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| FeaTures n n n n n LTC3857-1 Low IQ, Dual, 2-Phase Synchronous Step-Down Controller DescripTion TheLTC(R)3857-1isahighperformancedualstep-down switching regulator controller that drives all N-channel synchronouspowerMOSFETstages.Aconstantfrequency current mode architecture allows a phase-lockable frequencyofupto850kHz.Powerlossandnoiseduetothe ESRoftheinputcapacitorESRareminimizedbyoperating thetwocontrolleroutputstagesoutofphase. The50Ano-loadquiescentcurrentextendsoperatinglife inbattery-poweredsystems.TheLTC3857-1featuresaprecision0.8Vreferenceandapowergoodoutputindicator.A wide4Vto38Vinputsupplyrangeencompassesawiderange ofintermediatebusvoltagesandbatterychemistries. IndependentTRACK/SSpinsforeachcontrollerrampthe outputvoltagesduringstart-up.Currentfoldbacklimits MOSFETheatdissipationduringshort-circuitconditions. ThePLLIN/MODEpinselectsamongBurstModeoperation,pulse-skippingmode,orcontinuousinductorcurrent modeatlightloads. Foraleadless32-pinQFNpackagewiththeadditionalfeaturesofadjustablecurrentlimit,clockout,phasemodulationandtwoPGOODoutputs,seetheLTC3857datasheet. L,LT,LTC,LTM,BurstMode,OPTI-LOOP ,Module,LinearTechnologyandtheLinearlogo areregisteredtrademarksandNoRSENSEandUltraFastaretrademarksofLinearTechnology Corporation.Allothertrademarksarethepropertyoftheirrespectiveowners.Protectedby U.S.Patents,including5481178,5929620,6177787,6144194,5408150,6580258,5705919, 6100678. n n n n n n n n n n n n Low Operating IQ: 50A (One Channel On) Wide Output Voltage Range: 0.8V VOUT 24V Wide VIN Range: 4V to 38V RSENSE or DCR Current Sensing Out-of-PhaseControllersReduceRequiredInput CapacitanceandPowerSupplyInducedNoise OPTI-LOOP(R)CompensationMinimizesCOUT Phase-LockableFrequency(75kHz-850kHz) ProgrammableFixedFrequency(50kHz-900kHz) SelectableContinuous,Pulse-SkippingorLowRipple BurstMode(R)OperationatLightLoads VeryLowDropoutOperation:99%DutyCycle AdjustableOutputVoltageSoft-StartorTracking PowerGoodOutputVoltageMonitor OutputOvervoltageProtection LowShutdownIQ:<8A InternalLDOPowersGateDrivefromVINorEXTVCC NoCurrentFoldbackDuringStart-Up NarrowSSOPPackage applicaTions AutomotiveAlways-OnSystems BatteryOperatedDigitalDevices n DistributedDCPowerSystems n n Typical applicaTion High Efficiency Dual 3.3V/8.5V Step-Down Converter 1F TG1 3.3H 0.1F VIN INTVCC TG2 22F 50V VIN 9V TO 38V 100 Efficiency and Power Loss vs Output Current VIN = 12V 90 VOUT = 3.3V FIGURE 13 CIRCUIT 80 70 60 50 40 30 20 10 0 0.00001 0.0001 0.001 0.01 0.1 OUTPUT CURRENT (A) 1 10 3857 TA01b 10000 BOOST1 SW1 BG1 LTC3857-1 BOOST2 SW2 BG2 PGND SENSE2+ 0.1F 7.2H EFFICIENCY (%) 1000 POWER LOSS (mW) 100 10 SENSE1+ 0.007 VOUT1 3.3V 5A SENSE1- VFB1 ITH1 0.1F 0.010 SENSE2- VFB2 ITH2 0.1F VOUT2 8.5V 3.5A 150F 1 62.5k 150F 20k 193k 680pF 15k 20k 680pF 15k TRACK/SS1 SGND TRACK/SS2 0.1 38571 TA01 38571fa LTC3857-1 absoluTe MaxiMuM raTings (Note 1) pin conFiguraTion TOP VIEW ITH1 VFB1 SENSE1+ SENSE1- FREQ PLLIN/MODE SGND RUN1 RUN2 SENSE2+ 1 2 3 4 5 6 7 8 9 11 28 TRACK/SS1 27 PGOOD1 26 TG1 25 SW1 24 BOOST1 23 BG1 22 VIN 21 PGND 20 EXTVCC 19 INTVCC 18 BG2 17 BOOST2 16 SW2 15 TG2 InputSupplyVoltage(VIN)......................... -0.3Vto40V TopsideDriverVoltages BOOST1,BOOST2................................ -0.3Vto46V . SwitchVoltage(SW1,SW2)........................ -5Vto40V . (BOOST1-SW1),(BOOST2-SW2),INTVCC.. -0.3Vto6V RUN1,RUN2................................................ -0.3Vto8V MaximumCurrentSourcedintoPin fromSource>8V...............................................100A SENSE1+,SENSE2+,SENSE1- SENSE2-Voltages..................................... -0.3Vto28V . PLLIN/MODE,FREQVoltages.............. -0.3VtoINTVCC EXTVCC ..................................................... -0.3Vto14V . ITH1,ITH2,VFB1,VFB2Voltages...................... -0.3Vto6V PGOOD1Voltage......................................... -0.3Vto6V TRACK/SS1,TRACK/SS2Voltages.............. -0.3Vto6V OperatingJunctionTemperatureRange (Note2)................................................. -40Cto125C . MaximumJunctionTemperature(Note3)............ 125C StorageTemperatureRange.................. -65Cto150C . SENSE2- 10 VFB2 12 ITH2 13 TRACK/SS2 14 GN PACKAGE 28-LEAD PLASTIC SSOP TJMAX=125C,JA=90C/W orDer inForMaTion LEAD FREE FINISH LTC3857EGN-1#PBF LTC3857IGN-1#PBF TAPE AND REEL LTC3857EGN-1#TRPBF LTC3857IGN-1#TRPBF PART MARKING* LTC3857GN-1 LTC3857GN-1 PACKAGE DESCRIPTION 28-LeadPlasticSSOP 28-LeadPlasticSSOP TEMPERATURE RANGE -40Cto125C -40Cto125C ConsultLTCMarketingforpartsspecifiedwithwideroperatingtemperatureranges.*Thetemperaturegradeisidentifiedbyalabelontheshippingcontainer. ConsultLTCMarketingforinformationonnon-standardleadbasedfinishparts. Formoreinformationonleadfreepartmarking,goto:http://www.linear.com/leadfree/ Formoreinformationontapeandreelspecifications,goto:http://www.linear.com/tapeandreel/ 38571fa LTC3857-1 elecTrical characTerisTics SYMBOL VIN VFB1,2 IFB1,2 VREFLNREG VLOADREG PARAMETER InputSupplyOperatingVoltageRange RegulatedFeedbackVoltage (Note4)ITH1,2Voltage=1.2V -40Cto125C -40Cto85C (Note4) (Note4)VIN=4.5Vto38V (Note4) MeasuredinServoLoop, ITHVoltage=1.2Vto0.7V (Note4) MeasuredinServoLoop, ITHVoltage=1.2Vto2V gm1,2 IQ TransconductanceAmplifiergm InputDCSupplyCurrent Pulse-SkippingorForcedContinuous Mode(OneChannelOn) Pulse-SkippingorForcedContinuous Mode(BothChannelsOn) SleepMode(OneChannelOn) (Note4)ITH1,2=1.2V,Sink/Source=5A (Note5) RUN1=5VandRUN2=0Vor RUN1=0VandRUN2=5V, VFB1=0.83V(NoLoad) RUN1,2=5V,VFB1,2=0.83V(NoLoad) RUN1=5VandRUN2=0Vor RUN1=0VandRUN2=5V, VFB1=0.83V(NoLoad) RUN1,2=5V,VFB1,2=0.83V(NoLoad) RUN1,2=0V INTVCCRampingUp INTVCCRampingDown MeasuredatVFB1,2,RelativetoRegulatedVFB1,2 EachChannel EachChannel VSENSE- l l l l l The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted. CONDITIONS MIN 4 0.788 0.792 0.800 0.800 5 0.002 0.01 -0.01 2 2 TYP MAX 38 0.812 0.808 50 0.02 0.1 -0.1 UNITS V V V nA %/V % % mmho mA FeedbackCurrent ReferenceVoltageLineRegulation OutputVoltageLoadRegulation l 2 50 75 mA A SleepMode(BothChannelsOn) Shutdown UVLO VOVL ISENSE+ ISENSE- DFMAX ITRACK/SS1,2 VRUN1,2On VSENSE(MAX) Gate Driver TG1,2 BG1,2 TG1,2tr TG1,2tf BG1,2tr BG1,2tf Pull-UpOn-Resistance Pull-DownOn-Resistance Pull-UpOn-Resistance Pull-DownOn-Resistance TGTransistionTime: RiseTime FallTime BGTransistionTime: RiseTime FallTime UndervoltageLockout FeedbackOvervoltageProtection SENSE+PinCurrent SENSE-PinsCurrent 65 8 3.6 7 4.0 3.8 10 550 98 0.7 1.21 43 99.4 1.0 1.26 50 l 120 20 4.2 4 13 1 1 700 1.4 1.31 57 A A V V % A A A % A V mV mV ns ns ns ns 38571fa MaximumDutyFactor Soft-StartChargeCurrent RUNPinOnThreshold MaximumCurrentSenseThreshold VRUN1,2Hyst RUNPinHysteresis 50 2.5 1.5 2.4 1.1 (Note6) CLOAD=3300pF CLOAD=3300pF (Note6) CLOAD=3300pF CLOAD=3300pF 25 16 28 13 LTC3857-1 elecTrical characTerisTics SYMBOL TG/BGt1D BG/TGt1D tON(MIN) VINTVCCVIN VLDOVIN VINTVCCEXT VLDOEXT VEXTVCC VLDOHYS f25k f65k f105k fLOW fHIGH fSYNC VPGL IPGOOD VPG PARAMETER TopGateOfftoBottomGateOnDelay SynchronousSwitch-OnDelayTime BottomGateOfftoTopGateOnDelay TopSwitch-OnDelayTime MinimumOn-Time InternalVCCVoltage INTVCCLoadRegulation InternalVCCVoltage INTVCCLoadRegulation EXTVCCSwitchoverVoltage EXTVCCHysteresis ProgrammableFrequency ProgrammableFrequency ProgrammableFrequency LowFixedFrequency HighFixedFrequency SynchronizableFrequency PGOOD1VoltageLow PGOOD1LeakageCurrent PGOOD1TripLevel RFREQ=25k,PLLIN/MODE=DCVoltage RFREQ=65k,PLLIN/MODE=DCVoltage RFREQ=105k,PLLIN/MODE=DCVoltage VFREQ=0V,PLLIN/MODE=DCVoltage VFREQ=INTVCC,PLLIN/MODE=DCVoltage PLLIN/MODE=ExternalClock IPGOOD=2mA VPGOOD=5V VFBwithRespecttoSetRegulatedVoltage VFBRampingNegative Hysteresis VFBwithRespecttoSetRegulatedVoltage VFBRampingPositive Hysteresis tPG DelayforReportingaFault Note 1:StressesbeyondthoselistedunderAbsoluteMaximumRatings maycausepermanentdamagetothedevice.ExposuretoanyAbsolute MaximumRatingsforextendedperiodsmayaffectdevicereliabilityand lifetime. Note 2:TheLTC3857E-1isguaranteedtomeetperformancespecifications from0Cto85C.Specificationsoverthe-40Cto125Coperating junctiontemperaturerangeareassuredbydesign,characterizationand correlationwithstatisticalprocesscontrols.TheLTC3857I-1isguaranteed overthefull-40Cto125Coperatingjunctiontemperaturerange. Note 3:TJiscalculatedfromtheambienttemperatureTAandpower dissipationPDaccordingtothefollowingformula: TJ=TA+(PD*90C/W) -13 7 -10 2.5 10 2.5 25 l The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted. CONDITIONS CLOAD=3300pFEachDriver CLOAD=3300pFEachDriver (Note7) 6V Oscillator and Phase-Locked Loop PGOOD1 Output Note 4:TheLTC3857-1istestedinafeedbackloopthatservosVITH1,2to aspecifiedvoltageandmeasurestheresultantVFB1,2.Thespecificationat 85Cisnottestedinproduction.Thisspecificationisassuredbydesign, characterizationandcorrelationtoproductiontestingat125C. Note 5:Dynamicsupplycurrentishigherduetothegatechargebeing deliveredattheswitchingfrequency.SeeApplicationsinformation. Note 6:Riseandfalltimesaremeasuredusing10%and90%levels.Delay timesaremeasuredusing50%levels. Note 7:Theminimumon-timeconditionisspecifiedforaninductor peak-to-peakripplecurrent40%ofIMAX(SeeMinimumOn-Time ConsiderationsintheApplicationsInformationsection). 