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| FeaTures n n n n n LTC3868-1 Low IQ, Dual 2-Phase Synchronous Step-Down Controller DescripTion TheLTC(R)3868-1isahighperformancedualstep-down switching regulator controller that drives all N-channel synchronouspowerMOSFETstages.Aconstantfrequency current mode architecture allows a phase-lockable frequencyofupto850kHz.Powerlossandnoiseduetothe inputcapacitorESRareminimizedbyoperatingthetwo controlleroutputsoutofphase. The170Ano-loadquiescentcurrentextendsoperating lifeinbatterypoweredsystems.OPTI-LOOPcompensationallowsthetransientresponsetobeoptimizedover awiderangeofoutputcapacitanceandESRvalues.The LTC3868-1featuresaprecision0.8Vreferenceandapower goodoutputindicator.Awide4Vto24Vinputsupplyrange encompassesawiderangeofintermediatebusvoltages andbatterychemistries. Independentsoft-startpinsforeachcontrollerrampthe outputvoltagesduringstart-up.Currentfoldbacklimits MOSFETheatdissipationduringshort-circuitconditions. Theoutputshort-circuitlatchofffeaturefurtherprotects thecircuitinshort-circuitconditions. Foraleadless32-pinQFNpackagewiththeadditionalfeaturesofadjustablecurrentlimit,clockout,phasemodulationandtwoPGOODoutputs,seetheLTC3868datasheet. L,LT,LTC,LTM,BurstMode,OPTI-LOOP ,Module,LinearTechnologyandtheLinearlogo areregisteredtrademarksandNoRSENSEandUltraFastaretrademarksofLinearTechnology Corporation.Allothertrademarksarethepropertyoftheirrespectiveowners.ProtectedbyU.S. Patents,including5481178,5705919,5929620,6100678,6144194,6177787,6304066,6580258. n n n n n n n n n n n n n Low Operating IQ: 170A (One Channel On) Wide Output Voltage Range: 0.8V VOUT 14V Wide VIN Range: 4V to 24V RSENSE or DCR Current Sensing Out-of-PhaseControllersReduceRequiredInput CapacitanceandPowerSupplyInducedNoise OPTI-LOOP(R)CompensationMinimizesCOUT Phase-LockableFrequency(75kHzto850kHz) ProgrammableFixedFrequency(50kHzto900kHz) SelectableContinuous,Pulse-Skippingor BurstMode(R)OperationatLightLoads VeryLowDropoutOperation:99%DutyCycle AdjustableOutputVoltageSoft-Start PowerGoodOutputVoltageMonitor OutputOvervoltageProtection OutputLatchoffProtectionDuringShortCircuit LowShutdownIQ:8A InternalLDOPowersGateDrivefromVINorEXTVCC NoCurrentFoldbackDuringStart-Up Small4mmx5mmQFNandNarrowSSOPPackages NotebookandPalmtopComputers PortableInstruments BatteryOperatedDigitalDevices DistributedDCPowerSystems applicaTions n n n n Typical applicaTion TG1 3.3H 0.1F VIN High Efficiency Dual 8.5V/3.3V Step-Down Converter 4.7F INTVCC TG2 22F 50V VIN 9V TO 24V 100 90 7.2H EFFICIENCY (%) 80 70 60 50 40 30 20 10 Efficiency and Power Loss vs Load Current 10000 1000 POWER LOSS (mW) EFFICIENCY POWER LOSS BOOST1 SW1 BG1 LTC3868-1 BOOST2 SW2 BG2 PGND SENSE2+ 0.1F 100 10 SENSE1+ 0.007 VOUT1 3.3V 5A SENSE1- VFB1 ITH1 SS1 0.1F SGND 0.01 SENSE2- VFB2 ITH2 SS2 0.1F VOUT2 8.5V 3.5A 150F 62.5k 150F 20k 193k 680pF 15k 20k 680pF 15k 0 0.0001 VIN = 12V VOUT = 3.3V FIGURE 12 CIRCUIT 0.001 0.01 0.1 1 OUTPUT CURRENT (A) 10 38681 TA01b 1 0.1 38681 TA01 38681fb LTC3868-1 absoluTe MaxiMuM raTings (Note 1) InputSupplyVoltage(VIN)......................... -0.3Vto28V TopsideDriverVoltages BOOST1,BOOST2................................ -0.3Vto34V . SwitchVoltage(SW1,SW2)........................ -5Vto28V (BOOST1-SW1),(BOOST2-SW2)................ -0.3Vto6V RUN1,RUN2............................................... -0.3Vto8V MaximumCurrentSourcedintoPinfrom Source>8V......................................................100A SENSE1+,SENSE2+,SENSE1- SENSE2-Voltages..................................... -0.3Vto16V . PLLIN/MODE,FREQVoltages.............. -0.3VtoINTVCC EXTVCC ..................................................... -0.3Vto14V . ITH1,ITH2,VFB1,VFB2Voltages..................... -0.3Vto6V PGOOD1Voltage......................................... -0.3Vto6V SS1,SS2,INTVCCVoltages......................... -0.3Vto6V OperatingTemperatureRange(Note2).... -40Cto85C JunctionTemperature(Note3)............................. 125C . StorageTemperatureRange.................. -65Cto150C LeadTemperature(Soldering,10sec) SSOP................................................................ 300C pin conFiguraTion TOP VIEW PGOOD1 SW1 ITH1 VFB1 SENSE1+ 22 BOOST1 21 BG1 20 VIN 29 SGND 19 PGND 18 EXTVCC 17 INTVCC 16 BG2 15 BOOST2 9 10 11 12 13 14 SENSE2+ VFB2 ITH2 SS2 TG2 SW2 SENSE1- FREQ PLLIN/MODE SGND RUN1 RUN2 SENSE2- SENSE2+ 1 2 3 4 5 6 7 8 9 10 11 VFB1 ITH1 SS1 TG1 TOP VIEW 28 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 28 27 26 25 24 23 SENSE1+ 1 SENSE1- 2 FREQ 3 PLLIN/MODE 4 SGND 5 RUN1 6 RUN2 7 SENSE2- 8 VFB2 12 ITH2 13 SS2 14 TJMAX=125C,JA=43C/W EXPOSEDPAD(PIN29)ISSGND,MUSTBESOLDEREDTOPCB UFD PACKAGE 28-LEAD (4mm 5mm) PLASTIC QFN GN PACKAGE 28-LEAD PLASTIC SSOP TJMAX=125C,JA=90C/W orDer inForMaTion LEAD FREE FINISH LTC3868EUFD-1#PBF LTC3868IUFD-1#PBF LTC3868EGN-1#PBF LTC3868IGN-1#PBF TAPE AND REEL LTC3868EUFD-1#TRPBF LTC3868IUFD-1#TRPBF LTC3868EGN-1#TRPBF LTC3868IGN-1#TRPBF PART MARKING* 38681 38681 LTC3868GN-1 LTC3868GN-1 PACKAGE DESCRIPTION 28-Lead(4mmx5mm)PlasticQFN 28-Lead(4mmx5mm)PlasticQFN 28-LeadPlasticSSOP 28-LeadPlasticSSOP TEMPERATURE RANGE -40Cto85C -40Cto85C -40Cto85C -40Cto85C ConsultLTCMarketingforpartsspecifiedwithwideroperatingtemperatureranges.