes between high impedance states during power (VOL, Hi-Z), connect
1939f
LT1939 APPLICATIONS INFORMATION
synchronization circuitry LT1939 shown Typical Applications section. This will allow LT1939 start with switching frequency determined equivalent resistance from RT/SYNC ground. Shutdown Undervoltage Lockout Figure shows undervoltage lockout (UVLO) LT1939. Typically, UVLO used situations where input supply current limited, relatively high source resistance. switching regulator draws constant power from source, source current increases source voltage drops. This looks like negative resistance load source cause source current limit latch under source voltage conditions. UVLO prevents regulator from operating source voltages where these problems might occur.
2.5A SHDN 0.76V
0.76 0.76 2.5A
Turn-on threshold Turn-off threshold Example: switching should start until input above 4.75V stop input falls below 3.75V. 4.75V 3.75V 4.75 3.75 499k 0.76 71.5k 4.75 0.76 2.5A 499k Keep connections from resistors SHDN short make sure that interplane surface capacitance switching nodes minimized. high resistor values used, SHDN should bypassed with capacitor prevent coupling problems from switch node. Soft-Start outputs LT1939 regulate either voltage minus 100mV internally regulated 800mV, whichever lowest. capacitor from ground charged internal 2.75A current source resulting linear output ramp from regulated output whose duration given tRAMP 0.9V 2.75A
1939
Figure Undervoltage Lockout
internal comparator will force part into shutdown below minimum 2.8V. This feature used prevent excessive discharge battery-operated systems. adjustable UVLO threshold required, SHDN used. threshold voltage SHDN comparator 0.76V. 2.5A internal current source defaults open-pin condition operating (see Typical Performance Characteristics). Current hysteresis added above SHDN threshold. This used voltage hysteresis UVLO using following:
power-up, reset signal sets soft-start latch discharges approximately ensure proper start-up. When fully discharged latch reset internal 2.75A current source starts charge pin.
1939f
LT1939 APPLICATIONS INFORMATION
When voltage below 100mV, pulled which disables switching. voltage rises above 100mV, released outputs regulated voltage. When voltage minus 100mV exceeds internal 0.8V reference, outputs regulated reference. voltage will continue rise until clamped event undervoltage lockout, SHDN driven below 0.8V, internal temperature exceeding maximum rating during normal operation, soft-start latch set, triggering start-up sequence. addition, load exceeds maximum output switch current (switching regulator only), output will start drop causing clamp activated. long clamped, will discharged. result, output will regulated highest voltage that maximum output current support. example, output loaded will drop 0.5V, regulating output (typical current limit time load, Once overload condition removed, output will soft-start from temporary voltage level normal regulation point. Since clamped discharge 0.9V before taking control regulation, momentary overload conditions will tolerated without soft-start recovery. typical time before takes control SS(CONTROL) 1.1V 600A sink capability 400A when pins below threshold withstand when outputs regulation. typically connected output with resistor used error flag. resistor value should chosen allow voltage drop below 0.4V error condition. Example: VOUT1 PGSINK(MIN) 200A 0.4)/200A sink capability 800A when pins above threshold withstand when outputs regulation. typically used drive signal output disconnect device. pull-up resistor should sized same manner pull-up resistor. Linear Regulator LT1939 contains error amplifier output device which configured linear regulator linear regulator controller. With LDRV pins configured shown Figure LDRV outputs regulated voltage with typical current limit 13mA. LDRV voltage programmed with resistor divider between output pin. Choose resistors according VLDRV 0.8V
Power Good Indicators pins collector outputs internal comparator. comparator compares voltages pins reference voltage with 30mV hysterisis.
should 10.0k less avoid bias current errors. Reference designators refer Block Diagram Figure reference voltage linear regulator (LFB pin) will track same manner switching regulator.