38571fa LTC3857-1 Typical perForMance characTerisTics Efficiency and Power Loss vs Output Current 100 VIN = 12V 90 VOUT = 3.3V FIGURE 13 CIRCUIT 80 70 60 50 40 30 20 10 0 0.1 0.00001 0.0001 0.001 0.01 0.1 1 10 38571 G01 OUTPUT CURRENT (A) BURST EFFICIENCY BURST LOSS PULSE-SKIPPING PULSE-SKIPPING EFFICIENCY LOSS CCM EFFICIENCY CCM LOSS 1 10 100 10000 100 90 1000 POWER LOSS (mW) EFFICIENCY (%) 80 70 60 50 40 30 20 10 VOUT = 3.3V FIGURE 13 CIRCUIT 1 10 38571 G02 Efficiency vs Output Current 98 VIN = 5V VIN = 12V 96 94 EFFICIENCY (%) 92 90 88 86 84 82 80 Efficiency vs Input Voltage VOUT = 3.3V ILOAD = 5A EFFICIENCY (%) 0 0.1 0.00001 0.0001 0.001 0.01 OUTPUT CURRENT (A) 1 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40 38571 G03 Load Step (Burst Mode Operation) Load Step (Forced Continuous Mode) Load Step (Pulse-Skipping Mode) VOUT 100mV/DIV VOUT 100mV/DIV VOUT 100mV/DIV INDUCTOR CURRENT 2A/DIV INDUCTOR CURRENT 2A/DIV INDUCTOR CURRENT 2A/DIV VIN = 12V 20s/DIV VOUT = 3.3V FIGURE 13 CIRCUIT 38571 G04 20s/DIV VIN = 12V VOUT = 3.3V FIGURE 13 CIRCUIT 38571 G05 20s/DIV VIN = 12V VOUT = 3.3V FIGURE 13 CIRCUIT 38571 G06 Inductor Current at Light Load Soft Start-Up Tracking Start-Up FORCED CONTINUOUS MODE Burst Mode OPERATION 2A/DIV VOUT2 2V/DIV VOUT2 2V/DIV VOUT1 2V/DIV VOUT1 2V/DIV PULSESKIPPING MODE 5s/DIV VIN = 12V VOUT = 3.3V ILOAD = 200A FIGURE 13 CIRCUIT 38571 G07 20ms/DIV FIGURE 13 CIRCUIT 38571 G08 20ms/DIV FIGURE 13 CIRCUIT 38571 G09 38571fa LTC3857-1 Typical perForMance characTerisTics Total Input Supply Current vs Input Voltage 500 450 400 SUPPLY CURRENT (A) 350 300 250 200 150 100 50 0 5 10 500A 300A EXTVCC AND INTVCC VOLTAGE (V) VOUT1 = 3.3V RUN2 = 0V FIGURE 13 CIRCUIT 6.0 5.8 5.6 INTVCC EXTVCC RISING EXTVCC FALLING INTVCC VOLTAGE (v) 130 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 -45 -20 55 30 5 80 TEMPERATURE (C) 105 4.8 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40 5.1 EXTVCC Switchover and INTVCC Voltages vs Temperature 5.2 INTVCC Line Regulation 5.0 4.9 NO LOAD 25 20 15 30 INPUT VOLTAGE (V) 35 40 38571 G10 38571 G11 38571 G12 -50 -100 SENSE- CURRENT (A) -150 -200 -250 -300 -350 -400 -450 -500 -550 -600 60 PULSE-SKIPPING MODE 40 20 0 FORCED CONTINUOUS MODE -20 -40 Burst Mode OPERATION MAXIMUM CURRENT SENSE VOLTAGE (mV) 80 CURRENT SENSE THESHOLD (mV) Maximum Current Sense Voltage vs ITH Voltage 5% DUTY CYCLE SENSE- Pin Input Bias Current 0 80 Maximum Current Sense Threshold vs Duty Cycle 60 40 20 0 0.2 0.4 0.6 0.8 VITH (V) 1.0 1.2 1.4 0 10 15 20 25 VSENSE COMMON MODE VOLTAGE (V) 5 38571 G14 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 38571 G15 38571 G13 Foldback Current Limit MAXIMUM CURRENT SENSE VOLTAGE (mV) 90 80 QUIESCENT CURRENT (A) 70 60 50 40 30 20 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 FEEDBACK VOLTAGE (V) 38571 G16 Quiescent Current vs Temperature 80 75 70 65 60 55 50 45 40 -45 -20 5 55 80 30 TEMPERATURE (C) 105 130 4.95 INVCC VOLTAGE (V) 5.15 5.10 5.20 INTVCC and EXTVCC vs Load Current VIN = 12V EXTVCC = 0V 5.05 5.00 EXTVCC = 8.5V 0 20 40 60 80 100 120 140 160 180 200 LOAD CURRENT (mA) 38571 G18 38571 G17 38571fa LTC3857-1 Typical perForMance characTerisTics TRACK/SS Pull-Up Current vs Temperature 1.10 1.40 REGULATED FEEDBACK VOLTAGE (mV) 1.35 TRACK/SS CURRENT (A) RUN PIN VOLTAGE (V) 1.05 1.30 1.25 1.20 1.15 0.90 -45 1.10 -45 -20 RUN FALLING RUN RISING Shutdown (RUN) Threshold vs Temperature 800 806 804 802 800 798 796 794 Regulated Feedback Voltage vs Temperature 1.00 0.95 -20 80 5 30 55 TEMPERATURE (C) 105 130 55 30 80 5 TEMPERATURE (C) 105 130 792 -45 -20 5 55 80 30 TEMPERATURE (C) 105 130 38571 G19 38571 G20 3857 G21 SENSE- Pin Input Current vs Temperature 50 0 -50 -100 -150 -200 -250 -300 -350 -400 -450 -500 -550 -600 -45 30 VOUT < INTVCC - 0.5V INPUT CURRENT (A) 25 Shutdown Current vs Input Voltage 600 550 FREQUENCY (kHz) 500 450 400 350 Oscillator Frequency vs Temperature FREQ = INTVCC SENSE - CURRENT (A) 20 15 10 5 0 FREQ = GND VOUT > INTVCC - 0.5V -20 80 55 5 30 TEMPERATURE (C) 105 130 5 10 25 20 30 15 INPUT VOLTAGE (V) 35 40 300 -45 -20 55 30 80 5 TEMPERATURE (C) 105 130 38571 G22 38571 G23 38571 G24 Undervoltage Lockout Threshold vs Temperature 4.4 4.3 OSCILATOR FREQUENCY (kHz) 4.2 INTVCC VOLTAGE (V) 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 -45 -20 55 30 5 80 TEMPERATURE (C) 105 130 354 356 Oscillator Frequency vs Input Voltage FREQ = GND SHUTDOWN CURRENT (A) 20 18 16 14 12 10 8 6 5 10 25 20 30 15 INPUT VOLTAGE (V) 35 40 Shutdown Current vs Temperature 352 350 348 346 344 4 -45 -20 5 55 80 30 TEMPERATURE (C) 105 130 38571 G25 38571 G26 38571 G27 38571fa LTC3857-1 pin FuncTions ITH1, ITH2 (Pin 1, Pin 13):ErrorAmplifierOutputsand SwitchingRegulatorCompensationPoints.Eachassociated channel's current comparator trip point increases withthiscontrolvoltage. VFB1, VFB2 (Pin 2, Pin 12):Receivestheremotelysensed feedback voltage for each controller from an external resistivedivideracrosstheoutput. SENSE1+, SENSE2+ (Pin 3, Pin 11):The(+)inputtothe differentialcurrentcomparatorsarenormallyconnected toDCRsensingnetworksorcurrentsensingresistors. The ITH pin voltage and controlled offsets between the SENSE-andSENSE+pinsinconjunctionwithRSENSEset thecurrenttripthreshold. SENSE1-, SENSE2- (Pin 4, Pin 10): The (-) Input to theDifferentialCurrentComparators.Whengreaterthan INTVCC - 0.5V, the SENSE- pin supplies current to the currentcomparator. FREQ (Pin 5):TheFrequencyControlPinfortheInternal VCO.ConnectingthepintoGNDforcestheVCOtoafixed lowfrequencyof350kHz.ConnectingthepintoINTVCC forces the VCO to a fixed high frequency of 535kHz. Other frequencies between 50kHz and 900kHz can be programmedusingaresistorbetweenFREQandGND. Aninternal20Apull-upcurrentdevelopsthevoltageto beusedbytheVCOtocontrolthefrequency PLLIN/MODE (Pin 6):ExternalSynchronizationInputto PhaseDetectorandForcedContinuousModeInput.When anexternalclockisappliedtothispin,thephase-locked loopwillforcetherisingTG1signaltobesynchronized withtherisingedgeoftheexternalclock.Whennotsynchronizingtoanexternalclock,thisinput,whichactson bothcontrollers,determineshowtheLTC3857-1operates at light loads. Pulling this pin to ground selects Burst Modeoperation.Aninternal100kresistortogroundalso invokes Burst Mode operation when the pin is floated. TyingthispintoINTVCCforcescontinuousinductorcurrent operation.Tyingthispintoavoltagegreaterthan1.2Vand lessthanINTVCC-1.3Vselectspulse-skippingoperation. Thiscanbedonebyaddinga100kresistorbetweenthe PLLIN/MODEpinandINTVCC. SGND (Pin 7): Small-signal ground common to both controllers, must be routed separately from high current grounds to the common (-) terminals of the CIN capacitors. RUN1, RUN2 (Pin 8, Pin 9):DigitalRunControlInputs for Each Controller. Forcing either of these pins below 1.26Vshutsdownthatcontroller.Forcingbothofthese pinsbelow0.7VshutsdowntheentireLTC3857-1,reducingquiescentcurrenttoapproximately8A.Donotfloat thesepins. INTVCC (Pin 19):OutputoftheInternalLinearLowDropout Regulator. The driver and control circuits are powered fromthisvoltagesource.Mustbedecoupledtopower groundwithaminimumof4.7Fceramicorotherlow ESRcapacitor.DonotusetheINTVCCpinforanyother purpose. EXTVCC (Pin 20):ExternalPowerInputtoanInternalLDO ConnectedtoINTVCC.ThisLDOsuppliesINTVCCpower, bypassingtheinternalLDOpoweredfromVINwhenever EXTVCCishigherthan4.7V.SeeEXTVCCConnectionin theApplicationsInformationsection.Donotexceed14V onthispin. PGND (Pin 21):DriverPowerGround.Connectstothe sources of bottom (synchronous) N-channel MOSFETs andthe(-)terminal(s)ofCIN. VIN (Pin 22):MainSupplyPin.Abypasscapacitorshould betiedbetweenthispinandthesignalgroundpin. BG1, BG2 (Pin 23, Pin 18): High Current Gate Drives forBottom(Synchronous)N-ChannelMOSFETs.Voltage swingatthesepinsisfromgroundtoINTVCC. 38571fa LTC3857-1 pin FuncTions BOOST1, BOOST2 (Pin 24, Pin 17):BootstrappedSupplies totheTopsideFloatingDrivers.Capacitorsareconnected betweentheBOOSTandSWpinsandSchottkydiodesare tiedbetweentheBOOSTandINTVCCpins.Voltageswing attheBOOSTpinsisfromINTVCCto(VIN+INTVCC). SW1, SW2 (Pin 25, Pin 16):SwitchNodeConnections toInductors. TG1, TG2 (Pin 26, Pin 15):HighCurrentGateDrivesfor TopN-ChannelMOSFETs.ThesearetheoutputsoffloatingdriverswithavoltageswingequaltoINTVCC-0.5V superimposedontheswitchnodevoltageSW. PGOOD1 (Pin 27):Open-DrainLogicOutput.PGOOD1is pulledtogroundwhenthevoltageontheVFB1pinisnot within10%ofitssetpoint. TRACK/SS1, TRACK/SS2 (Pin 28, Pin 14): ExternalTracking and Soft-Start Input. The LTC3857-1 regulates the VFB1,2voltagetothesmallerof0.8Vorthevoltageonthe TRACK/SS1,2pin.Aninternal1Apull-upcurrentsource is connected to this pin. A capacitor to ground at this pinsetstheramptimetofinalregulatedoutputvoltage. Alternatively,aresistordivideronanothervoltagesupply connectedtothispinallowstheLTC3857-1outputtotrack theothersupplyduringstart-up. 38571fa LTC3857-1 FuncTional DiagraM INTVCC DUPLICATE FOR SECOND CONTROLLER CHANNEL BOOST DB CB D SW INTVCC BG PGND VCO CLK2 CLK1 - + 0.425V SLEEP IR - L RSENSE CIN VIN FREQ 20A -+ +- 3mV PLLIN/MODE 100k SYNC DET 2.7V 0.55V SLOPE COMP + VIN EXTVCC OV 5.1V LDO EN + 5.1V LDO EN 0.5A SHDN RST 2(VFB) FOLDBACK 11V 0 - 4.7V SHDN SGND INTVCC RUN 38571 FD + - - + + - + + - - PGOOD1 0.88V VFB1 0.72V S R Q Q DROP OUT DET TOP BOT TOP ON SWITCH LOGIC BOT TG SHDN COUT VOUT CLP PFD ICMP SENSE+ SENSE- VFB EA 0.80V TRACK/SS RA RB 0.88V ITH CC 1A TRACK/SS CC2 RC CSS 38571fa LTC3857-1 operaTion (Refer to the Functional Diagram) Main Control Loop TheLTC3857-1usesaconstantfrequency,currentmode step-downarchitecturewiththetwocontrollerchannels operating 180 degrees out of phase. During normal operation,eachexternaltopMOSFETisturnedonwhen theclockforthatchannelsetstheRSlatch,andisturned offwhenthemaincurrentcomparator,ICMP ,resetsthe RSlatch.ThepeakinductorcurrentatwhichICMPtrips andresetsthelatchiscontrolledbythevoltageontheITH pin,whichistheoutputoftheerroramplifier,EA.Theerror amplifiercomparestheoutputvoltagefeedbacksignalat theVFBpin,(whichisgeneratedwithanexternalresistor divider connected across the output voltage, VOUT, to ground)totheinternal0.800Vreferencevoltage.Whenthe loadcurrentincreases,itcausesaslightdecreaseinVFB relativetothereference,whichcausestheEAtoincrease theITHvoltageuntiltheaverageinductorcurrentmatches thenewloadcurrent. AfterthetopMOSFETisturnedoffeachcycle,thebottom MOSFETisturnedonuntileithertheinductorcurrentstarts toreverse,asindicatedbythecurrentcomparatorIR,or thebeginningofthenextclockcycle. INTVCC/EXTVCC Power PowerforthetopandbottomMOSFETdriversandmost otherinternalcircuitryisderivedfromtheINTVCCpin.When theEXTVCCpinisleftopenortiedtoavoltagelessthan 4.7V,theVINLDO(lowdropoutlinearregulator)supplies 5.1VfromVINtoINTVCC.IfEXTVCCistakenabove4.7V, theVINLDOisturnedoffandanEXTVCCLDOisturnedon. Onceenabled,theEXTVCCLDOsupplies5.1VfromEXTVCC