*Thetemperaturegradeisidentifiedbyalabelontheshipping container.ConsultLTCMarketingforinformationonnon-standardleadbasedfinishparts. Formoreinformationonleadfreepartmarking,goto:http://www.linear.com/leadfree/ Formoreinformationontapeandreelspecifications,goto:http://www.linear.com/tapeandreel/ 38681fb LTC3868-1 elecTrical characTerisTics SYMBOL VIN VFB1,2 IFB1,2 VREFLNREG VLOADREG PARAMETER InputSupplyOperatingVoltageRange RegulatedFeedbackVoltage FeedbackCurrent ReferenceVoltageLineRegulation OutputVoltageLoadRegulation (Note4)ITH1,2Voltage=1.2V (Note4) (Note4)VIN=4.5Vto24V (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,RelativetoRegulated VFB1,2 EachChannel EachChannel VOUT1,2 l l l l l The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted. CONDITIONS MIN 4 0.788 0.8 5 0.002 0.01 -0.01 2 1.3 TYP MAX 24 0.812 50 0.02 0.1 -0.1 UNITS V V nA %/V % % mmho mA l 2 mA 170 250 A SleepMode(BothChannelsOn) Shutdown UVLO VOVL ISENSE+ ISENSE - 300 8 3.6 7 4 3.8 10 450 25 4.2 4 13 1 A A V V % A A A % A V mV V V A mV UndervoltageLockout FeedbackOvervoltageProtection SENSE+PinCurrent SENSE -PinsCurrent 540 98 0.7 1.21 1.9 1.3 7 43 99.4 1 1.26 50 2 1.5 10 50 2.5 1.5 2.4 1.1 1 700 1.4 1.31 2.1 1.7 13 57 DFMAX ISS1,2 VRUN1,2On VSS1,2LA VSS1,2LT IDSC1,2LT MaximumDutyFactor Soft-StartChargeCurrent RUNPinOnThreshold SSPinLatchoffArmingThreshold SSPinLatchoffThreshold SSDischargeCurrent VRUN1,2Hyst RUNPinHysteresis VSENSE(MAX) MaximumCurrentSenseThreshold Gate Driver TG1,2 BG1,2 Pull-UpOn-Resistance Pull-DownOn-Resistance Pull-UpOn-Resistance Pull-DownOn-Resistance 38681fb LTC3868-1 elecTrical characTerisTics SYMBOL TG1,2tr TG1,2t f BG1,2tr BG1,2t f TG/BGt1D BG/TGt1D tON(MIN) VINTVCCVIN VLDOVIN VINTVCCEXT VLDOEXT VEXTVCC VLDOHYS f 25k f 65k f105k fLOW fHIGH fSYNC VPGL IPGOOD VPG PARAMETER TGTransitionTime: RiseTime FallTime BGTransitionTime: RiseTime FallTime 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(PGOODLow) Note 1:StressesbeyondthoselistedunderAbsoluteMaximumRatings maycausepermanentdamagetothedevice.ExposuretoanyAbsolute MaximumRatingsforextendedperiodsmayaffectdevicereliabilityand lifetime. Note 2:TheLTC3868E-1isguaranteedtomeetperformancespecifications from0Cto85C.Specificationsoverthe-40Cto85Coperating temperaturerangeareassuredbydesign,characterizationandcorrelation withstatisticalprocesscontrols.TheLTC3868I-1isguaranteedoverthe full-40Cto85Coperatingtemperaturerange. Note 3:TJiscalculatedfromtheambienttemperatureTAandpower dissipationPDaccordingtothefollowingformula: TJ=TA+(PD*JA) -13 7 -10 2.5 10 2.5 25 l The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted. CONDITIONS (Note6) CLOAD=3300pF CLOAD=3300pF (Note6) CLOAD=3300pF CLOAD=3300pF CLOAD=3300pFEachDriver CLOAD=3300pFEachDriver (Note7) 6V Oscillator and Phase-Locked Loop PGOOD1 Output whereJA=43CfortheQFNpackageandJA=90CfortheSSOP package. Note 4:TheLTC3868-1istestedinafeedbackloopthatservosVITH1,2toa specifiedvoltageandmeasurestheresultantVFB1,2. Note 5:Dynamicsupplycurrentishigherduetothegatechargebeing deliveredattheswitchingfrequency.SeeApplicationsinformation. Note 6:Riseandfalltimesaremeasuredusing10%and90%levels.Delay timesaremeasuredusing50%levels. Note 7:Theminimumon-timeconditionisspecifiedforaninductor peak-to-peakripplecurrent40%ofIMAX(SeeMinimumOn-Time ConsiderationsintheApplicationsInformationsection). 38681fb LTC3868-1 Typical perForMance characTerisTics Efficiency and Power Loss vs Output Current 100 FIGURE 12 CIRCUIT 90 VIN = 12V VOUT = 3.3V 80 70 60 50 40 30 20 10 0 0.0001 0.001 Burst Mode OPERATION PULSESKIPPING FCM 0.01 0.1 1 OUTPUT CURRENT (A) 10 38681 G01 Efficiency vs Load Current 10000 100 90 1000 POWER LOSS (mW) EFFICIENCY (%) 80 70 60 50 40 30 20 10 0.1 0 0.0001 0.001 VOUT = 3.3V FIGURE 12 CIRCUIT 0.01 0.1 1 OUTPUT CURRENT (A) 10 38681 G02 VIN = 5V VIN = 12V EFFICIENCY (%) 100 10 1 Efficiency vs Input Voltage 98 96 94 EFFICIENCY (%) 92 90 88 86 84 82 80 0 5 20 10 15 INPUT VOLTAGE (V) 25 28 IL 2A/DIV FIGURE 12 CIRCUIT VOUT = 3.3V IOUT = 4A VOUT 100mV/DIV ACCOUPLED Load Step (Burst Mode Operation) VOUT 100mV/DIV ACCOUPLED Load Step (Forced Continuous Mode) IL 2A/DIV VOUT = 3.3V 20s/DIV FIGURE 12 CIRCUIT 38681 G04 20s/DIV VOUT = 3.3V FIGURE 12 CIRCUIT 38681 G05 38681 G03 Load Step (Pulse-Skipping Mode) VOUT 100mV/DIV ACCOUPLED Inductor Current at Light Load Soft Start-Up FORCED CONTINUOUS MODE Burst Mode OPERATION 2A/DIV PULSESKIPPING MODE VOUT = 3.3V 20s/DIV FIGURE 12 CIRCUIT 38681 G06 VOUT2 2V/DIV VOUT1 2V/DIV IL 2A/DIV VOUT = 3.3V 2s/DIV ILOAD = 200A FIGURE 12 CIRCUIT 38681 G07 20ms/DIV FIGURE 12 CIRCUIT 38681 G08 38681fb LTC3868-1 Typical perForMance characTerisTics Total Input Supply Current vs Input Voltage 400 350 SUPPLY CURRENT (A) 300 250 200 150 100 50 0 5 10 20 INPUT VOLTAGE (V) 15 25 28 NO LOAD EXTVCC AND INTVCC VOLTAGE (V) FIGURE 12 CIRCUIT VOUT = 3.3V ONE CHANNEL ON 300A LOAD 5.6 5.4 INTVCC VOLTAGE (V) 5.2 5.0 4.8 4.6 4.4 4.2 4.0 -45 -20 5 80 55 30 TEMPERATURE (C) 105 130 5.0 0 5 10 15 20 INPUT VOLTAGE (V) 25 28 38681 G12 EXTVCC Switchover and INTVCC Voltages vs Temperature 5.2 INTVCC Line Regulation INTVCC EXTVCC RISING EXTVCC FALLING 5.2 5.1 5.1 38681 G10 38681 G11 CURRENT SENSE THRESHOLD (mV) -50 -100 SENSE- CURRENT (A) -150 -200 -250 -300 -350 -400 -450 -500 -550 -600 0 10 5 VSENSE COMMON MODE VOLTAGE (V) 15 60 40 20 0 -20 -40 MAXIMUM CURRENT SENSE VOLTAGE (mV) 80 Maximum Current Sense Voltage vs ITH Voltage PULSE-SKIPPING FORCED CONTINUOUS Burst Mode OPERATION (FALLING) Burst Mode OPERATION (RISING) 0 SENSE- Pins Input Bias Current 80 Maximum Current Sense Threshold vs Duty Cycle 60 40 20 5% DUTY CYCLE 0 0.2 0.4 0.6 0.8 1.0 ITH PIN VOLTAGE 1.2 1.4 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 38681 G15 38681 G13 38681 G14 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) 38681 G16 Quiescent Current vs Temperature 240 230 PLLIN/MODE = 0 V = 12V 220 VIN = 3.3V OUT 210 ONE CHANNEL ON 200 190 180 170 160 150 140 130 120 110 80 55 5 -45 -20 30 TEMPERATURE (C) 10 9 8 7 6 5 Shutdown Current vs Temperature SHUTDOWN CURRENT (A) 105 130 4 -45 -20 55 30 80 5 TEMPERATURE (C) 105 130 38681 G17 38681 G18 38681fb LTC3868-1 Typical perForMance characTerisTics Soft-Start Pull-Up Current vs Temperature 1.20 1.15 SS PULL-UP CURRENT (A) RUN PIN VOLTAGE (V) 1.10 1.05 1.00 0.95 0.90 0.85 0.80 -45 -20 5 80 55 30 TEMPERATURE (C) 105 130 1.40 REGULATED FEEDBACK VOLTAGE (mV) 1.35 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0.95 0.90 -45 -20 55 30 5 80 TEMPERATURE (C) 105 130 Shutdown (RUN) Threshold vs Temperature 808 806 804 802 800 798 796 794 Regulated Feedback Voltage vs Temperature 792 -45 -20 5 80 55 30 TEMPERATURE (C) 105 130 38681 G19 38681 G20 38681 G21 SENSE- Pin Input