1939f
LT1939 APPLICATIONS INFORMATION
VOUT COUPLED 20mV/DIV
LOAD STEP 2.5mA 7.5mA 5mA/DIV 20s/DIV
1939
increases overall efficiency system. However, minimum increases plus full load transistor. Additionally, lack beta current limiting, shorted output cause switcher output LT1939 collapse. Since collector LDRV connected internally VIN, must consider impact LDRV current efficiency temperature when configuring linear regulator/controller. example, with 25V, LDRV 3.3V ILDRV 10mA, power dissipation will 217mW. typical 3.3V/1A switcher application, this represents additional efficiency loss approximately degrees rise temperature. linear output LT1939 used, LDRV should shorted Pin. Layout proper operation minimum EMI, care must taken during printed circuit board (PCB) layout. Figure shows high di/dt paths buck regulator circuit. Note that large switched currents flow power switch, catch diode input capacitor. loop formed these components should small possible. These components, along with inductor output capacitor, should placed same side circuit board their connections should made that layer. Place local, unbroken ground plane below these components, this ground plane system ground
Figure Linear Regulator Transient Response
compensate linear regulator, simply ceramic capacitor from LDRV ground. Typical values range from 0.01F Figure illustrates transient response with 0.47F output capacitor. Linear Controller adding external follower (NPN NMOS), LDRV pins configured controller (Figure dropout regulator with increased output capability. output current capability Figure 10's circuit product LDRV current limit beta external which normally less than current capability LT1939. dropout voltage circuit saturation voltage external NPN, which typically 300mV. minimum circuit function properly plus base emitter drop external NPN. Replacing Figure with NMOS transistor reduce dropout voltage down RDS(ON) NMOS times output current regulator. This also
BAT54 4.5V 2.2F LT1939 SHDN RT/SYNC LDRV 24.9k 8.06k
1939
0.47F 3.3H B240A 27.4k
VOUT1 3.5V
0.47F
8.06k
220pF 40.2k
49.9k
VOUT2 3.3V
Figure Linear Controller
1939f
LT1939 APPLICATIONS INFORMATION
LT1939 LT1939 LT1939
1939
(11a)
(11b)
(11c)
Figure Subtracting Current when Switch (11a) from Current when Switch (11b) Reveals Path High Frequency Switching Current (11c). Keep this Loop Small. Voltage Traces will Also Switched; Keep These Traces Short Possible. Finally, Make Sure Circuit Shielded with Local Ground Plane
package must soldered ground plane. This ground should tied other copper layers below with thermal vias; these layers will spread heat dissipated LT1939. Place additional vias near catch diodes. Adding more copper bottom layers tying this copper internal planes with vias further reduce thermal resistance. With these steps, thermal resistance from junction) ambient reduced 45°C/W. Power dissipation within LT1939 estimated calculating total power loss from efficiency measurement subtracting catch diode loss. temperature calculated multiplying LT1939 power dissipation thermal resistance from junction ambient. power dissipation other power components such catch diodes, boost diodes inductors, cause additional copper heating further increase what sees ambient temperature. LT1767 data sheet's Thermal Considerations section. Other Linear Technology Publications Application notes AN19, AN35 AN44 contain more detailed descriptions design information buck regulators other switching regulators. LT1376 data sheet more extensive discussion output ripple, loop compensation stability testing. Design note DN100 shows generate dual output supply using buck regulator.
Figure LT1939 Demonstration Circuit Board DC1293A
location, ideally ground terminal output capacitor Additionally, traces should kept short possible. topside metal from DC1069A demonstration board Figure illustrates proper component placement trace routing. Thermal Considerations must also provide heat sinking keep LT1939 cool. exposed metal bottom
1939f
LT1939 TYPICAL APPLICATIONS
High Efficiency Linear Regulator
BAT54 4.5V 2.2F LT1939 SHDN RT/SYNC LDRV 220pF 40.2k 49.9k 24.9k 8.06k
1939 TA02a
Efficiency Load Current
B240A
25.5k
0.47F
8.06k
EFFICIENCY ZXMN2A03E6 VOUT1
0.47F 3.3H
LOAD CURRENT
1939 TA02b
5V/1.5A, 3.3V/0.5A Step-Down with Output Disconnect
BAT54 2.2F LT1939 SHDN RT/SYNC LDRV 220pF 40.2k 49.9k 8.06k 24.9k
1939 TA03
0.47F 4.7H B240A 42.2k 100k
VOUT1 1.5A
0.47F
8.06k
ZXTCM322
VOUT2 3.3V 0.5A ZXMP3A17E6
5V/2A Step-Down with Power Good
BAT54 2.2F LT1939 SHDN RT/SYNC LDRV 49.9k B240A 42.2k 8.06k 8.06k 42.2k 8.06k 100k ZXM61N02F
1939 TA04
0.47F 4.7H
VOUT1
0.47F
220pF 40.2k
1939f
LT1939 PACKAGE DESCRIPTION
Package 12-Lead Plastic (3mm 3mm)
(Reference 05-08-1725
0.70 ±0.05
3.50 ±0.05 2.10 ±0.05
2.38 ±0.05 1.65 ±0.05 PACKAGE OUTLINE
0.25 0.05 0.45 2.25 RECOMMENDED SOLDER PITCH DIMENSIONS APPLY SOLDER MASK AREAS THAT SOLDERED
0.115
0.40 0.10
3.00 ±0.10 SIDES) MARK (SEE NOTE
2.38 ±0.10 1.65 0.10 NOTCH 0.20 0.25 CHAMFER
0.200 0.75 ±0.05 2.25 0.00 0.05
0.23 0.05 0.45
(DD12) 0106
BOTTOM VIEW-EXPOSED NOTE: DRAWING JEDEC PACKAGE OUTLINE DRAWING SCALE DIMENSIONS MILLIMETERS DIMENSIONS EXPOSED BOTTOM PACKAGE INCLUDE MOLD FLASH. MOLD FLASH, PRESENT, SHALL EXCEED 0.15mm SIDE EXPOSED BARS SHALL SOLDER PLATED SHADED AREA ONLY REFERENCE LOCATION BOTTOM PACKAGE
1939f
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.