toINTVCC.UsingtheEXTVCCpinallowstheINTVCCpower tobederivedfromahighefficiencyexternalsourcesuch asoneoftheLTC3857-1switchingregulatoroutputs. Each top MOSFET driver is biased from the floating bootstrapcapacitorCB,whichnormallyrechargesduring eachcyclethroughanexternaldiodewhenthetopMOSFET turnsoff.Iftheinputvoltage,VIN,decreasestoavoltage closetoVOUT,theloopmayenterdropoutandattempt to turn on the top MOSFET continuously. The dropout detectordetectsthisandforcesthetopMOSFETofffor aboutone-twelfthoftheclockperiodeverytenthcycleto allowCBtorecharge. Shutdown and Start-Up (RUN1, RUN2 and TRACK/ SS1, TRACK/SS2 Pins) ThetwochannelsoftheLTC3857-1canbeindependently shutdownusingtheRUN1andRUN2pins.Pullingeitherof thesepinsbelow1.26Vshutsdownthemaincontrolloop forthatcontroller.Pullingbothpinsbelow0.7Vdisables bothcontrollersandmostinternalcircuits,includingthe INTVCCLDOs.Inthisstate,theLTC3857-1drawsonly8A ofquiescentcurrent. TheRUNpinmaybeexternallypulledupordrivendirectly bylogic.WhendrivingtheRUNpinwithalowimpedance source,donotexceedtheabsolutemaximumratingof 8V.TheRUNpinhasaninternal11Vvoltageclampthat allowstheRUNpintobeconnectedthrougharesistortoa highervoltage(forexample,VIN),solongasthemaximum currentintotheRUNpindoesnotexceed100A. Thestart-upofeachcontroller'soutputvoltageVOUTis controlledbythevoltageontheTRACK/SSpinforthat channel.WhenthevoltageontheTRACK/SSpinisless thanthe0.8Vinternalreference,theLTC3857-1regulates theVFBvoltagetotheTRACK/SSpinvoltageinsteadofthe 0.8Vreference.ThisallowstheTRACK/SSpintobeused toprogramasoft-startbyconnectinganexternalcapacitor fromtheTRACK/SSpintoSGND.Aninternal1Apull-up currentchargesthiscapacitorcreatingavoltagerampon theTRACK/SSpin.AstheTRACK/SSvoltageriseslinearly from0Vto0.8V(andbeyonduptotheabsolutemaximum ratingof6V),theoutputvoltageVOUTrisessmoothlyfrom zerotoitsfinalvalue. AlternativelytheTRACK/SSpincanbeusedtocausethe start-upofVOUTtotrackthatofanothersupply.Typically, thisrequiresconnectingtotheTRACK/SSpinanexternal resistor divider from the other supply to ground (see ApplicationsInformationsection). 38571fa LTC3857-1 operaTion (Refer to the Functional Diagram) Light Load Current Operation (Burst Mode Operation, Pulse-Skipping or Forced Continuous Mode) (PLLIN/MODE Pin) TheLTC3857-1canbeenabledtoenterhighefficiency BurstModeoperation,constantfrequencypulse-skipping mode,orforcedcontinuousconductionmodeatlowload currents.ToselectBurstModeoperation,tiethePLLIN/ MODEpintoGND.Toselectforcedcontinuousoperation, tiethePLLIN/MODEpintoINTVCC.Toselectpulse-skipping mode,tiethePLLIN/MODEpintoaDCvoltagegreater than1.2VandlessthanINTVCC-1.3V. WhenacontrollerisenabledforBurstModeoperation, theminimumpeakcurrentintheinductorissettoapproximately 15% of the maximum sense voltage even thoughthevoltageontheITHpinindicatesalowervalue. If the average inductor current is higher than the load current,theerroramplifier,EA,willdecreasethevoltage ontheITHpin.WhentheITHvoltagedropsbelow0.425V, theinternalsleepsignalgoeshigh(enablingsleepmode) andbothexternalMOSFETsareturnedoff.TheITHpinis thendisconnectedfromtheoutputoftheEAandparked at0.450V. Insleepmode,muchoftheinternalcircuitryisturnedoff, reducingthequiescentcurrentthattheLTC3857-1draws. Ifonechannelisshutdownandtheotherchannelisin sleepmode,theLTC3857-1drawsonly50Aofquiescent current.Ifbothchannelsareinsleepmode,theLTC3857-1 draws only 80A of quiescent current. In sleep mode, theloadcurrentissuppliedbytheoutputcapacitor.As theoutputvoltagedecreases,theEA'soutputbeginsto rise.Whentheoutputvoltagedropsenough,theITHpin isreconnectedtotheoutputoftheEA,thesleepsignal goeslow,andthecontrollerresumesnormaloperation byturningonthetopexternalMOSFETonthenextcycle oftheinternaloscillator. WhenacontrollerisenabledforBurstModeoperation,the inductorcurrentisnotallowedtoreverse.Thereversecurrentcomparator,IR,turnsoffthebottomexternalMOSFET justbeforetheinductorcurrentreacheszero,preventing itfromreversingandgoingnegative.Thus,thecontroller operatesindiscontinuousoperation. Inforcedcontinuousoperationorclockedbyanexternal clocksourcetousethephase-lockedloop(seeFrequency Selection and Phase-Locked Loop section), the inductorcurrentisallowedtoreverseatlightloadsorunder largetransientconditions.Thepeakinductorcurrentis determinedbythevoltageontheITHpin,justasinnormal operation.Inthismode,theefficiencyatlightloadsislower thaninBurstModeoperation.However,continuousoperationhastheadvantageofloweroutputvoltagerippleand lessinterferencetoaudiocircuitry.Inforcedcontinuous mode,theoutputrippleisindependentofloadcurrent. WhenthePLLIN/MODEpinisconnectedforpulse-skipping mode, the LTC3857-1 operates in PWM pulse-skipping mode at light loads. In this mode, constant frequency operation is maintained down to approximately 1% of designedmaximumoutputcurrent.Atverylightloads,the currentcomparator,ICMP ,mayremaintrippedforseveral cyclesandforcetheexternaltopMOSFETtostayofffor thesamenumberofcycles(i.e.,skippingpulses).The inductorcurrentisnotallowedtoreverse(discontinuous operation).Thismode,likeforcedcontinuousoperation, exhibitslowoutputrippleaswellaslowaudionoiseand reduced RF interference as compared to Burst Mode operation.Itprovideshigherlowcurrentefficiencythan forcedcontinuousmode,butnotnearlyashighasBurst Modeoperation. Frequency Selection and Phase-Locked Loop (FREQ and PLLIN/MODE Pins) Theselectionofswitchingfrequencyisatradeoffbetween efficiency and component size. Low frequency operationincreasesefficiencybyreducingMOSFETswitching losses,butrequireslargerinductanceand/orcapacitance tomaintainlowoutputripplevoltage. TheswitchingfrequencyoftheLTC3857-1'scontrollers canbeselectedusingtheFREQpin. IfthePLLIN/MODEpinisnotbeingdrivenbyanexternal clocksource,theFREQpincanbetiedtoSGND,tiedto INTVCCorprogrammedthroughanexternalresistor.Tying FREQtoSGNDselects350kHzwhiletyingFREQtoINTVCC selects 535kHz. Placing a resistor between FREQ and 38571fa LTC3857-1 operaTion (Refer to the Functional Diagram) SGNDallowsthefrequencytobeprogrammedbetween 50kHzand900kHz. Aphase-lockedloop(PLL)isavailableontheLTC3857-1 tosynchronizetheinternaloscillatortoanexternalclock source that is connected to the PLLIN/MODE pin. The phasedetectoradjuststhevoltage(throughaninternal lowpassfilter)oftheVCOinputtoaligntheturn-onof controller1'sexternaltopMOSFETtotherisingedgeof thesynchronizingsignal.Thus,theturn-onofcontroller 2'sexternaltopMOSFETis180degreesoutofphaseto therisingedgeoftheexternalclocksource. The VCO input voltage is prebiased to the operating frequencysetbytheFREQpinbeforetheexternalclock isapplied.Ifprebiasedneartheexternalclockfrequeny, thePLLlooponlyneedstomakeslightchangestothe VCOinputinordertosynchronizetherisingedgeofthe externalclock'stotherisingedgeofTG1.Theabilityto prebiastheloopfilterallowsthePLLtolock-inrapidly withoutdeviatingfarfromthedesiredfrequency. Thetypicalcapturerangeofthephase-lockedloopisfrom approximately55kHzto1MHz,withaguaranteeoverall manufacturingvariationstobebetween75kHzand850kHz. Inotherwords,theLTC3857-1'sPLLisguaranteedtolock toanexternalclocksourcewhosefrequencyisbetween 75kHzand850kHz. ThetypicalinputclockthresholdsonthePLLIN/MODE pinare1.6V(rising)and1.1V(falling). Output Overvoltage Protection Anovervoltagecomparatorguardsagainsttransientovershootsaswellasothermoreseriousconditionsthatmay overvoltagetheoutput.WhentheVFBpinrisesbymore than10%aboveitsregulationpointof0.800V,thetop MOSFETisturnedoffandthebottomMOSFETisturned onuntiltheovervoltageconditioniscleared. Power Good (PGOOD1 Pin) ThePGOOD1pinisconnectedtoanopendrainofaninternal N-channelMOSFET.TheMOSFETturnsonandpullsthe PGOOD1pinlowwhenthecorrespondingVFB1pinvoltageisnotwithin10%ofthe0.8Vreferencevoltage.The PGOOD1pinisalsopulledlowwhenthecorresponding RUN1pinislow(shutdown).WhentheVFB1pinvoltage iswithinthe10%requirement,theMOSFETisturned offandthepinisallowedtobepulledupbyanexternal resistortoasourcenogreaterthan6V. Foldback Current When the output voltage falls to less than 70% of its nominallevel,foldbackcurrentlimitingisactivated,progressivelyloweringthepeakcurrentlimitinproportionto theseverityoftheovercurrentorshort-circuitcondition. Foldbackcurrentlimitingisdisabledduringthesoft-start interval(aslongastheVFBvoltageiskeepingupwiththe TRACK/SSvoltage). Theory and Benefits of 2-Phase Operation Whytheneedfor2-phaseoperation?Upuntilthe2-phase family, constant-frequency dual switching regulators operated both channels in phase (i.e., single phase operation).Thismeansthatbothswitchesturnedonat thesametime,causingcurrentpulsesofuptotwicethe amplitudeofthoseforoneregulatortobedrawnfromthe inputcapacitorandbattery.Theselargeamplitudecurrent pulsesincreasedthetotalRMScurrentflowingfromthe inputcapacitor,requiringtheuseofmoreexpensiveinput capacitorsandincreasingbothEMIandlossesintheinput capacitorandbattery. With 2-phase operation, the two channels of the dual switchingregulatorareoperated180degreesoutofphase. Thiseffectivelyinterleavesthecurrentpulsesdrawnbythe switches,greatlyreducingtheoverlaptimewheretheyadd 38571fa LTC3857-1 operaTion (Refer to the Functional Diagram) together.TheresultisasignificantreductionintotalRMS inputcurrent,whichinturnallowslessexpensiveinput capacitorstobeused,reducesshieldingrequirementsfor EMIandimprovesrealworldoperatingefficiency. Figure1comparestheinputwaveformsforasingle-phase dual switching regulator to a 2-phase dual switching regulator.AnactualmeasurementoftheRMSinputcurrentundertheseconditionsshowsthat2-phaseoperation droppedtheinputcurrentfrom2.53ARMSto1.55ARMS. Whilethisisanimpressivereductioninitself,remember thatthepowerlossesareproportionaltoIRMS2,meaning thattheactualpowerwastedisreducedbyafactorof2.66. Thereducedinputripplevoltagealsomeanslesspoweris lostintheinputpowerpath,whichcouldincludebatteries,switches,trace/connectorresistancesandprotection circuitry.Improvementsinbothconductedandradiated EMIalsodirectlyaccrueasaresultofthereducedRMS inputcurrentandvoltage. Ofcourse,theimprovementaffordedby2-phaseoperationisafunctionofthedualswitchingregulator'srelative dutycycleswhich,inturn,aredependentupontheinput voltageVIN(DutyCycle=VOUT/VIN).Figure2showshow theRMSinputcurrentvariesforsinglephaseand2-phase operationfor3.3Vand5Vregulatorsoverawideinput voltagerange. Itcanreadilybeseenthattheadvantagesof2-phaseoperationarenotjustlimitedtoanarrowoperatingrange, formostapplicationsisthat2-phaseoperationwillreduce theinputcapacitorrequirementtothatforjustonechannel operatingatmaximumcurrentand50%dutycycle. 