Current vs Temperature 50 0 -50 -100 -150 -200 -250 -300 -350 -400 -450 -500 -550 -600 -45 14 VOUT = 3.3V INPUT CURRENT (A) 12 Shutdown Current vs Input Voltage 800 700 600 FREQUENCY (kHz) 500 400 300 200 100 5 10 20 15 INPUT VOLTAGE (V) 25 28 Oscillator Frequency vs Temperature SENSE- CURRENT (A) 10 8 6 4 2 0 FREQ = INTVCC FREQ = GND VOUT = 28V -20 80 55 5 30 TEMPERATURE (C) 105 130 0 -45 -20 5 80 55 30 TEMPERATURE (C) 105 130 38681 G22 38681 G23 38681 G24 Oscillator Frequency vs Input Voltage 356 OSCILLATOR FREQUENCY (kHz) 354 352 350 348 346 344 INTVCC VOLTAGE (V) 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 5 10 20 15 INPUT VOLTAGE (V) 25 28 Undervoltage Lockout Threshold vs Temperature 3.4 -45 -20 55 30 5 80 TEMPERATURE (C) 105 130 38681 G28 38681 G25 38681fb LTC3868-1 Typical perForMance characTerisTics INTVCC vs Load Current 5.20 VIN = 12V 2.3 2.2 2.1 INTVCC VOLTAGE (V) 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 4.95 0 20 40 60 80 100 120 140 160 180 200 LOAD CURRENT (mA) 38681 G26 Latchoff Thresholds vs Temperature 5.15 INTVCC VOLTAGE (V) 5.10 EXTVCC = 0V 5.05 5.00 EXTVCC = 8V ARMING THRESHOLD LATCH-OFF THRESHOLD 1.2 -45 -20 55 30 5 80 TEMPERATURE (C) 105 130 38681 G27 pin FuncTions (QFN/SSOP) SENSE1-, SENSE2- (Pin 2, Pin 4/Pin 8, Pin 10):The(-) InputtotheDifferentialCurrentComparators.Whengreater thanINTVCC-0.5V,theSENSE-pinsuppliescurrentto thecurrentcomparator. FREQ (Pin 3/Pin 5):TheFrequencyControlPinforthe InternalVCO.ConnectingthispintoGNDforcestheVCO toafixedlowfrequencyof350kHz.Connectingthispin to INTVCC forces the VCO to a fixed high frequency of 535kHz.Otherfrequenciesbetween50kHzand900kHzcan beprogrammedusingaresistorbetweenFREQandGND. Aninternal20Apull-upcurrentdevelopsthevoltageto beusedbytheVCOtocontrolthefrequency PLLIN/MODE (Pin 4/Pin 6): External Synchronization Input to Phase Detector and Forced Continuous Mode Input.Whenanexternalclockisappliedtothispin,the phase-lockedloopwillforcetherisingTG1signaltobe synchronizedwiththerisingedgeoftheexternalclock. Whennotsynchronizingtoanexternalclock,thisinput, which acts on both controllers, determines how the LTC3868-1 operates at light loads. Pulling this pin to groundselectsBurstModeoperation.Aninternal100k resistor to ground also invokes Burst Mode operation whenthepinisfloated.TyingthispintoINTVCCforces continuousinductorcurrentoperation.Tyingthispinto avoltagegreaterthan1.2VandlessthanINTVCC-1.3V selectspulse-skippingoperation. SGND (Pin 5, Exposed Pad Pin 29/Pin 7):Small-signal ground common to both controllers, must be routed separatelyfromhighcurrentgroundstothecommon(-) terminalsoftheCINcapacitors.Theexposedpad(QFN only) must be soldered to the PCB for rated thermal performance. RUN1, RUN2 (Pin 6, Pin 8/Pin 7, Pin 9): Digital Run ControlInputsforEachController.Forcingeitherofthese pinsbelow1.26Vshutsdownthatcontroller.Forcingboth ofthesepinsbelow0.7VshutsdowntheentireLTC3868-1, reducingquiescentcurrenttoapproximately8A.Donot floatthesepins. 38681fb LTC3868-1 pin FuncTions (QFN/SSOP) INTVCC (Pin 17/Pin 19):OutputoftheInternalLinearLow Dropout Regulator. The driver and control circuits are poweredfromthisvoltagesource.Mustbedecoupledto powergroundwithaminimumof4.7Fceramicorother low ESR capacitor. Do not use the INTVCC pin for any otherpurpose. EXTVCC (Pin 18/Pin 20): External Power Input to an Internal LDO Connected to INTVCC. This LDO supplies INTVCCpower,bypassingtheinternalLDOpoweredfrom VIN whenever EXTVCC is higher than 4.7V. See EXTVCC ConnectionintheApplicationsInformationsection.Do notexceed14Vonthispin. PGND (Pin 19/Pin 21):DriverPowerGround.Connectsto thesourcesofbottom(synchronous)N-channelMOSFETs andthe(-)terminal(s)ofCIN. VIN (Pin 20/Pin 22):MainSupplyPin.Abypasscapacitorshouldbetiedbetweenthispinandthesignalground pin. BG1, BG2 (Pin 21, Pin 23/Pin 16, Pin 18):HighCurrent Gate Drives for Bottom (Synchronous) N-Channel MOSFETs. Voltage swing at these pins is from ground toINTVCC. BOOST1, BOOST2 (Pin 22, Pin 24/Pin 15, Pin 17): Bootstrapped Supplies to the Topside Floating Drivers. CapacitorsareconnectedbetweentheBOOSTandSWpins andSchottkydiodesaretiedbetweentheBOOSTandINTVCC pins.VoltageswingattheBOOSTpinsisfromINTVCCto (VIN+INTVCC). SW1, SW2 (Pin 23, Pin 25/Pin 14, Pin 16):SwitchNode ConnectionstoInductors. TG1, TG2 (Pin 24, Pin 26/Pin 13, Pin 15):HighCurrent GateDrivesforTopN-ChannelMOSFETs.Thesearethe outputsoffloatingdriverswithavoltageswingequalto INTVCC-0.5Vsuperimposedontheswitchnodevoltage SW. PGOOD1 (Pin 25/Pin 27): Open-Drain Logic Output. PGOOD1ispulledtogroundwhenthevoltageontheVFB1 pinisnotwithin10%ofitssetpoint. SS1, SS2 (Pin 26, Pin 28/Pin 12, Pin 14):ExternalSoftStartInput.TheLTC3868-1regulatestheVFB1,2voltage tothesmallerof0.8VorthevoltageontheSS1,2pin.An internal1Apull-upcurrentsourceisconnectedtothis pin.Acapacitortogroundatthispinsetstheramptime tofinalregulatedoutputvoltage.Thispinisalsousedas theshort-circuitlatchofftimer. ITH1, ITH2 (Pin 27, Pin 1/Pin 11, Pin 13):ErrorAmplifier OutputsandSwitchingRegulatorCompensationPoints. Eachassociatedchannel'scurrentcomparatortrippoint increaseswiththiscontrolvoltage. VFB1, VFB2 (Pin 28, Pin 2/Pin 10, Pin 12):Receivesthe remotelysensedfeedbackvoltageforeachcontrollerfrom anexternalresistivedivideracrosstheoutput. SENSE1+, SENSE2+ (Pin 1, Pin 3/Pin 9, Pin 11):The(+) inputtothedifferentialcurrentcomparatorsarenormally connectedtoDCRsensingnetworksorcurrentsensing resistors.TheITHpinvoltageandcontrolledoffsetsbetween theSENSE-andSENSE+pinsinconjunctionwithRSENSE setthecurrenttripthreshold. 