LT1939 TYPICAL APPLICATION
1.8V/2A Step-Down Regulator
4.5V 2.2F LDRV LT1939 0.47F 40.2k 220pF SHDN 0.47F RT/SYNC 2.2H 8.06k VOUT1 1.8V
1939 TA05
24.9k 8.06k
VOUT2 3.3V 10mA
49.9k
RELATED PARTS
PART NUMBER LT1766 LT1933 LT1936 LT1940 LTC3407/LTC3407-2 LT3434/LT3435 LT3437 LT3493 LT3501 LT3502/LT3502A LT3503 LT3505 LT3506/LT3506A LT3508 LT3510 LTC3548 LT3680 LT3500 DESCRIPTION 60V, 1.2A (IOUT), 200kHz High Efficiency Step-Down DC/DC Converter 36V, 500mA (IOUT), 500kHz Step-Down Switching Regulator SOT-23 36V, 1.4A (IOUT), 500kHz High Efficiency Step-Down DC/DC Converter Dual 25V, 1.4A (IOUT), 1.1MHz High Efficiency Step-Down DC/DC Converter Dual 600mA/800mA, 1.5MHz/2.25MHz Synchronous Step-Down DC/DC Converter 60V, 2.4A (IOUT), 200kHz/500kHz High Efficiency Step-Down DC/DC Converters with Burst Mode Operation 60V, 400mA (IOUT), Micropower Step-Down DC/DC Converter with Burst Mode Operation 36V, 1.4A (IOUT), 750kHz High Efficiency Step-Down DC/DC Converter Dual 25V, (IOUT), 1.5MHz High Efficiency Step-Down DC/DC Converter 40V, 500mA (IOUT), 1.1MHz/2.2MHz High Efficiency Step-Down DC/DC Converter COMMENTS VIN: 5.5V 60V, VOUT(MIN) 1.20V, 2.5mA, 25A, 16-Lead TSSOPE Package VIN: 3.6V 36V, VOUT(MIN) 1.2V, 1.6mA, ThinSOTPackage VIN: 3.6V 36V, VOUT(MIN) 1.2V, 1.9mA, 8-Lead MS8E Package VIN: 3.6V 25V, VOUT(MIN) 1.20V, 3.8mA, 30A, 16-Lead TSSOPE Package VIN: 2.5V 5.5V, VOUT(MIN) 0.6V, 40A, 10-Lead MS10E Packages VIN: 3.3V 60V, VOUT(MIN) 1.20V, 100A, 16-Lead TSSOPE Package VIN: 3.3V 60V, VOUT(MIN) 1.25V, 100A, 10-Lead DFN, 16-Lead TSSOPE Package VIN: 3.6V 36V, VOUT(MIN) 0.8V, 1.9mA, 6-Lead Package VIN: 3.3V 25V, VOUT(MIN) 0.8V, 3.7mA, 10A, 20-Lead TSSOPE Package VIN: 40V, VOUT(MIN) 0.8V, 1.5mA, 8-Lead Package
20V, (IOUT), 2.2MHz High Efficiency Step-Down DC/DC Converter VIN: 3.6V 20V, VOUT(MIN) 0.78V, 1.9mA, 6-Lead Package 36V, 1.2A (IOUT), 3MHz High Efficiency Step-Down DC/DC Converter VIN: 3.6V 36V, VOUT(MIN) 0.78V, 2mA, 8-Lead Packages Dual 25V, 1.6A (IOUT), 575kHz/1.1MHz High Efficiency Step-Down DC/DC Converters Dual 36V, 1.4A (IOUT), 2.5MHz High Efficiency Step-Down DC/DC Converter Dual 25V, (IOUT), 1.5MHz High Efficiency Step-Down DC/DC Converter Dual 400mA/800mA, 2.25MHz Synchronous Step-Down DC/DC Converter 36V, 3.5A (IOUT), 2.4MHz High Efficiency Step-Down DC/DC Converter VIN: 3.6V 25V, VOUT(MIN) 0.8V, 3.8mA, 30A, 16-Lead TSSOPE Packages VIN: 3.6V 36V, VOUT(MIN) 0.8V, 4.3mA, 24-Lead 16-Lead TSSOPE Packages VIN: 3.3V 25V, VOUT(MIN) 0.8V, 3.7mA, 10A, 20-Lead TSSOPE Package VIN: 2.5V 5.5V, VOUT(MIN) 0.6V, 40A, 10-Lead Packages VIN: 3.6V 36V, VOUT(MIN) 0.79V, 75A, MS10E Packages
40V, (IOUT), 2.2MHz High Efficiency Step-Down DC/DC Converter VIN: 36V, VOUT(MIN) 0.8V, 75A, 12A,
1939f 0108 PRINTED
Linear Technology Corporation
(408) 432-1900
1630 McCarthy Blvd., Milpitas, 95035-7417
FAX: (408) 434-0507 www.linear.com
LINEAR TECHNOLOGY CORPORATION 2008
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