3.0 2.5 INPUT RMS CURRENT (A) 2.0 1.5 1.0 0.5 0 SINGLE PHASE DUAL CONTROLLER 2-PHASE DUAL CONTROLLER VO1 = 5V/3A VO2 = 3.3V/3A 0 10 20 30 INPUT VOLTAGE (V) 40 38571 F02 Figure 2. RMS Input Current Comparison 5V SWITCH 20V/DIV 3.3V SWITCH 20V/DIV INPUT CURRENT 5A/DIV INPUT VOLTAGE 500mV/DIV IIN(MEAS) = 2.53ARMS IIN(MEAS) = 1.55ARMS 38571 F01 Figure 1. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efficiency 38571fa LTC3857-1 applicaTions inForMaTion The Typical Application on the first page is a basic LTC3857-1applicationcircuit.LTC3857-1canbeconfigured touseeitherDCR(inductorresistance)sensingorlow valueresistorsensing.Thechoicebetweenthetwocurrent sensing schemes is largely a design trade-off between cost, power consumption, and accuracy. DCR sensing isbecomingpopularbecauseitsavesexpensivecurrent sensingresistorsandismorepowerefficient,especially in high current applications. However, current sensing resistorsprovidethemostaccuratecurrentlimitsforthe controller.Otherexternalcomponentselectionisdriven bytheloadrequirement,andbeginswiththeselectionof RSENSE(ifRSENSEisused)andinductorvalue.Next,the powerMOSFETsandSchottkydiodesareselected.Finally, inputandoutputcapacitorsareselected. SENSE+ and SENSE- Pins comparators.Thecommonmodevoltagerangeonthese pinsis0Vto24V(absmax),enablingtheLTC3857-1to regulateoutputvoltagesuptoanominal24V(allowing marginfortolerancesandtransients). TheSENSE+andSENSE-pinsaretheinputstothecurrent programmedcurrentlimitunpredictable.IfinductorDCR sensingisused(Figure4b),resistorR1shouldbeplaced closetotheswitchingnode,topreventnoisefromcoupling intosensitivesmall-signalnodes. TO SENSE FILTER, NEXT TO THE CONTROLLER 38571 F03 COUT INDUCTOR OR RSENSE Figure 3. Sense Lines Placement with Inductor or Sense Resistor VIN INTVCC BOOST TG SW LTC3857-1 BG VIN RSENSE VOUT SENSE+ SENSE- SGND PLACE CAPACITOR NEAR SENSE PINS TheSENSE+pinishighimpedanceoverthefullcommon moderange,drawingatmost1A.Thishighimpedance allows the current comparators to be used in inductor DCRsensing. TheimpedanceoftheSENSE-pinchangesdependingon thecommonmodevoltage.WhenSENSE-islessthan INTVCC-0.5V,asmallcurrentoflessthan1Aflowsout ofthepin.WhenSENSE-isaboveINTVCC+0.5V,ahigher current(~550A)flowsintothepin.BetweenINTVCC-0.5V andINTVCC+0.5V,thecurrenttransitionsfromthesmaller currenttothehighercurrent. Filtercomponentsmutualtothesenselinesshouldbe placedclosetotheLTC3857-1,andthesenselinesshould runclosetogethertoaKelvinconnectionunderneaththe currentsenseelement(showninFigure3).Sensingcurrent elsewhere can effectively add parasitic inductance andcapacitancetothecurrentsenseelement,degrading the information at the sense terminals and making the 38571 F04a (4a) Using a Resistor to Sense Current VIN INTVCC BOOST TG SW LTC3857-1 BG R1 C1* SENSE- SGND *PLACE C1 NEAR SENSE PINS (R1||R2) * C1 = L DCR RSENSE(EQ) = DCR R2 R1 + R2 38571 F04b VIN INDUCTOR L DCR VOUT SENSE+ R2 (4b) Using the Inductor DCR to Sense Current Figure 4. Current Sensing Methods 38571fa LTC3857-1 applicaTions inForMaTion Low Value Resistor Current Sensing Atypicalsensingcircuitusingadiscreteresistorisshown in Figure 4a. RSENSE is chosen based on the required outputcurrent. The current comparator has a maximum threshold VSENSE(MAX).Thecurrentcomparatorthresholdvoltage setsthepeakoftheinductorcurrent,yieldingamaximum averageoutputcurrent,IMAX,equaltothepeakvalueless halfthepeak-to-peakripplecurrent,IL.Tocalculatethe senseresistorvalue,usetheequation: RSENSE = VSENSE(MAX ) IMAX + IL 2 acrosstheexternalcapacitorisequaltothedropacross theinductorDCRmultipliedbyR2/(R1+R2).R2scalesthe voltageacrossthesenseterminalsforapplicationswhere theDCRisgreaterthanthetargetsenseresistorvalue. Toproperlydimensiontheexternalfiltercomponents,the DCRoftheinductormustbeknown.Itcanbemeasured using a good RLC meter, but the DCR tolerance is not alwaysthesameandvarieswithtemperature;consultthe manufacturers'datasheetsfordetailedinformation. UsingtheinductorripplecurrentvaluefromtheInductor ValueCalculationsection,thetargetsenseresistorvalue is: RSENSE(EQUIV ) = VSENSE(MAX ) IMAX + IL 2 Whenusingthecontrollerinverylowdropoutconditions, themaximumoutputcurrentlevelwillbereducedduetothe internalcompensationrequiredtomeetstabilitycriterion forbuckregulatorsoperatingatgreaterthan50%duty factor.AcurveisprovidedintheTypicalPerformanceCharacteristicssectiontoestimatethisreductioninpeakoutput currentdependingupontheoperatingdutyfactor. Inductor DCR Sensing Forapplicationsrequiringthehighestpossibleefficiency athighloadcurrents,theLTC3857-1iscapableofsensing thevoltagedropacrosstheinductorDCR,asshownin Figure4b.TheDCRoftheinductorrepresentsthesmall amountofDCresistanceofthecopperwire,whichcanbe lessthan1mfortoday'slowvalue,highcurrentinductors. Inahighcurrentapplicationrequiringsuchaninductor, powerlossthroughasenseresistorwouldcostseveral pointsofefficiencycomparedtoinductorDCRsensing. IftheexternalR1||R2*C1timeconstantischosentobe exactlyequaltotheL/DCRtimeconstant,thevoltagedrop Toensurethattheapplicationwilldeliverfullloadcurrent over the full operating temperature range, choose the minimumvalueforthemaximumcurrentsensethreshold voltage(VSENSE(MAX)). Next,determinetheDCRoftheinductor.Whenprovided, usethemanufacturer'smaximumvalue,usuallygivenat 20C.Increasethisvaluetoaccountforthetemperature coefficientofcopperresistance,whichisapproximately 0.4%/C.AconservativevalueforTL(MAX)is100C. ToscalethemaximuminductorDCRtothedesiredsense resistor(RD)value,usethedividerratio: RD = RSENSE(EQUIV ) DCRMAX at TL(MAX ) C1isusuallyselectedtobeintherangeof0.1Fto0.47F . ThisforcesR1||R2toaround2k,reducingerrorthatmight havebeencausedbytheSENSE+pin's1Acurrent. 38571fa LTC3857-1 applicaTions inForMaTion TheequivalentresistanceR1||R2isscaledtotheroom temperatureinductanceandmaximumDCR: R1|| R2 = L DCR at 20C * C1 AcceptinglargervaluesofILallowstheuseoflowinductances,butresultsinhigheroutputvoltagerippleand greatercorelosses.Areasonablestartingpointforsetting ripplecurrentisIL=0.3(IMAX).ThemaximumILoccurs atthemaximuminputvoltage. Theinductorvaluealsohassecondaryeffects.ThetransitiontoBurstModeoperationbeginswhentheaverage inductorcurrentrequiredresultsinapeakcurrentbelow 15% of the current limit determined by RSENSE. Lower inductorvalues(higherIL)willcausethistooccurat lowerloadcurrents,whichcancauseadipinefficiencyin theupperrangeoflowcurrentoperation.InBurstMode operation,lowerinductancevalueswillcausetheburst frequencytodecrease. Inductor Core Selection OncethevalueforLisknown,thetypeofinductormust beselected.Highefficiencyconvertersgenerallycannot affordthecorelossfoundinlowcostpowderedironcores, forcingtheuseofmoreexpensiveferriteormolypermalloy cores.Actualcorelossisindependentofcoresizefora fixedinductorvalue,butitisverydependentoninductance valueselected.Asinductanceincreases,corelossesgo down.Unfortunately,increasedinductancerequiresmore turnsofwireandthereforecopperlosseswillincrease. Ferritedesignshaveverylowcorelossandarepreferred for high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates hard, which means that inductancecollapsesabruptlywhenthepeakdesigncurrent isexceeded.Thisresultsinanabruptincreaseininductor ripplecurrentandconsequentoutputvoltageripple.Do notallowthecoretosaturate! Power MOSFET and Schottky Diode (Optional) Selection TwoexternalpowerMOSFETsmustbeselectedforeach controllerintheLTC3857-1:oneN-channelMOSFETfor thetop(main)switch,andoneN-channelMOSFETforthe bottom(synchronous)switch. ( ) Thesenseresistorvaluesare: R1 = R1 * RD R1|| R2 ;R2 = RD 1 - RD ThemaximumpowerlossinR1isrelatedtodutycycle, andwilloccurincontinuousmodeatthemaximuminput voltage: PLOSS R1 = ( VIN(MAX) - VOUT ) * VOUT R1 EnsurethatR1hasapowerratinghigherthanthisvalue. Ifhighefficiencyisnecessaryatlightloads,considerthis powerlosswhendecidingwhethertouseDCRsensingor senseresistors.Lightloadpowerlosscanbemodestly higherwithaDCRnetworkthanwithasenseresistor,due totheextraswitchinglossesincurredthroughR1.However, DCRsensingeliminatesasenseresistor,reducesconductionlossesandprovideshigherefficiencyatheavyloads. Peakefficiencyisaboutthesamewitheithermethod. Inductor Value Calculation Theoperatingfrequencyandinductorselectionareinterrelatedinthathigheroperatingfrequenciesallowtheuse ofsmallerinductorandcapacitorvalues.Sowhywould anyoneeverchoosetooperateatlowerfrequencieswith larger components? The answer is efficiency. A higher frequency generally results in lower efficiency because ofMOSFETgatechargelosses.Inadditiontothisbasic trade-off,theeffectofinductorvalueonripplecurrentand lowcurrentoperationmustalsobeconsidered. Theinductorvaluehasadirecteffectonripplecurrent. Theinductorripplecurrent,IL,decreaseswithhigher inductanceorhigherfrequencyandincreaseswithhigher VIN: IL = ( )( ) V 1 VOUT 1- OUT VIN fL 38571fa LTC3857-1 applicaTions inForMaTion Thepeak-to-peakdrivelevelsaresetbytheINTVCCvoltage. Thisvoltageistypically5.1Vduringstart-up(seeEXTVCC Pin Connection). Consequently, logic-level threshold MOSFETsmustbeusedinmostapplications.Theonly exceptionisiflowinputvoltageisexpected(VIN<4V); then,sub-logiclevelthresholdMOSFETs(VGS(TH)<3V) shouldbeused.PaycloseattentiontotheBVDSSspecificationfortheMOSFETsaswell;manyofthelogic-level MOSFETsarelimitedto30Vorless. SelectioncriteriaforthepowerMOSFETsincludetheonresistance, RDS(ON), Miller capacitance, CMILLER, input voltageandmaximumoutputcurrent.Millercapacitance, CMILLER,canbeapproximatedfromthegatechargecurve usually provided on the MOSFET manufacturers' data sheet. CMILLER is equal to the increase in gate charge