38681fb LTC3868-1 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(VFB) 0.45V SLOPE COMP + VIN EXTVCC OV 5.1V LDO EN + 5.1V LDO EN 0.5A SHDN RST 2(VFB) FOLDBACK 1A SS 0 - 4.7V SGND INTVCC RUN 11V SHORT CKT LATCH-OFF SHDN 10A 38681 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 PFD ICMP SENSE+ SENSE- VFB EA 0.80V TRACK/SS RA RB 0.88V ITH CC CC2 RC CSS 38681fb LTC3868-1 operaTion (Refer to the Functional Diagram) Main Control Loop TheLTC3868-1usesaconstantfrequency,currentmode step-downarchitecturewiththetwocontrollerchannels operating180degreesoutofphase.Duringnormaloperation,eachexternaltopMOSFETisturnedonwhenthe clockforthatchannelsetstheRSlatch,andisturnedoff whenthemaincurrentcomparator,ICMP ,resetstheRS latch.ThepeakinductorcurrentatwhichICMPtripsand resetsthelatchiscontrolledbythevoltageontheITHpin, 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 asoneoftheLTC3868-1switchingregulatoroutputs. EachtopMOSFETdriverisbiasedfromthefloatingbootstrapcapacitor,CB,whichnormallyrechargesduringeach cyclethroughanexternaldiodewhenthetopMOSFET turnsoff.IftheinputvoltageVINdecreasestoavoltage closetoVOUT,theloopmayenterdropoutandattempt to turn on the top MOSFET continuously. The dropout detectordetectsthisandforcesthetopMOSFETofffor aboutone-twelfthoftheclockperiodeverytenthcycleto allowCBtorecharge. Shutdown and Start-Up (RUN1, RUN2 and SS1, SS2 Pins) ThetwochannelsoftheLTC3868-1canbeindependently shutdownusingtheRUN1andRUN2pins.Pullingeitherof thesepinsbelow1.26Vshutsdownthemaincontrolloop forthatcontroller.Pullingbothpinsbelow0.7Vdisables bothcontrollersandmostinternalcircuits,includingthe INTVCCLDOs.Inthisstate,theLTC3868-1drawsonly8A ofquiescentcurrent. TheRUNpinmaybeexternallypulledupordrivendirectly bylogic.WhendrivingtheRUNpinwithalowimpedance source,donotexceedtheabsolutemaximumratingof 8V.TheRUNpinhasaninternal11Vvoltageclampthat allowstheRUNpintobeconnectedthrougharesistortoa highervoltage(forexample,VIN),solongasthemaximum currentintotheRUNpindoesnotexceed100A. Thestart-upofeachcontroller'soutputvoltageVOUTis controlledbythevoltageontheSSpinforthatchannel. When the voltage on the SS pin is less than the 0.8V internalreference,theLTC3868-1regulatestheVFBvoltagetotheSSpinvoltageinsteadofthe0.8Vreference. ThisallowstheSSpintobeusedtoprogramasoft-start byconnectinganexternalcapacitorfromtheSSpinto SGND.Aninternal1Apull-upcurrentchargesthiscapacitorcreatingavoltagerampontheSSpin.AstheSS voltageriseslinearlyfrom0Vto0.8V(andbeyondupto theabsolutemaximumratingof6V),theoutputvoltage VOUTrisessmoothlyfromzerotoitsfinalvalue. Short-Circuit Latchoff Afterthecontrollerhasbeenstartedandbeengivenadequatetimetorampuptheoutputvoltage,theSScapacitorisusedinashort-circuittimeoutcircuit.Specifically, oncethevoltageontheSSpinrisesabove2V(thearming threshold),theshort-circuittimeoutcircuitisenabled(see Figure1).Iftheoutputvoltagefallsbelow70%ofitsnominalregulatedvoltage,theSScapacitorbeginsdischargingwithanet9Apulldowncurrentontheassumption thattheoutputisinanovercurrentand/orshort-circuit condition.Iftheconditionlastslongenoughtoallowthe SSpinvoltagetofallbelow1.5V(thelatchoffthreshold), thecontrollerwillshutdown(latchoff)untiltheRUNpin voltageortheVINvoltageisrecycled. 38681fb LTC3868-1 operaTion (Refer to the Functional Diagram) INTVCC SS VOLTAGE 2V 0.8V LATCHOFF COMMAND 0V 1A -9A OUTPUT VOLTAGE LATCHOFF ENABLE 38681 F01 1.5V currentisprovidedduetointernalcurrentfoldbackand actualpowerwastedislowduetotheefficientnatureof thecurrentmodeswitchingregulator.Foldbackcurrent limitingisdisabledduringthesoft-startinterval(aslong astheVFBvoltageiskeepingupwiththeSSvoltage). Light Load Current Operation (Burst Mode Operation, Pulse-Skipping or Forced Continuous Mode) (PLLIN/MODE Pin) TheLTC3868-1canbeenabledtoenterhighefficiency BurstModeoperation,constantfrequencypulse-skipping mode,orforcedcontinuousconductionmodeatlowload currents.ToselectBurstModeoperation,tiethePLLIN/ MODEpintoground.Toselectforcedcontinuousoperation,tiethePLLIN/MODEpintoINTVCC.Toselectpulseskippingmode,tiethePLLIN/MODEpintoaDCvoltage greaterthan1.2VandlessthanINTVCC-1.3V. WhenacontrollerisenabledforBurstModeoperation, theminimumpeakcurrentintheinductorissettoapproximately 30% 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. Insleepmode,muchoftheinternalcircuitryisturnedoff, reducingthequiescentcurrent.Ifonechannelisshutdown andtheotherchannelisinsleepmode,theLTC3868-1 drawsonly170Aofquiescentcurrent.Ifbothchannels areinsleepmode,theLTC3868-1drawsonly300Aofquiescentcurrent.Insleepmode,theloadcurrentissupplied bytheoutputcapacitor.Astheoutputvoltagedecreases, theEA'soutputbeginstorise.Whentheoutputvoltage dropsenough,theITHpinisreconnectedtotheoutput oftheEA,thesleepsignalgoeslow,andthecontroller resumesnormaloperationbyturningonthetopexternal MOSFETonthenextcycleoftheinternaloscillator. WhenacontrollerisenabledforBurstModeoperation, theinductorcurrentisnotallowedtoreverse.Thereverse SS PIN CURRENT 1A ARMING SOFT-START INTERVAL tLATCH Figure 1. Latchoff Timing Diagram Thedelaytimefromwhenashort-circuitoccursuntilthe controllerlatchesoffcanbecalculatedusingthefollowingequation tLATCH~CSS(VSS-1.5V)/9A whereVSSistheinitialvoltage(mustbegreaterthan2V) ontheSSpinatthetimetheshort-circuitoccurs.Normally theSSpinvoltagewillhavebeenpulleduptotheINTVCC voltage(5.1V)bytheinternal1Apull-upcurrent. NotethatthetwocontrollersontheLTC3868-1haveseparate,independentshort-circuitlatchoffcircuits.Latchoff canbeoverridden/defeatedbyconnectingaresistor150k orlessfromtheSSpintoINTVCC.Thisresistorprovides enoughpull-upcurrenttoovercomethe9Apull-down currentpresentduringashort-circuit.Notethatthisresistoralsoshortensthesoft-startperiod. Foldback Current Ontheotherhand,whentheoutputvoltagefallstoless than70%ofitsnominallevel,foldbackcurrentlimiting isalsoactivated,progressivelyloweringthepeakcurrent limitinproportiontotheseverityoftheovercurrentor short-circuitcondition.Evenifashort-circuitispresent and the short-circuit latchoff is not yet enabled (when SSvoltagehasnotyetreached2V),asafe,lowoutput 38681fb LTC3868-1 operaTion (Refer to the Functional Diagram) current comparator, IR, turns off the bottom external MOSFETjustbeforetheinductorcurrentreacheszero, preventing it from reversing and going negative. Thus, thecontrollerisindiscontinuousoperation. In forced continuous operation or when clocked by an externalclocksourcetousethephase-lockedloop(see Frequency Selection and Phase-Locked Loop section), theinductorcurrentisallowedtoreverseatlightloads or under large transient conditions. The peak inductor currentisdeterminedbythevoltageontheITHpin,just asinnormaloperation.Inthismode,theefficiencyatlight loads is lower than in Burst Mode operation. However, continuousoperationhastheadvantagesofloweroutput voltagerippleandlessinterferencetoaudiocircuitry.In forcedcontinuousmode,theoutputrippleisindependent ofloadcurrent. WhenthePLLIN/MODEpinisconnectedforpulse-skipping mode, the LTC3868-1 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 reducedRFinterferencewhencomparedtoBurstMode operation. It provides higher light load efficiency than 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. TheswitchingfrequencyoftheLTC3868-1'scontrollers canbeselectedusingtheFREQpin. IfthePLLIN/MODEpinisnotbeingdrivenbyanexternal clocksource,theFREQpincanbetiedtoSGND,tiedto INTVCCorprogrammedthroughanexternalresistor.Tying FREQtoSGNDselects350kHzwhiletyingFREQtoINTVCC selects 535kHz. Placing a resistor between FREQ and SGNDallowsthefrequencytobeprogrammedbetween 50kHzand900kHz. Aphase-lockedloop(PLL)isavailableontheLTC3868-1 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. TheVCOinputvoltageisprebiasedtotheoperatingfrequencysetbytheFREQpinbeforetheexternalclockis applied.Ifprebiasedneartheexternalclockfrequency, thePLLlooponlyneedstomakeslightchangestothe VCOinputinordertosynchronizetherisingedgeofthe externalclock'stotherisingedgeofTG1.Theabilityto prebiastheloopfilterallowsthePLLtolock-inrapidly withoutdeviatingfarfromthedesiredfrequency. Thetypicalcapturerangeofthephase-lockedloopisfrom approximately55kHzto1MHz,withaguaranteeoverall manufacturingvariationstobebetween75kHzand850kHz. Inotherwords,theLTC3868-1'sPLLisguaranteedtolock toanexternalclocksourcewhosefrequencyisbetween 75kHzand850kHz. ThetypicalinputclockthresholdsonthePLLIN/MODE pinare1.6V(rising)and1.1V(falling). Output Overvoltage Protection Anovervoltagecomparatorguardsagainsttransientovershootsaswellasothermoreseriousconditionsthatmay overvoltagetheoutput.WhentheVFBpinrisesbymore than10%aboveitsregulationpointof0.800V,thetop MOSFETisturnedoffandthebottomMOSFETisturned onuntiltheovervoltageconditioniscleared. 38681fb LTC3868-1 operaTion (Refer to the Functional Diagram) Power Good (PGOOD) Pin The PGOOD1 pin is connected to an open drain of an internalN-channelMOSFET.TheMOSFETturnsonand pullsthePGOOD1pinlowwhenthecorrespondingVFB1pin voltageisnotwithin10%ofthe0.8Vreferencevoltage. ThePGOOD1pinisalsopulledlowwhentheRUN1pin islow(shutdown).WhentheVFB1pinvoltageiswithin the10%requirement,theMOSFETisturnedoffandthe pinisallowedtobepulledupbyanexternalresistortoa sourcenogreaterthan6V. Theory and Benefits of 2-Phase Operation Why the need for 2-phase operation? Up until the 2-phasefamily,constantfrequencydualswitchingregulatorsoperatedbothchannelsinphase(i.e.,singlephase 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 together.TheresultisasignificantreductionintotalRMS inputcurrent,whichinturnallowslessexpensiveinput capacitorstobeused,reducesshieldingrequirementsfor EMIandimprovesrealworldoperatingefficiency. Figure2comparestheinputwaveformsforarepresentative singlephasedualswitchingregulatortotheLTC3868-1 2-phase dual switching regulator. An actual measurementoftheRMSinputcurrentundertheseconditions showsthat2-phaseoperationdroppedtheinputcurrent from2.53ARMSto1.55ARMS.Whilethisisanimpressive reductioninitself,rememberthatthepowerlossesare 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 38681 F01 Figure 2. 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 38681fb LTC3868-1 operaTion (Refer to the Functional Diagram) proportionaltoIRMS2,meaningthattheactualpowerwasted isreducedbyafactorof2.66.Thereducedinputripple voltagealsomeanslesspowerislostintheinputpower path,whichcouldincludebatteries,switches,trace/connectorresistancesandprotectioncircuitry.Improvements inbothconductedandradiatedEMIalsodirectlyaccrueas aresultofthereducedRMSinputcurrentandvoltage. Ofcourse,theimprovementaffordedby2-phaseoperationisafunctionofthedualswitchingregulator'srelative dutycycleswhich,inturn,aredependentupontheinput voltageVIN(DutyCycle=VOUT/VIN).Figure3showshow theRMSinputcurrentvariesforsingle-phaseand2-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 38681 F03 Figure 3. RMS Input Current Comparison 38681fb LTC3868-1 applicaTions inForMaTion The Typical Application on the first page is a basic LTC3868-1applicationcircuit.LTC3868-1canbeconfigured touseeitherDCR(inductorresistance)sensingorlow valueresistorsensing.Thechoicebetweenthetwocurrentsensingschemesislargelyadesigntradeoffbetween cost,powerconsumptionandaccuracy.DCRsensingis becoming popular because it saves expensive current 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 TheSENSE+andSENSE-pinsaretheinputstothecurrentcomparators.Thecommonmodevoltagerangeon thesepinsis0Vto16V(AbsoluteMaximum),enabling theLTC3868-1toregulateoutputvoltagesuptoanominal 14V(allowingmarginfortolerancesandtransients). 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 placedclosetotheLTC3868-1,andthesenselinesshould runclosetogethertoaKelvinconnectionunderneaththe currentsenseelement(showninFigure4).Sensingcurrent elsewhere can effectively add parasitic inductance andcapacitancetothecurrentsenseelement,degrading the information at the sense terminals and making the programmedcurrentlimitunpredictable.IfinductorDCR sensingisused(Figure5b),resistorR1shouldbeplaced closetotheswitchingnode,topreventnoisefromcoupling intosensitivesmall-signalnodes. TO SENSE FILTER, NEXT TO THE CONTROLLER 38681 F04 COUT INDUCTOR OR RSENSE Figure 4. Sense Lines Placement with Inductor or Sense Resistor VIN INTVCC BOOST TG SW LTC3868-1 BG VOUT VIN SENSE+ SENSE- SGND PLACE CAPACITOR NEAR SENSE PINS 38681 F05a (5a) Using a Resistor to Sense Current VIN INTVCC BOOST TG SW LTC3868-1 BG R1 C1* SENSE- SGND *PLACE C1 NEAR SENSE PINS (R1||R2) * C1 = L DCR RSENSE(EQ) = DCR R2 R1 + R2 38681 F05b VIN INDUCTOR L DCR VOUT SENSE+ R2 (5b) Using the Inductor DCR to Sense Current Figure 5. Current Sensing Methods 38681fb LTC3868-1 applicaTions inForMaTion Low Value Resistor Current Sensing Atypicalsensingcircuitusingadiscreteresistorisshown in Figure 5a. RSENSE is chosen based on the required outputcurrent. The current comparator has a maximum threshold VSENSE(MAX)of50mV.Thecurrentcomparatorthreshold voltagesetsthepeakoftheinductorcurrent,yieldinga maximumaverageoutputcurrent,IMAX,equaltothepeak value less half the peak-to-peak ripple current, IL. To calculatethesenseresistorvalue,usetheequation: RSENSE = VSENSE(MAX) IMAX + IL 2 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, themaximumoutputcurrentlevelwillbereduceddueto theinternalcompensationrequiredtomeetstabilitycriterionforbuckregulatorsoperatingatgreaterthan50% dutyfactor.AcurveisprovidedintheTypicalPerformance Characteristics section to estimate this reduction in peakoutputcurrentdependingupontheoperatingduty factor. Inductor DCR Sensing Forapplicationsrequiringthehighestpossibleefficiency athighloadcurrents,theLTC3850iscapableofsensing thevoltagedropacrosstheinductorDCR,asshownin Figure5b.TheDCRoftheinductorrepresentsthesmall amountofDCresistanceofthecopperwire,whichcanbe lessthan1mfortoday'slowvalue,highcurrentinductors. Inahighcurrentapplicationrequiringsuchaninductor, powerlossthroughasenseresistorwouldcostseveral pointsofefficiencycomparedtoinductorDCRsensing. IftheexternalR1||R2*C1timeconstantischosentobe exactlyequaltotheL/DCRtimeconstant,thevoltagedrop acrosstheexternalcapacitorisequaltothedropacross theinductorDCRmultipliedbyR2/(R1+R2).R2scalesthe voltageacrossthesenseterminalsforapplicationswhere theDCRisgreaterthanthetargetsenseresistorvalue. Toproperlydimensiontheexternalfiltercomponents,the DCRoftheinductormustbeknown.Itcanbemeasured Toensurethattheapplicationwilldeliverfullloadcurrent over the full operating temperature range, choose the minimumvalueforthemaximumcurrentsensethreshold voltage (VSENSE(MAX)) in the Electrical Characteristics table. 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. TheequivalentresistanceR1||R2isscaledtotheroom temperatureinductanceandmaximumDCR: R1|| R2 = ( L DCR at 20C * C1 ) Thesenseresistorvaluesare: R1 = R1 * RD R1|| R2 ;R2 = RD 1 - RD 38681fb LTC3868-1 applicaTions inForMaTion ThemaximumpowerlossinR1isrelatedtodutycycle, andwilloccurincontinuousmodeatthemaximuminput voltage: PLOSS R1 = inductorcurrentrequiredresultsinapeakcurrentbelow 30% 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 controllerintheLTC3868-1:oneN-channelMOSFETfor thetop(main)switch,andoneN-channelMOSFETforthe bottom(synchronous)switch. 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. 38681fb ( 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. The inductor ripple current IL decreases with higher inductanceorhigherfrequencyandincreaseswithhigher VIN: IL = V 1 VOUT 1- OUT ( f) (L) VIN Accepting larger values of IL allows the use of low inductances,butresultsinhigheroutputvoltageripple andgreatercorelosses.Areasonablestartingpointfor settingripplecurrentisIL=0.3(IMAX).Themaximum ILoccursatthemaximuminputvoltage. Theinductorvaluealsohassecondaryeffects.ThetransitiontoBurstModeoperationbeginswhentheaverage LTC3868-1 applicaTions inForMaTion Selection criteria for the power MOSFETs include the on-resistance,RDS(ON),Millercapacitance,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 synchronousMOSFETlossesaregreatestathighinput voltagewhenthetopswitchdutyfactorisloworduring ashort-circuitwhenthesynchronousswitchisonclose to100%oftheperiod. Theterm(1+)isgenerallygivenforaMOSFETinthe formofanormalizedRDS(ON)vsTemperaturecurve,but =0.005/Ccanbeusedasanapproximationforlow voltageMOSFETs. TheoptionalSchottkydiodesD1andD2showninFigure10 conductduringthedead-timebetweentheconductionof thetwopowerMOSFETs.Thispreventsthebodydiodeof thebottomMOSFETfromturningon,storingchargeduring thedead-timeandrequiringareverserecoveryperiodthat couldcostasmuchas3%inefficiencyathighVIN.A1A to3ASchottkyisgenerallyagoodcompromiseforboth regionsofoperationduetotherelativelysmallaverage current.Largerdiodesresultinadditionaltransitionlosses duetotheirlargerjunctioncapacitance. CIN and COUT Selection TheselectionofCINissimplifiedbythe2-phasearchitectureanditsimpactontheworst-caseRMScurrentdrawn throughtheinputnetwork(battery/fuse/capacitor).Itcanbe shownthattheworst-casecapacitorRMScurrentoccurs whenonlyonecontrollerisoperating.Thecontrollerwith thehighest(VOUT)(IOUT)productneedstobeusedinthe formulashowninEquation1todeterminethemaximum RMScapacitorcurrentrequirement.Increasingtheoutputcurrentdrawnfromtheothercontrollerwillactually decreasetheinputRMSripplecurrentfromitsmaximum value.Theout-of-phasetechniquetypicallyreducesthe inputcapacitor'sRMSripplecurrentbyafactorof30% to70%whencomparedtoasinglephasepowersupply solution. Incontinuousmode,thesourcecurrentofthetopMOSFET isasquarewaveofdutycycle(VOUT)/(VIN).Toprevent largevoltagetransients,alowESRcapacitorsizedforthe maximumRMScurrentofonechannelmustbeused.The maximumRMScapacitorcurrentisgivenby: CIN RequiredIRMS 1/2 IMAX ( VOUT ) ( VIN - VOUT ) (1) VIN 38681fb Synchronous SwitchDuty Cycle = The MOSFET power dissipations at maximum output currentaregivenby: V 2 PMAIN = OUT (IMAX ) (1+ ) RDS(ON) + VIN 2 I ( VIN) MAX (RDR ) (CMILLER ) * 2 1 1 + ( f) VINTVCC - VTHMIN VTHMIN PSYNC = VIN - VOUT 2 (IMAX ) (1+ )RDS(ON) 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 LTC3868-1 applicaTions inForMaTion Equation1hasamaximumatVIN=2VOUT,whereIRMS =IOUT/2.Thissimpleworst-caseconditioniscommonly usedfordesignbecauseevensignificantdeviationsdonot offermuchrelief.Notethatcapacitormanufacturers'ripple currentratingsareoftenbasedononly2000hoursoflife. Thismakesitadvisabletofurtherderatethecapacitor,or tochooseacapacitorratedatahighertemperaturethan required. Several capacitors may be paralleled to meet