alongthehorizontalaxiswhilethecurveisapproximately flatdividedbythespecifiedchangeinVDS.Thisresultis thenmultipliedbytheratiooftheapplicationappliedVDS totheGatechargecurvespecifiedVDS.WhentheICis operatingincontinuousmodethedutycyclesforthetop andbottomMOSFETsaregivenby: MainSwitchDuty Cycle = VOUT VIN VIN - VOUT VIN whereisthetemperaturedependencyofRDS(ON)and RDR(approximately2)istheeffectivedriverresistance attheMOSFET'sMillerthresholdvoltage.VTHMINisthe typicalMOSFETminimumthresholdvoltage. BothMOSFETshaveI2RlosseswhilethetopsideN-channel equationincludesanadditionaltermfortransitionlosses, whicharehighestathighinputvoltages.ForVIN<20V thehighcurrentefficiencygenerallyimproveswithlarger MOSFETs,whileforVIN>20Vthetransitionlossesrapidly increasetothepointthattheuseofahigherRDS(ON)device withlowerCMILLERactuallyprovideshigherefficiency.The synchronousMOSFETlossesaregreatestathighinput voltagewhenthetopswitchdutyfactorisloworduring ashort-circuitwhenthesynchronousswitchisonclose to100%oftheperiod. Theterm(1+)isgenerallygivenforaMOSFETinthe formofanormalizedRDS(ON)vsTemperaturecurve,but =0.005/Ccanbeusedasanapproximationforlow voltageMOSFETs. The optional Schottky diodes D1 and D2 shown in Figure11 conduct during the dead-time between the conductionofthetwopowerMOSFETs.Thisprevents thebodydiodeofthebottomMOSFETfromturningon, storing charge during the dead-time and requiring a reverserecoveryperiodthatcouldcostasmuchas3% inefficiencyathighVIN.A1Ato3ASchottkyisgenerally agoodcompromiseforbothregionsofoperationdue to the relatively small average current. Larger diodes resultinadditionaltransitionlossesduetotheirlarger junctioncapacitance. CIN and COUT Selection TheselectionofCINissimplifiedbythe2-phasearchitectureanditsimpactontheworst-caseRMScurrentdrawn throughtheinputnetwork(battery/fuse/capacitor).Itcanbe shownthattheworst-casecapacitorRMScurrentoccurs whenonlyonecontrollerisoperating.Thecontrollerwith thehighest(VOUT)(IOUT)productneedstobeusedinthe formulashowninEquation1todeterminethemaximum Synchronous SwitchDuty Cycle = The MOSFET power dissipations at maximum output currentaregivenby: PMAIN = VOUT I VIN MAX ( )2 (1+ )RDS(ON) + ( )( ) () 2I VIN MAX RDR CMILLER * 2 1 1 + f VINTVCC - VTHMIN VTHMIN PSYNC = VIN - VOUT IMAX VIN () ( )2 (1+ )RDS(ON) 38571fa LTC3857-1 applicaTions inForMaTion RMScapacitorcurrentrequirement.Increasingtheoutputcurrentdrawnfromtheothercontrollerwillactually decreasetheinputRMSripplecurrentfromitsmaximum value.Theout-of-phasetechniquetypicallyreducesthe inputcapacitor'sRMSripplecurrentbyafactorof30% to70%whencomparedtoasinglephasepowersupply solution. Incontinuousmode,thesourcecurrentofthetopMOSFET isasquarewaveofdutycycle(VOUT)/(VIN).Toprevent largevoltagetransients,alowESRcapacitorsizedforthe maximumRMScurrentofonechannelmustbeused.The maximumRMScapacitorcurrentisgivenby: CIN Re quiredIRMS 1/ 2 IMAX VOUT VIN - VOUT (1) VIN 1cmofeachotherandshareacommonCIN(s).Separating thedrainsandCINmayproduceundesirablevoltageand currentresonancesatVIN. Asmall(0.1Fto1F)bypasscapacitorbetweenthechip VIN pin and ground, placed close to the LTC3857-1, is alsosuggested.A10resistorplacedbetweenCIN(C1) and the VIN pin provides further isolation between the twochannels. The selection of COUT is driven by the effective series resistance (ESR). Typically, once the ESR requirement issatisfied,thecapacitanceisadequateforfiltering.The outputripple(VOUT)isapproximatedby: 1 VOUT IL ESR + 8 * f * COUT where f is the operating frequency, COUT is the output capacitanceandIListheripplecurrentintheinductor. Theoutputrippleishighestatmaximuminputvoltage sinceILincreaseswithinputvoltage. Setting Output Voltage TheLTC3857-1outputvoltagesareeachsetbyanexternalfeedbackresistordividercarefullyplacedacrossthe output,asshowninFigure5.Theregulatedoutputvoltage isdeterminedby: R VOUT = 0.8 V 1+ B RA To improve the frequency response, a feedforward capacitor,CFF,maybeused.Greatcareshouldbetakento routetheVFBlineawayfromnoisesources,suchasthe inductorortheSWline. VOUT 1/2 LTC3857-1 VFB RA 38571 F05 ( )( ) ThisformulahasamaximumatVIN=2VOUT,whereIRMS =IOUT/2.Thissimpleworst-caseconditioniscommonly usedfordesignbecauseevensignificantdeviationsdonot offermuchrelief.Notethatcapacitormanufacturers'ripple currentratingsareoftenbasedononly2000hoursoflife. Thismakesitadvisabletofurtherderatethecapacitor,or tochooseacapacitorratedatahighertemperaturethan required. Several capacitors may be paralleled to meet sizeorheightrequirementsinthedesign.Duetothehigh operatingfrequencyoftheLTC3857-1,ceramiccapacitors canalsobeusedforCIN.Alwaysconsultthemanufacturer ifthereisanyquestion. ThebenefitoftheLTC3857-12-phaseoperationcanbe calculatedbyusingEquation1forthehigherpowercontrollerandthencalculatingthelossthatwouldhaveresulted ifbothcontrollerchannelsswitchedonatthesametime. ThetotalRMSpowerlostislowerwhenbothcontrollers areoperatingduetothereducedoverlapofcurrentpulses requiredthroughtheinputcapacitor'sESR.Thisiswhy theinputcapacitor'srequirementcalculatedaboveforthe worst-casecontrollerisadequateforthedualcontroller design.Also,theinputprotectionfuseresistance,battery resistance,andPCboardtraceresistancelossesarealso reducedduetothereducedpeakcurrentsina2-phase system.Theoverallbenefitofamultiphasedesignwill onlybefullyrealizedwhenthesourceimpedanceofthe powersupply/batteryisincludedintheefficiencytesting. ThedrainsofthetopMOSFETsshouldbeplacedwithin RB CFF Figure 5. Setting Output Voltage 38571fa LTC3857-1 applicaTions inForMaTion Tracking and Soft-Start (TRACK/SS Pins) Thestart-upofeachVOUTiscontrolledbythevoltageon therespectiveTRACK/SSpin.Whenthevoltageonthe TRACK/SSpinislessthantheinternal0.8Vreference,the LTC3857-1regulatestheVFBpinvoltagetothevoltageon theTRACK/SSpininsteadof0.8V.TheTRACK/SSpincan beusedtoprogramanexternalsoft-startfunctionorto allowVOUTtotrackanothersupplyduringstart-up. Soft-start is enabled by simply connecting a capacitor fromtheTRACK/SSpintoground,asshowninFigure6. An internal 1A current source charges the capacitor, providingalinearrampingvoltageattheTRACK/SSpin. TheLTC3857-1willregulatetheVFBpin(andhenceVOUT) accordingtothevoltageontheTRACK/SSpin,allowing VOUTtorisesmoothlyfrom0Vtoitsfinalregulatedvalue. Thetotalsoft-starttimewillbeapproximately: tSS = CSS * 0.8 V 1A 1/2 LTC3857-1 TRACK/SS CSS SGND 38571 F06 VX(MASTER) OUTPUT VOLTAGE VOUT(SLAVE) TIME 38571 F07a (7a) Coincident Tracking VX(MASTER) OUTPUT VOLTAGE VOUT(SLAVE) TIME 38571 F07b (7b) Ratiometric Tracking Figure 7. Two Different Modes of Output Voltage Tracking Vx VOUT Figure 6. Using the TRACK/SS Pin to Program Soft-Start RB RA RTRACKB 1/2 LTC3857-1 VFB Alternatively,theTRACK/SSpincanbeusedtotracktwo (ormore)suppliesduringstart-up,asshownqualitatively inFigures7aand7b.Todothis,aresistordividershould beconnectedfromthemastersupply(VX)totheTRACK/ SSpinoftheslavesupply(VOUT),asshowninFigure8. Duringstart-upVOUTwilltrackVXaccordingtotheratio setbytheresistordivider: + RTRACKB R VX RA = * TRACKA VOUT RTRACKA RA + RB TRACK/SS RTRACKA 38571 F08 Figure 8. Using the TRACK/SS Pin for Tracking INTVCC Regulators TheLTC3857-1featurestwoseparateinternalP-channel low dropout linear regulators (LDO) that supply power attheINTVCCpinfromeithertheVINsupplypinorthe EXTVCCpindependingontheconnectionoftheEXTVCC pin. INTVCC powers the gate drivers and much of the LTC3857-1'sinternalcircuitry.TheVINLDOandtheEXTVCC 38571fa Forcoincidenttracking(VOUT=VXduringstart-up): RA=RTRACKA RB=RTRACKB 0 LTC3857-1 applicaTions inForMaTion LDOregulateINTVCCto5.1V.Eachofthesecansupplya peakcurrentof50mAandmustbebypassedtoground withaminimumof4.7Fceramiccapacitor.Nomatter what type of bulk capacitor is used, an additional 1F ceramiccapacitorplaceddirectlyadjacenttotheINTVCC andPGNDpinsishighlyrecommended.Goodbypassing isneededtosupplythehightransientcurrentsrequired bytheMOSFETgatedriversandtopreventinteraction betweenthechannels. HighinputvoltageapplicationsinwhichlargeMOSFETs arebeingdrivenathighfrequenciesmaycausethemaximumjunctiontemperatureratingfortheLTC3857-1tobe exceeded.TheINTVCCcurrent,whichisdominatedbythe gatechargecurrent,maybesuppliedbyeithertheVINLDO ortheEXTVCCLDO.WhenthevoltageontheEXTVCCpin islessthan4.7V,theVINLDOisenabled.PowerdissipationfortheICinthiscaseishighestandisequaltoVIN* IINTVCC.Thegatechargecurrentisdependentonoperating frequencyasdiscussedintheEfficiencyConsiderations section.Thejunctiontemperaturecanbeestimatedbyusing theequationsgiveninNote3oftheElectricalCharacteristics.Forexample,theLTC3857-1INTVCCcurrentislimited tolessthan15mAfroma40Vsupplywhennotusingthe EXTVCCsupplyata70Cambienttemperature: TJ=70C+(15mA)(40V)(90C/W)=125C Topreventthemaximumjunctiontemperaturefrombeingexceeded,theinputsupplycurrentmustbechecked whileoperatinginforcedcontinuousmode(PLLIN/MODE =INTVCC)atmaximumVIN. WhenthevoltageappliedtoEXTVCCrisesabove4.7V,the VINLDOisturnedoffandtheEXTVCCLDOisenabled.The EXTVCCLDOremainsonaslongasthevoltageappliedto EXTVCCremainsabove4.5V.TheEXTVCCLDOattempts toregulatetheINTVCCvoltageto5.1V,sowhileEXTVCC islessthan5.1V,theLDOisindropoutandtheINTVCC voltageisapproximatelyequaltoEXTVCC.WhenEXTVCC isgreaterthan5.1V,uptoanabsolutemaximumof14V, INTVCCisregulatedto5.1V. Using the EXTVCC LDO allows the MOSFET driver and controlpowertobederivedfromoneoftheLTC3857-1's switchingregulatoroutputs(4.7VVOUT14V)during normaloperationandfromtheVINLDOwhentheoutputisoutofregulation(e.g.,start-up,short-circuit).If morecurrentisrequiredthroughtheEXTVCCLDOthan is specified, an external Schottky diode can be added betweentheEXTVCCandINTVCCpins.Inthiscase,do notapplymorethan6VtotheEXTVCCpinandmakesure thatEXTVCCVIN. Significantefficiencyandthermalgainscanberealized bypoweringINTVCCfromtheoutput,sincetheVINcurrentresultingfromthedriverandcontrolcurrentswillbe scaledbyafactorof(DutyCycle)/(SwitcherEfficiency). For5Vto14Vregulatoroutputs,thismeansconnecting theEXTVCCpindirectlytoVOUT.TyingtheEXTVCCpinto an8.5Vsupplyreducesthejunctiontemperatureinthe previousexamplefrom125Cto: TJ=70C+(15mA)(8.5V)(90C/W)=82C However,for3.3Vandotherlowvoltageoutputs,additionalcircuitryisrequiredtoderiveINTVCCpowerfrom theoutput. ThefollowinglistsummarizesthefourpossibleconnectionsforEXTVCC: 1.EXTVCCLeftOpen(orGrounded).ThiswillcauseINTVCC tobepoweredfromtheinternal5.1Vregulatorresultinginanefficiencypenaltyofupto10%athighinput voltages. 2.EXTVCCConnecteddirectlytoVOUT.Thisisthenormal connectionfora5Vto14Vregulatorandprovidesthe highestefficiency. 3.EXTVCCConnectedtoanExternalsupply.Ifanexternal supplyisavailableinthe5Vto14Vrange,itmaybe usedtopowerEXTVCC.EnsurethatEXTVCC LTC3857-1 applicaTions inForMaTion CIN VIN MTOP TG1 1/2 LTC3857-1 EXTVCC SW MBOT BG1 PGND D COUT 38571 F09 BAT85 BAT85 BAT85 VN2222LL L RSENSE VOUT voltagefallsbelow70%ofitsnominaloutputlevel,then themaximumsensevoltageisprogressivelyloweredto abouthalfofitsmaximumselectedvalue.Undershortcircuitconditionswithverylowdutycycles,theLTC3857-1 willbegincycleskippinginordertolimittheshort-circuit current.InthissituationthebottomMOSFETwillbedissipatingmostofthepowerbutlessthaninnormaloperation.Theshort-circuitripplecurrentisdeterminedbythe minimumon-time.tON(MIN),oftheLTC3857-1(90ns), theinputvoltageandinductorvalue: V IL(SC) = tON(MIN) IN L Figure 9. Capacitive Charge Pump for EXTVCC Topside MOSFET Driver Supply (CB, DB) Externalbootstrapcapacitors,CB,connectedtotheBOOST pinssupplythegatedrivevoltagesforthetopsideMOSFETs. CapacitorCBintheFunctionalDiagramischargedthough externaldiodeDBfromINTVCCwhentheSWpinislow. WhenoneofthetopsideMOSFETsistobeturnedon,the driverplacestheCBvoltageacrossthegate-sourceofthe desiredMOSFET.ThisenhancestheMOSFETandturnson thetopsideswitch.Theswitchnodevoltage,SW,risesto VINandtheBOOSTpinfollows.WiththetopsideMOSFET on,theboostvoltageisabovetheinputsupply:VBOOST= VIN+VINTVCC.Thevalueoftheboostcapacitor,CB,needs tobe100timesthatofthetotalinputcapacitanceofthe topsideMOSFET(s).Thereversebreakdownoftheexternal SchottkydiodemustbegreaterthanVIN(MAX). Whenadjustingthegatedrivelevel,thefinalarbiteristhe totalinputcurrentfortheregulator.Ifachangeismade andtheinputcurrentdecreases,thentheefficiencyhas improved.Ifthereisnochangeininputcurrent,thenthere isnochangeinefficiency. Fault Conditions: Current Limit and Current Foldback TheLTC3857-1includescurrentfoldbacktohelplimitload currentwhentheoutputisshortedtoground.Iftheoutput Theresultingaverageshort-circuitcurrentis: 1 ISC = 50% * ILIM(MAX ) - IL(SC) 2 Fault Conditions: Overvoltage Protection (Crowbar) Theovervoltagecrowbarisdesignedtoblowasystem inputfusewhentheoutputvoltageoftheregulatorrises muchhigherthannominallevels.Thecrowbarcauseshuge currentstoflow,thatblowthefusetoprotectagainsta shortedtopMOSFETiftheshortoccurswhilethecontrollerisoperating. Acomparatormonitorstheoutputforovervoltageconditions. The comparator detects faults greater than 10% abovethenominaloutputvoltage.Whenthiscondition issensed,thetopMOSFETisturnedoffandthebottom MOSFETisturnedonuntiltheovervoltageconditionis cleared. The bottom MOSFET remains on continuously foraslongastheovervoltageconditionpersists;ifVOUT returns to a safe level, normal operation automatically resumes. AshortedtopMOSFETwillresultinahighcurrentcondition whichwillopenthesystemfuse.Theswitchingregulator willregulateproperlywithaleakytopMOSFETbyaltering thedutycycletoaccommodatetheleakage. 38571fa LTC3857-1 applicaTions inForMaTion Phase-Locked Loop and Frequency Synchronization TheLTC3857-1hasaninternalphase-lockedloop(PLL) comprisedofaphasefrequencydetector,alowpassfilter, andavoltage-controlledoscillator(VCO).Thisallowsthe turn-onofthetopMOSFETofcontroller1tobelockedto therisingedgeofanexternalclocksignalappliedtothe PLLIN/MODEpin.Theturn-onofcontroller2'stopMOSFET isthus180degreesoutofphasewiththeexternalclock. Thephasedetectorisanedgesensitivedigitaltypethat provideszerodegreesphaseshiftbetweentheexternal andinternaloscillators.Thistypeofphasedetectordoes notexhibitfalselocktoharmonicsoftheexternalclock. Iftheexternalclockfrequencyisgreaterthantheinternal oscillator'sfrequency,fOSC,thencurrentissourcedcontinuouslyfromthephasedetectoroutput,pullinguptheVCO input.WhentheexternalclockfrequencyislessthanfOSC, currentissunkcontinuously,pullingdowntheVCOinput. Iftheexternalandinternalfrequenciesarethesamebut exhibitaphasedifference,thecurrentsourcesturnonfor anamountoftimecorrespondingtothephasedifference. ThevoltageattheVCOinputisadjusteduntilthephase andfrequencyoftheinternalandexternaloscillatorsare identical.Atthestableoperatingpoint,thephasedetector outputishighimpedanceandtheinternalfiltercapacitor, CLP ,holdsthevoltageattheVCOinput. NotethattheLTC3857-1canonlybesynchronizedtoan external clock whose frequency is within range of the LTC3857-1's internal VCO, which is nominally 55kHz to1MHz.Thisisguaranteedtobebetween75kHzand 850kHz. Typically, the external clock (on the PLLIN/MODE pin) inputhighthresholdis1.6V,whiletheinputlowthreshold is1.1V. RapidphaselockingcanbeachievedbyusingtheFREQ pin to set a free-running frequency near the desired synchronizationfrequency.TheVCO'sinputvoltageis 1000 900 800 FREQUENCY (kHz) 700 600 500 400 300 200 100 0 15 25 35 45 55 65 75 85 95 105 115 125 FREQ PIN RESISTOR (k ) 38571 F10 Figure 10. Relationship Between Oscillator Frequency and Resistor Value at the FREQ Pin prebiasedatafrequencycorrespondingtothefrequency setbytheFREQpin.Onceprebiased,thePLLonlyneeds to adjust the frequency slightly to achieve phase lock andsynchronization.Althoughitisnotrequiredthatthe free-runningfrequencybenearexternalclockfrequency, doingsowillpreventtheoperatingfrequencyfrompassing throughalargerangeoffrequenciesasthePLLlocks. Table2summarizesthedifferentstatesinwhichtheFREQ pincanbeused. Table 2 FREQ PIN 0V INTVCC Resistor AnyoftheAbove PLLIN/MODE PIN DCVoltage DCVoltage DCVoltage ExternalClock FREQUENCY 350kHz 535kHz 50kHz-900kHz Phase-Lockedto ExternalClock Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time durationthattheLTC3857-1iscapableofturningonthetop MOSFET.Itisdeterminedbyinternaltimingdelaysandthe 38571fa LTC3857-1 applicaTions inForMaTion gatechargerequiredtoturnonthetopMOSFET.Lowduty cycleapplicationsmayapproachthisminimumon-time limitandcareshouldbetakentoensurethat tON(MIN) < VOUT VIN f 2.INTVCCcurrentisthesumoftheMOSFETdriverand control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs.EachtimeaMOSFETgateisswitchedfrom lowtohightolowagain,apacketofcharge,dQ,moves fromINTVCCtoground.TheresultingdQ/dtisacurrent out of INTVCC that is typically much larger than the controlcircuitcurrent.Incontinuousmode,IGATECHG =f(QT+QB),whereQTandQBarethegatechargesof thetopsideandbottomsideMOSFETs. SupplyingINTVCCfromanoutput-derivedpowersource through EXTVCC will scale the VIN current required forthedriverandcontrolcircuitsbyafactorof(Duty Cycle)/(Efficiency).Forexample,ina20Vto5Vapplication,10mAofINTVCCcurrentresultsinapproximately 2.5mAofVINcurrent.Thisreducesthemidcurrentloss from10%ormore(ifthedriverwaspowereddirectly fromVIN)toonlyafewpercent. 3.I2RlossesarepredictedfromtheDCresistancesofthe fuse(ifused),MOSFET,inductor,currentsenseresistor,andinputandoutputcapacitorESR.Incontinuous modetheaverageoutputcurrentflowsthroughLand RSENSE,butischoppedbetweenthetopsideMOSFET andthesynchronousMOSFET.IfthetwoMOSFETshave approximatelythesameRDS(ON),thentheresistance ofoneMOSFETcansimplybesummedwiththeresistancesofL,RSENSEandESRtoobtainI2Rlosses.For example,ifeachRDS(ON)=30m,RL=50m,RSENSE =10mandRESR=40m(sumofbothinputand output capacitance losses), then the total resistance is130m.Thisresultsinlossesrangingfrom3%to 13%astheoutputcurrentincreasesfrom1Ato5Afor a5Voutput,ora4%to20%lossfora3.3Voutput. EfficiencyvariesastheinversesquareofVOUTforthe sameexternalcomponentsandoutputpowerlevel.The combinedeffectsofincreasinglyloweroutputvoltages andhighercurrentsrequiredbyhighperformancedigital systemsisnotdoublingbutquadruplingtheimportance oflosstermsintheswitchingregulatorsystem! () Ifthedutycyclefallsbelowwhatcanbeaccommodated bytheminimumon-time,thecontrollerwillbegintoskip cycles.Theoutputvoltagewillcontinuetoberegulated, buttheripplevoltageandcurrentwillincrease. Theminimumon-timefortheLTC3857-1isapproximately 95ns.However,asthepeaksensevoltagedecreasesthe minimumon-timegraduallyincreasesuptoabout130ns. Thisisofparticularconcerninforcedcontinuousapplicationswithlowripplecurrentatlightloads.Ifthedutycycle dropsbelowtheminimumon-timelimitinthissituation, asignificantamountofcycleskippingcanoccurwithcorrespondinglylargercurrentandvoltageripple. Efficiency Considerations Thepercentefficiencyofaswitchingregulatorisequalto theoutputpowerdividedbytheinputpowertimes100%. Itisoftenusefultoanalyzeindividuallossestodetermine whatislimitingtheefficiencyandwhichchangewould producethemostimprovement.Percentefficiencycan beexpressedas: %Efficiency=100%-(L1+L2+L3+...) whereL1,L2,etc.aretheindividuallossesasapercentageofinputpower. Althoughalldissipativeelementsinthecircuitproduce losses,fourmainsourcesusuallyaccountformostofthe lossesinLTC3857-1circuits:1)ICVINcurrent,2)INTVCC regulator current, 3) I2R losses, 4) topside MOSFET transitionlosses. 