sizeorheightrequirementsinthedesign.Duetothehigh operatingfrequencyoftheLTC3868-1,ceramiccapacitors canalsobeusedforCIN.Alwaysconsultthemanufacturer ifthereisanyquestion. ThebenefitoftheLTC3868-12-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. ThesourcesofthetopMOSFETsshouldbeplacedwithin 1cmofeachotherandshareacommonCIN(s).Separating thesourcesandCINmayproduceundesirablevoltageand currentresonancesatVIN. Asmall(0.1Fto1F)bypasscapacitorbetweenthechip VIN pin and ground, placed close to the LTC3868-1, 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: VOUT 1 IL ESR + 8 * f * COUT where f is the operating frequency, COUT is the output capacitanceandIListheripplecurrentintheinductor. Theoutputrippleishighestatmaximuminputvoltage sinceILincreaseswithinputvoltage. Setting Output Voltage TheLTC3868-1outputvoltagesareeachsetbyanexternalfeedbackresistordividercarefullyplacedacrossthe output,asshowninFigure6.Theregulatedoutputvoltage isdeterminedby: R VOUT = 0.8V 1+ B RA To improve the frequency response, a feedforward capacitor,CFF,maybeused.Greatcareshouldbetakento routetheVFBlineawayfromnoisesources,suchasthe inductorortheSWline. VOUT 1/2 LTC3868-1 VFB RA 38681 F05 RB CFF Figure 6. Setting Output Voltage Soft-Start (SS Pins) Thestart-upofeachVOUTiscontrolledbythevoltageon therespectiveSSpin.WhenthevoltageontheSSpin islessthantheinternal0.8Vreference,theLTC3868-1 regulatestheVFBpinvoltagetothevoltageontheSSpin insteadof0.8V.TheSSpincanbeusedtoprograman externalsoft-startfunction. Soft-startisenabledbysimplyconnectingacapacitorfrom theSSpintoground,asshowninFigure7.Aninternal 1A current source charges the capacitor, providing a 1/2 LTC3868-1 SS CSS SGND 38681 F06 Figure 7. Using the SS Pin to Program Soft-Start 38681fb 0 LTC3868-1 applicaTions inForMaTion linearrampingvoltageattheSSpin.TheLTC3868-1will regulatetheVFBpin(andhenceVOUT)accordingtothe voltageontheSSpin,allowingVOUTtorisesmoothlyfrom 0Vtoitsfinalregulatedvalue.Thetotalsoft-starttimewill beapproximately: 0.8 V tSS = CSS * 1A INTVCC Regulators TheLTC3868-1featurestwoseparateinternalP-channel low dropout linear regulators (LDO) that supply power attheINTVCCpinfromeithertheVINsupplypinorthe EXTVCCpindependingontheconnectionoftheEXTVCC pin. INTVCC powers the gate drivers and much of the LTC3868-1'sinternalcircuitry.TheVINLDOandtheEXTVCC LDOregulateINTVCCto5.1V.Eachofthesecansupplya peakcurrentof50mAandmustbebypassedtoground withaminimumof4.7FlowESRcapacitor.Nomatter whattypeofbulkcapacitorisused,anadditional1FceramiccapacitorplaceddirectlyadjacenttotheINTVCCand PGNDICpinsishighlyrecommended.Goodbypassing isneededtosupplythehightransientcurrentsrequired bytheMOSFETgatedriversandtopreventinteraction betweenthechannels. HighinputvoltageapplicationsinwhichlargeMOSFETs are being driven at high frequencies may cause the maximumjunctiontemperatureratingfortheLTC3868-1 tobeexceeded.TheINTVCCcurrent,whichisdominated by the gate charge current, may be supplied by either the VIN LDO or the EXTVCC LDO. When the voltage on theEXTVCCpinislessthan4.7V,theVINLDOisenabled. PowerdissipationfortheICinthiscaseishighestandis equaltoVIN*IINTVCC.ThegatechargecurrentisdependentonoperatingfrequencyasdiscussedintheEfficiency Considerationssection.Thejunctiontemperaturecanbe estimatedbyusingtheequationsgiveninNote2ofthe Electrical Characteristics. For example, the LTC3868-1 INTVCCcurrentislimitedtolessthan22mAfroma28V supplywhennotusingtheEXTVCCsupplyat70Cambient temperatureintheSSOPpackage: TJ=70C+(22mA)(28V)(90C/W)=125C 38681fb 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 controlpowertobederivedfromoneoftheLTC3868-1'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 a 8.5V supply reduces the junction temperature in the previousexamplefrom125Cto: TJ=70C+(45mA)(8.5V)(90C/W)=87C However,for3.3Vandotherlowvoltageoutputs,additionalcircuitryisrequiredtoderiveINTVCCpowerfrom theoutput. LTC3868-1 applicaTions inForMaTion ThefollowinglistsummarizesthefourpossibleconnectionsforEXTVCC: 1.EXTVCCLeftOpen(orGrounded).ThiswillcauseINTVCC tobepoweredfromtheinternal5.1Vregulatorresultinginanefficiencypenaltyofupto10%athighinput voltages. 2.EXTVCCConnectedDirectlytoVOUT.Thisisthenormal connectionfora5Vto14Vregulatorandprovidesthe highestefficiency. 3.EXTVCCConnectedtoanExternalSupply.Ifanexternal supplyisavailableinthe5Vto14Vrange,itmaybe usedtopowerEXTVCC.EnsurethatEXTVCC 38681 F08 and the BOOST pin follows. With the topside MOSFET 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 Whentheoutputcurrenthitsthecurrentlimit,theoutput voltagebeginstodrop.Iftheoutputvoltagefallsbelow 70%ofitsnominaloutputlevel,thenthemaximumsense voltageisprogressivelyloweredtoaboutone-halfofits maximumselectedvalue.Undershort-circuitconditions withverylowdutycycles,theLTC3868-1willbegincycle skippinginordertolimittheshort-circuitcurrent.Inthis situationthebottomMOSFETwillbedissipatingmostof thepowerbutlessthaninnormaloperation.Theshortcircuitripplecurrentisdeterminedbytheminimumontime,tON(MIN),oftheLTC3868-1(90ns),theinputvoltage andinductorvalue: V IL(SC) = tON(MIN) IN L Theresultingaverageshort-circuitcurrentis: ISC = 50% * ILIM(MAX) RSENSE 1 - IL(SC) 2 BAT85 BAT85 BAT85 VN2222LL L RSENSE VOUT Figure 8. Capacitive Charge Pump for EXTVCC Topside MOSFET Driver Supply (CB, DB) Externalbootstrapcapacitors,CB,connectedtotheBOOST pinssupplythegatedrivevoltagesforthetopsideMOSFETs. CapacitorCBintheFunctionalDiagramischargedthough externaldiodeDBfromINTVCCwhentheSWpinislow. WhenoneofthetopsideMOSFETsistobeturnedon,the driverplacestheCBvoltageacrossthegate-sourceofthe desiredMOSFET.ThisenhancesthetopMOSFETswitch andturnsiton.Theswitchnodevoltage,SW,risestoVIN Fault Conditions: Overvoltage Protection (Crowbar) Theovervoltagecrowbarisdesignedtoblowasystem inputfusewhentheoutputvoltageoftheregulatorrises muchhigherthannominallevels.Thecrowbarcauseshuge currentstoflow,thatblowthefusetoprotectagainsta shortedtopMOSFETiftheshortoccurswhilethecontrollerisoperating. Acomparatormonitorstheoutputforovervoltageconditions. The comparator detects faults greater than 10% 38681fb LTC3868-1 applicaTions inForMaTion 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. Phase-Locked Loop and Frequency Synchronization TheLTC3868-1hasaninternalphase-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. Typically, the external clock (on the PLLIN/MODE pin) inputhighthresholdis1.6V,whiletheinputlowthreshold is1.1V. 