1.TheVINcurrentistheDCinputsupplycurrentgiven intheElectricalCharacteristicstable,whichexcludes MOSFETdriverandcontrolcurrents.VINcurrenttypicallyresultsinasmall(<0.1%)loss. 38571fa LTC3857-1 applicaTions inForMaTion 4.TransitionlossesapplyonlytothetopsideMOSFET(s), and become significant only when operating at high input voltages (typically 15V or greater). Transition lossescanbeestimatedfrom: TransitionLoss=(1.7)*VIN*2*IO(MAX)*CRSS*f Otherhiddenlossessuchascoppertraceandinternal batteryresistancescanaccountforanadditional5% to10%efficiencydegradationinportablesystems.It isveryimportanttoincludethesesystemlevellosses duringthedesignphase.Theinternalbatteryandfuse resistancelossescanbeminimizedbymakingsurethat CINhasadequatechargestorageandverylowESRat theswitchingfrequency.A25Wsupplywilltypically require a minimum of 20F to 40F of capacitance having a maximum of 20m to 50m of ESR. The LTC3857-1 2-phase architecture typically halves this input capacitance requirement over competing solutions.OtherlossesincludingSchottkyconductionlosses duringdead-timeandinductorcorelossesgenerally accountforlessthan2%totaladditionalloss. Checking Transient Response Theregulatorloopresponsecanbecheckedbylookingat theloadcurrenttransientresponse.Switchingregulators takeseveralcyclestorespondtoastepinDC(resistive) loadcurrent.Whenaloadstepoccurs,VOUTshiftsby anamountequaltoILOAD(ESR),whereESRistheeffectiveseriesresistanceofCOUT.ILOADalsobeginsto chargeordischargeCOUTgeneratingthefeedbackerror signalthatforcestheregulatortoadapttothecurrent changeandreturnVOUTtoitssteady-statevalue.During thisrecoverytimeVOUTcanbemonitoredforexcessive overshoot or ringing, which would indicate a stability problem.OPTI-LOOPcompensationallowsthetransient responsetobeoptimizedoverawiderangeofoutput capacitanceandESRvalues.The availability of the ITH pin not only allows optimization of control loop behavior, but it also provides a DC coupled and AC filtered closed-loop response test point. The DC step, rise time and settling at this test point truly reflects the closed-loop response. Assumingapredominantlysecondordersystem,phase marginand/ordampingfactorcanbeestimatedusingthe percentageofovershootseenatthispin.Thebandwidth canalsobeestimatedbyexaminingtherisetimeatthe pin. The ITH external components shown in Figure 13 circuitwillprovideanadequatestartingpointformost applications. TheITHseriesRC-CCfiltersetsthedominantpole-zero loopcompensation.Thevaluescanbemodifiedslightly (from0.5to2timestheirsuggestedvalues)tooptimize transientresponseoncethefinalPClayoutisdoneand theparticularoutputcapacitortypeandvaluehavebeen determined. The output capacitors need to be selected becausethevarioustypesandvaluesdeterminetheloop gainandphase.Anoutputcurrentpulseof20%to80% offull-loadcurrenthavingarisetimeof1sto10swill produceoutputvoltageandITHpinwaveformsthatwill giveasenseoftheoverallloopstabilitywithoutbreaking thefeedbackloop. Placing a resistive load and a power MOSFET directly acrosstheoutputcapacitoranddrivingthegatewithan appropriatesignalgeneratorisapracticalwaytoproduce arealisticloadstepcondition.Theinitialoutputvoltage stepresultingfromthestepchangeinoutputcurrentmay notbewithinthebandwidthofthefeedbackloop,sothis signalcannotbeusedtodeterminephasemargin.This iswhyitisbettertolookattheITHpinsignalwhichisin the feedback loop and is the filtered and compensated controlloopresponse. ThegainoftheloopwillbeincreasedbyincreasingRC andthebandwidthoftheloopwillbeincreasedbydecreasingCC.IfRCisincreasedbythesamefactorthatCC isdecreased,thezerofrequencywillbekeptthesame, thereby keeping the phase shift the same in the most criticalfrequencyrangeofthefeedbackloop.Theoutput voltagesettlingbehaviorisrelatedtothestabilityofthe closed-loopsystemandwilldemonstratetheactualoverall supplyperformance. Asecond,moreseveretransientiscausedbyswitching inloadswithlarge(>1F)supplybypasscapacitors.The dischargedbypasscapacitorsareeffectivelyputinparallel withCOUT,causingarapiddropinVOUT.Noregulatorcan alteritsdeliveryofcurrentquicklyenoughtopreventthis suddenstepchangeinoutputvoltageiftheloadswitch resistanceislowanditisdrivenquickly.Iftheratioof 38571fa LTC3857-1 applicaTions inForMaTion CLOADtoCOUTisgreaterthan1:50,theswitchrisetime shouldbecontrolledsothattheloadrisetimeislimited toapproximately25*CLOAD.Thusa10Fcapacitorwould requirea250srisetime,limitingthechargingcurrent toabout200mA. Design Example As a design example for one channel, assume VIN = 12V(nominal),VIN=22V(max),VOUT=3.3V,IMAX=6A, VSENSE(MAX)=50mVandf=350kHz. Theinductancevalueischosenfirstbasedona30%ripple currentassumption.Thehighestvalueofripplecurrent occursatthemaximuminputvoltage.TietheFREQpin to GND, generating 350kHz operation. The minimum inductancefor30%ripplecurrentis: IL(NOM) = VOUT VOUT 1- * L VIN(NOM) ThepowerdissipationonthetopsideMOSFETcanbeeasily estimated.ChoosingaFairchildFDS6982SdualMOSFET .At resultsin:RDS(ON)=0.035/0.022,CMILLER=215pF maximuminputvoltagewithT(estimated)=50C: PMAIN = 2 3.3V 6 A 1+ 0.005 50C - 25C 22V 2 6A 2.5 215pF * 0.035 + 22V 2 1 1 5V - 2.3V + 2.3V 350kHz = 433mW () ( )( ) ( )( ) ( )( ) ( ) Ashort-circuittogroundwillresultinafoldedbackcurrentof: ISC = 25mV 1 95ns 22V - = 3.9 A 0.006 2 3.9H ( ) A3.9Hinductorwillproduce29%ripplecurrent.The peakinductorcurrentwillbethemaximumDCvalueplus onehalftheripplecurrent,or6.88A.Increasingtheripple currentwillalsohelpensurethattheminimumon-time of95nsisnotviolated.Theminimumon-timeoccursat maximumVIN: 3.3V tON(MIN) = = 429ns = VIN(MAX ) 22V 350kHz VOUT withatypicalvalueofRDS(ON)and=(0.005/C)(25C) = 0.125. The resulting power dissipated in the bottom MOSFETis: PSYNC = 3.9 A ( )2 (1.125)(0.022) = 376mW whichislessthanunderfull-loadconditions. CINischosenforanRMScurrentratingofatleast3Aat temperatureassumingonlythischannelison.COUTis chosenwithanESRof0.02forlowoutputripple.The outputrippleincontinuousmodewillbehighestatthe maximuminputvoltage.Theoutputvoltagerippledueto ESRisapproximately: VORIPPLE=RESR(IL)=0.02(1.75A)=35mVP-P ( ) TheequivalentRSENSEresistorvaluecanbecalculatedby usingtheminimumvalueforthemaximumcurrentsense threshold(43mV): RSENSE 43mV = 0.006 6.88 A Choosing1%resistors:RA=25kandRB=80.6kyields anoutputvoltageof3.33V. 38571fa LTC3857-1 applicaTions inForMaTion PC Board Layout Checklist Whenlayingouttheprintedcircuitboard,thefollowing checklistshouldbeusedtoensureproperoperationof theIC.Theseitemsarealsoillustratedgraphicallyinthe layoutdiagramofFigure11.Figure12illustratesthecurrent waveformspresentinthevariousbranchesofthe2-phase synchronousregulatorsoperatinginthecontinuousmode. Checkthefollowinginyourlayout: 1.ArethetopN-channelMOSFETsMTOP1andMTOP2 locatedwithin1cmofeachotherwithacommondrain connectionatCIN?Donotattempttosplittheinput decouplingforthetwochannelsasitcancausealarge resonantloop. 2.Arethesignalandpowergroundskeptseparate?The combinedICsignalgroundpinandthegroundreturn ofCINTVCCmustreturntothecombinedCOUT(-)terminals.ThepathformedbythetopN-channelMOSFET, SchottkydiodeandtheCINcapacitorshouldhaveshort leadsandPCtracelengths.Theoutputcapacitor(-) terminals should be connected as close as possible tothe(-)terminalsoftheinputcapacitorbyplacing thecapacitorsnexttoeachotherandawayfromthe Schottkyloopdescribedabove. 3.DotheLTC3857-1VFBpins'resistivedividersconnectto the(+)terminalsofCOUT?Theresistivedividermustbe connectedbetweenthe(+)terminalofCOUTandsignal ground.Thefeedbackresistorconnectionsshouldnot bealongthehighcurrentinputfeedsfromtheinput capacitor(s). 4.AretheSENSE-andSENSE+leadsroutedtogetherwith minimumPCtracespacing?Thefiltercapacitorbetween SENSE+andSENSE-shouldbeascloseaspossible totheIC.EnsureaccuratecurrentsensingwithKelvin connectionsattheSENSEresistor. 5.Is the INTVCC decoupling capacitor connected close totheIC,betweentheINTVCCandthepowerground pins?ThiscapacitorcarriestheMOSFETdrivers'currentpeaks.Anadditional1Fceramiccapacitorplaced immediatelynexttotheINTVCCandPGNDpinscanhelp improvenoiseperformancesubstantially. 6.Keeptheswitchingnodes(SW1,SW2),topgatenodes (TG1,TG2),andboostnodes(BOOST1,BOOST2)away from sensitive small-signal nodes, especially from the opposites channel's voltage and current sensing feedbackpins.Allofthesenodeshaveverylargeand fastmovingsignalsandthereforeshouldbekepton theoutput sideoftheLTC3857-1andoccupyminimum PCtracearea. 7.Useamodifiedstar groundtechnique:alowimpedance, largecopperareacentralgroundingpointonthesame sideofthePCboardastheinputandoutputcapacitors withtie-insforthebottomoftheINTVCCdecoupling capacitor,thebottomofthevoltagefeedbackresistive dividerandtheSGNDpinoftheIC. PC Board Layout Debugging Startwithonecontrolleronatatime.Itishelpfultouse aDC-50MHzcurrentprobetomonitorthecurrentinthe inductor while testing the circuit. Monitor the output switchingnode(SWpin)tosynchronizetheoscilloscope totheinternaloscillatorandprobetheactualoutputvoltage aswell.Checkforproperperformanceovertheoperating voltageandcurrentrangeexpectedintheapplication.The frequencyofoperationshouldbemaintainedovertheinput voltagerangedowntodropoutanduntiltheoutputload dropsbelowthelowcurrentoperationthreshold--typically15%ofthemaximumdesignedcurrentlevelinBurst Modeoperation. Thedutycyclepercentageshouldbemaintainedfromcycle tocycleinawell-designed,lownoisePCBimplementation. Variationinthedutycycleatasubharmonicratecansuggestnoisepickupatthecurrentorvoltagesensinginputs orinadequateloopcompensation.Overcompensationof theloopcanbeusedtotameapoorPClayoutifregulator bandwidth optimization is not required. Only after eachcontrollerischeckedforitsindividualperformance shouldbothcontrollersbeturnedonatthesametime. A particularly difficult region of operation is when one controllerchannelisnearingitscurrentcomparatortrip pointwhentheotherchannelisturningonitstopMOSFET. Thisoccursaround50%dutycycleoneitherchanneldue tothephasingoftheinternalclocksandmaycauseminor dutycyclejitter. 38571fa LTC3857-1 applicaTions inForMaTion ITH1 VFB1 SENSE1+ TRACK/SS1 PGOOD1 TG1 RPU1 PGOOD1 VPULL-UP L1 RSENSE VOUT1 SW1 SENSE1- LTC3857-1 BOOST1 BG1 FREQ fIN PLLIN/MODE RUN1 RUN2 SGND SENSE2- SENSE2+ VFB2 ITH2 TRACK/SS2 VIN PGND EXTVCC INTVCC BG2 BOOST2 SW2 TG2 CB1 M1 M2 D1 RIN CVIN 1F CERAMIC COUT1 + GND + CINTVCC VIN 1F CERAMIC M3 CB2 CIN + COUT2 M4 D2 RSENSE VOUT2 L2 38571 F11 Figure 11. Recommended Printed Circuit Layout Diagram Reduce VIN from its nominal level to verify operation oftheregulatorindropout.Checktheoperationofthe undervoltagelockoutcircuitbyfurtherloweringVINwhile monitoringtheoutputstoverifyoperation. Investigatewhetheranyproblemsexistonlyathigheroutputcurrentsoronlyathigherinputvoltages.Ifproblems coincidewithhighinputvoltagesandlowoutputcurrents, lookforcapacitivecouplingbetweentheBOOST,SW,TG, and possibly BG connections and the sensitive voltage andcurrentpins.Thecapacitorplacedacrossthecurrent sensingpinsneedstobeplacedimmediatelyadjacentto thepinsoftheIC.Thiscapacitorhelpstominimizethe effectsofdifferentialnoiseinjectionduetohighfrequency capacitive coupling. If problems are encountered with highcurrentoutputloadingatlowerinputvoltages,look forinductivecouplingbetweenCIN,Schottkyandthetop 38571fa LTC3857-1 applicaTions inForMaTion SW1 L1 RSENSE1 VOUT1 D1 COUT1 RL1 VIN RIN CIN SW2 L2 RSENSE2 VOUT2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. D2 COUT2 RL2 3857 F12 Figure 12. Branch Current Waveforms MOSFETcomponentstothesensitivecurrentandvoltage sensingtraces.Inaddition,investigatecommonground pathvoltagepickupbetweenthesecomponentsandthe SGNDpinoftheIC. An embarrassing problem, which can be missed in an otherwiseproperlyworkingswitchingregulator,results whenthecurrentsensingleadsarehookedupbackwards. Theoutputvoltageunderthisimproperhookupwillstill bemaintainedbuttheadvantagesofcurrentmodecontrol willnotberealized.Compensationofthevoltageloopwill be much more sensitive to component selection. This behaviorcanbeinvestigatedbytemporarilyshortingout thecurrentsensingresistor--don'tworry,theregulator willstillmaintaincontroloftheoutputvoltage. 38571fa LTC3857-1 Typical applicaTions RB1 215k CF1 15pF RA1 68.1k CITH1A 150pF RITH1 15k CITH1 820pF C1 1nF LTC3857-1 SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 TG1 D1 VIN TRACK/SS1 INTVCC CINT 4.7F D2 MTOP2 CB2 0.47F L2 7.2H RSENSE2 8m VOUT2 8.5V COUT2 3A 150F CIN 22F VIN 9V TO 38V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 3.3H RSENSE1 6m COUT1 150F VOUT1 3.3V 5A CSS1 0.1F CSS2 0.1F CITH2 680pF PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ TRACK/SS2 SW2 BG2 RITH2 27k ITH2 MBOT2 CITH2A 100pF RA2 44.2k CF2 39pF RB2 422k VFB2 SENSE2- SENSE2+ C2 1nF COUT1, COUT2: SANYO 10TPD150M D1, D2: CENTRAL SEMI CMDSH-4E L1: SUMIDA CDEP105-3R2M L2: SUMIDA CDEP105-7R2M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP 38581 F12 Figure 13. High Efficiency Dual 3.3V/8.5V Step-Down Converter 38571fa 0 LTC3857-1 Typical applicaTions High Efficiency Dual 2.5V/3.3V Step-Down Converter RB1 144k CF1 22pF RA1 68.1k CITH1A 100pF RITH1 22k CITH1 820pF C1 1nF LTC3857-1 SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 TG1 INTVCC 100k MBOT1 CB1 0.47F MTOP1 D1 VIN CIN 22F VIN 4V TO 38V L1 2.4H RSENSE1 6m COUT1 150F VOUT1 2.5V 5A CSS1 0.01F TRACK/SS1 INTVCC CSS2 0.01F CITH2 820pF PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ TRACK/SS2 SW2 BG2 CINT 4.7F D2 MTOP2 CB2 0.47F L2 3.2H RSENSE2 6m RITH2 15k ITH2 MBOT2 VOUT2 3.3V COUT2 5A 150F CITH2A 150pF RA2 68.1k CF2 15pF RB2 215k VFB2 SENSE2- SENSE2+ C2 1nF COUT1, COUT2: SANYO 4TPE150M D1, D2: CENTRAL SEMI CMDSH-4E L1: SUMIDA CDEP105-2R5 L2: SUMIDA CDEP105-3R2M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP 38571 TA02 38571fa LTC3857-1 Typical applicaTions High Efficiency Dual 12V/5V Step-Down Converter RB1 475k CF1 33pF RA1 34k CITH1A 100pF RITH1 10k CITH1 680pF CSS1 0.01F C1 1nF SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3857-1 TRACK/SS1 VIN INTVCC PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ TRACK/SS2 RITH2 17k ITH2 SW2 BG2 MBOT2 CINT 4.7F D2 MTOP2 CB2 0.47F L2 4.3H RSENSE2 6m VOUT2 5V COUT2 5.5A 150F TG1 D1 CIN 22F VIN 12.5V TO 38V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 8.8H RSENSE1 9m COUT1 47F VOUT1 12V 3A RFREQ 60k CSS2 0.01F CITH2 680pF CITH2A 100pF RA2 75k CF2 15pF RB2 392k VFB2 SENSE2- SENSE2+ C2 1nF COUT1: KEMET T525D476M016E035 COUT2: SANYO 10TPD150M L1: SUMIDA CDEP105-5R7M L2: SUMIDA CDEP105-4R3M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP 38571 TA03 38571fa LTC3857-1 Typical applicaTions High Efficiency Dual 24V/5V Step-Down Converter RB1 487k CF1 18pF RA1 16.9k CITH1A 100pF RITH1 46k CITH1 680pF CSS1 0.01F C1 1nF SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3857-1 TRACK/SS1 VIN INTVCC PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ TRACK/SS2 RITH2 17k ITH2 SW2 BG2 MBOT2 CINT 4.7F D2 MTOP2 CB2 0.47F L2 4.3H RSENSE2 6m VOUT2 5V COUT2 5A 150F TG1 D1 CIN 22F VIN 28V TO 38V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 22H RSENSE1 25m VOUT1 24V 1A COUT1 22F 25V 2 CERAMIC RFREQ 60k CSS2 0.01F CITH2 680pF CITH2A 100pF RA2 75k CF2 15pF RB2 392k VFB2 SENSE2- SENSE2+ COUT2: SANYO 10TPD150M L1: SUMIDA CDR7D43MN L2: SUMIDA CDEP105-4R3M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP C2 1nF 38571 TA04 38571fa LTC3857-1 Typical applicaTions High Efficiency Dual 1V/1.2V Step-Down Converter RB1 28.7k CF1 56pF RA1 115k CITH1A 220pF RITH1 3.93k CITH1 1000pF C1 1nF SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3857-1 VIN TRACK/SS1 INTVCC CINT 4.7F D2 MTOP2 CB2 0.47F L2 0.47H RSENSE2 3.5m VOUT2 1.2V COUT2 8A 220F 2 TG1 D1 CIN 22F VIN 12V CB1 0.47F MTOP1 INTVCC 100k L1 MBOT1 0.47H RSENSE1 3.5m COUT1 220F 2 VOUT1 1V 8A CSS1 0.01F RFREQ 60k CSS2 0.01F CITH2 1000pF PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ TRACK/SS2 SW2 BG2 RITH2 3.43k ITH2 MBOT2 CITH2A 220pF RA2 115k CF2 56pF RB2 57.6k VFB2 SENSE2- SENSE2+ C2 1nF COUT1, COUT2: SANYO 2R5TPE220M L1: SUMIDA CDEP105-3R2M L2: SUMIDA CDEP105-7R2M MTOP1, MTOP2: RENESAS RJK0305 MBOT1, MBOT2: RENESAS RJK0328 38571 TA05 38571fa LTC3857-1 Typical applicaTions High Efficiency Dual 1V/1.2V Step-Down Converter with Inductor DCR Current Sensing RB1 28.7k CF1 56pF RA1 115k CITH1A 200pF RITH1 3.93k CITH1 1000pF C1 0.1F RS1 1.18k SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3857-1 VIN TRACK/SS1 INTVCC PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ TRACK/SS2 SW2 BG2 MBOT2 D2 MTOP2 CB2 0.47F L2 0.47H VOUT2 1.2V COUT2 8A 220F 2 CINT 4.7F TG1 D1 CIN 22F VIN 12V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 0.47H COUT1 220F 2 VOUT1 1V 8A CSS1 0.01F RFREQ 65k CSS2 0.01F CITH2 1000pF RITH2 3.43k ITH2 CITH2A 220pF RA2 115k CF2 56pF RB2 57.6k VFB2 SENSE2- SENSE2+ RS2 1.18k 38571 TA06 C2 0.1F COUT1, COUT2: SANYO 2R5TPE220M L1, L2: SUMIDA IHL ERR47M06 MTOP1, MTOP2: RENESAS RJK0305 MBOT1, MBOT2: RENESAS RJK0328 38571fa LTC3857-1 package DescripTion GN Package 28-Lead Plastic SSOP (Narrow .150 Inch) (ReferenceLTCDWG#05-08-1641) .045 .005 .386 - .393* (9.804 - 9.982) 28 27 26 25 24 23 22 21 20 19 18 17 1615 .033 (0.838) REF .254 MIN .150 - .165 .229 - .244 (5.817 - 6.198) .150 - .157** (3.810 - 3.988) .0165 .0015 RECOMMENDED SOLDER PAD LAYOUT .0250 BSC 1 45 23 4 56 7 8 9 10 11 12 13 14 .004 - .0098 (0.102 - 0.249) .015 .004 (0.38 0.10) .0075 - .0098 (0.19 - 0.25) .016 - .050 (0.406 - 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 0 - 8 TYP .0532 - .0688 (1.35 - 1.75) .008 - .012 (0.203 - 0.305) TYP .0250 (0.635) BSC GN28 (SSOP) 0204 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 38571fa LTC3857-1 revision hisTory REV A DATE 12/09 DESCRIPTION ChangetoAbsoluteMaximumRatings ChangestoElectricalCharacteristics ChangetoTypicalPerformanceCharacteristics ChangetoPinFunctions TextChangestoOperationSection TextChangestoApplicationsInformationSection ChangetoTable2 ChangetoFigure11 ChangestoRelatedParts PAGE NUMBER 2 3,4 6 8,9 11,12,13 21,22,23,26 23 28 38 38571fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However,noresponsibilityisassumedforitsuse.LinearTechnologyCorporationmakesnorepresentationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights. LTC3857-1 relaTeD parTs PART NUMBER LTC3858/LTC3858-1 LTC3868/LTC3868-1 LTC3834/LTC3834-1 LTC3835/LTC3835-1 LT3845 LT3800 LTC3824 LTC3850/LTC3850-1 LTC3850-2 LTC3855 DESCRIPTION LowIQ,DualOutput2-PhaseSynchronousStep-Down DC/DCControllerwith99%DutyCycle LowIQ,DualOutput2-PhaseSynchronousStep-Down DC/DCControllerwith99%DutyCycle LowIQ,SynchronousStep-DownDC/DCController LowIQ,SynchronousStep-DownDC/DCController LowIQ,HighVoltageSynchronousStep-Down DC/DCController LowIQ,HighVoltageSynchronousStep-Down DC/DCController COMMENTS Phase-LockableFixedOperatingFrequency50kHzto900kHz, 4VVIN38V,0.8VVOUT24V,IQ=170A, Phase-LockableFixedOperatingFrequency50kHzto900kHz, 4VVIN24V,0.8VVOUT14V,IQ=170A, Phase-LockableFixedOperatingFrequency140kHzto650kHz, 4VVIN36V,0.8VVOUT10V,IQ=30A, Phase-LockableFixedOperatingFrequency140kHzto650kHz, 4VVIN36V,0.8VVOUT10V,IQ=80A, AdjustableFixedOperatingFrequency100kHzto500kHz, 4VVIN60V,1.23VVOUT36V,IQ=120A,TSSOP-16 Fixed200kHzOperatingFrequency,4VVIN60V,1.23VVOUT36V, IQ=100A,TSSOP-16 LowIQ,HighVoltageDC/DCController,100%DutyCycle SelectableFixed200kHzto600kHzOperatingFrequency, 4VVIN60V,0.8VVOUTVIN,IQ=40A,MSOP-10E Dual2-Phase,HighEfficiencySynchronousStep-Down Phase-LockableFixedOperatingFrequency250kHzto780kHz, DC/DCControllers,RSENSEorDCRCurrentSensingand 4VVIN30V,0.8VVOUT5.25V Tracking Dual,Multiphase,SynchronousDC/DCStep-Down ControllerwithDiffampandDCRTemperature Compensation TripleOutput,MultiphaseSynchronousStep-Down DC/DCController,RSENSEorDCRCurrentSensingand Tracking SmallFootprintWideVINRangeSynchronous Step-DownDC/DCController Phase-LockableFixedOperatingFrequency250kHzto770kHz, 4.5VVIN38V,0.8VVOUT12.5V Phase-LockableFixedOperatingFrequency250kHzto750kHz, 4VVIN24V,VOUTUpto13.5V Fixed400kHzOperatingFrequency4.5VVIN38V, 0.8VVOUT5.25V,2mmx3mmQFN-12,MSOP-12 LTC3853 LTC3854 LTC3775 LTC3851A/ LTC3851A-1 LTC3878/LTC3879 LTM4600HV LTM4601AHV HighFrequencySynchronousVoltageModeStep-Down FastTransientResponse,tON(MIN)=30ns,4VVIN38V, DC/DCController 0.6VVOUT0.8VIN,MSOP-16E,3mmx3mmQFN-16 NoRSENSETMWideVINRangeSynchronousStep-Down DC/DCController NoRSENSEConstantOn-TimeSynchronousStep-Down DC/DCController 10ADC/DCModule(R)CompletePowerSupply 12ADC/DCModuleCompletePowerSupply Phase-LockableFixedOperatingFrequency250kHzto750kHz, 4VVIN38V,0.8VVOUT5.25V,MSOP-16E,3mmx3mmQFN-16, SSOP-16 VeryFastTransientResponse,tON(MIN)=43ns,4VVIN38V, VOUTUp90%ofVIN,MSOP-16E,3mmx3mmQFN-16,SSOP-16 HighEfficiency,CompactSize,UltraFastTMTransientResponse, 4.5VVIN28V,0.8VVOUT5V,15mmx15mmx2.8mm HighEfficiency,CompactSize,UltraFastTransientResponse, 4.5VVIN28V,0.8VVOUT5V,15mmx15mmx2.8mm 38571fa Linear Technology Corporation (408)432-1900 FAX: (408) 434-0507 www.linear.com LT 0110 REV A * PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 LINEAR TECHNOLOGY CORPORATION 2009 |
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