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 ) 38681 F09 Figure 9. Relationship Between Oscillator Frequency and Resistor Value at the FREQ Pin RapidphaselockingcanbeachievedbyusingtheFREQ pin to set a free-running frequency near the desired synchronizationfrequency.TheVCO'sinputvoltageis prebiasedatafrequencycorrespondingtothefrequency setbytheFREQpin.Onceprebiased,thePLLonlyneeds to adjust the frequency slightly to achieve phase lock andsynchronization.Althoughitisnotrequiredthatthe free-runningfrequencybenearexternalclockfrequency, doingsowillpreventtheoperatingfrequencyfrompassing throughalargerangeoffrequenciesasthePLLlocks. NotethattheLTC3868-1canonlybesynchronizedtoan external clock whose frequency is within range of the LTC3868-1's internal VCO, which is nominally 55kHz to1MHz.Thisisguaranteedtobebetween75kHzand 850kHz. 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 38681fb LTC3868-1 applicaTions inForMaTion Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time durationthattheLTC3868-1iscapableofturningonthetop MOSFET.Itisdeterminedbyinternaltimingdelaysandthe gatechargerequiredtoturnonthetopMOSFET.Lowduty cycleapplicationsmayapproachthisminimumon-time limitandcareshouldbetakentoensurethat: tON(MIN) < VOUT VIN f 1.TheVINcurrentistheDCinputsupplycurrentgiven intheElectricalCharacteristicstable,whichexcludes MOSFETdriverandcontrolcurrents.VINcurrenttypicallyresultsinasmall(<0.1%)loss. 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! 38681fb () Ifthedutycyclefallsbelowwhatcanbeaccommodated bytheminimumon-time,thecontrollerwillbegintoskip cycles.Theoutputvoltagewillcontinuetoberegulated, buttheripplevoltageandcurrentwillincrease. Theminimumon-timefortheLTC3868-1isapproximately 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, four main sources usually account for most of the losses in LTC3868-1 circuits: 1) IC VIN current, 2) INTVCCregulatorcurrent,3)I2Rlosses,4)topsideMOSFET transitionlosses. LTC3868-1 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 LTC3868-1 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 12 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 38681fb LTC3868-1 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 V 1- OUT ( f) (L) VIN(NOM) ThepowerdissipationonthetopsideMOSFETcanbeeasily estimated.ChoosingaFairchildFDS6982SdualMOSFET .At resultsin:RDS(ON)=0.035/0.022,CMILLER=215pF maximuminputvoltagewithT(estimated)=50C: PMAIN = 2 3.3V 6A 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: 25mV 1 95ns 22V ISC = - = 3.9A 0.006 2 3.9H withatypicalvalueofRDS(ON)and=(0.005/C)(25C) = 0.125. The resulting power dissipated in the bottom MOSFETis: P = 3.9A SYNC ( ) A3.9Hinductorwillproduce29%ripplecurrent.The peakinductorcurrentwillbethemaximumDCvalueplus onehalftheripplecurrent,or6.88A.Increasingtheripple currentwillalsohelpensurethattheminimumon-time of95nsisnotviolated.Theminimumon-timeoccursat maximumVIN: 3.3V tON(MIN) = = = 429ns 9 VIN(MAX ) ( f) 22V (350kHz ) VOUT ( )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): 43mV RSENSE = 0.006 6.88A Choosing1%resistors:RA=25kandRB=78.1kyields anoutputvoltageof3.299V. 38681fb LTC3868-1 applicaTions inForMaTion PC Board Layout Checklist Whenlayingouttheprintedcircuitboard,thefollowing checklistshouldbeusedtoensureproperoperationof theIC.Theseitemsarealsoillustratedgraphicallyinthe layoutdiagramofFigure10.Figure11illustratesthecurrent 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.DotheLTC3868-1VFBpins'resistivedividersconnect to the (+) terminals of COUT? The resistive divider mustbeconnectedbetweenthe(+)terminalofCOUT andsignalground.Thefeedbackresistorconnections shouldnotbealongthehighcurrentinputfeedsfrom theinputcapacitor(s). v 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 sideoftheLTC3868-1andoccupyminimum 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--typically10%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. 38681fb LTC3868-1 applicaTions inForMaTion Reduce VIN from its nominal level to verify operation oftheregulatorindropout.Checktheoperationofthe undervoltagelockoutcircuitbyfurtherloweringVINwhile monitoringtheoutputstoverifyoperation. Investigatewhetheranyproblemsexistonlyathigheroutputcurrentsoronlyathigherinputvoltages.Ifproblems coincidewithhighinputvoltavgesandlowoutputcurrents, 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 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. SS1 LTC3868-1 ITH1 VFB1 SENSE1+ SENSE1- FREQ PGOOD1 TG1 SW1 CB1 BOOST1 BG1 fIN PLLIN/MODE RUN1 RUN2 SGND SENSE2- SENSE2+ VFB2 ITH2 SS2 VIN PGND EXTVCC INTVCC BG2 BOOST2 SW2 TG2 L2 CB2 RSENSE VOUT2 VOUT1 1F CERAMIC COUT1 M1 M2 D1 RPU1 PGOOD1 VPULL-UP (<6V) L1 RSENSE VOUT1 CVIN RIN + GND + CIN + CINTVCC VIN 1F CERAMIC M3 COUT2 M4 D2 38681 F10 Figure 10. Recommended Printed Circuit Layout Diagram 38681fb LTC3868-1 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 38681 F11 Figure 11. Branch Current Waveforms 38681fb LTC3868-1 Typical applicaTions RB1 215k CF1 15pF RA1 68.1k CITH1A 150pF RITH1 15k CITH1 820pF C1 1nF LTC3868-1 SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 TG1 D1 VIN SS1 INTVCC PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 RITH2 27k ITH2 SW2 BG2 MBOT2 D2 MTOP2 CB2 0.47F L2 7.2H RSENSE2 10m VOUT2 8.5V 3A COUT2 150F CINT 4.7F CIN 22F VIN 9V TO 24V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 3.3H RSENSE1 7m COUT1 150F VOUT1 3.3V 5A CSS1 0.1F CSS2 0.1F CITH2 680pF CITH2A 100pF RA2 44.2k CF2 39pF RB2 442k VFB2 SENSE2- SENSE2+ C2 1nF COUT1, COUT2: SANYO 10TPD150M L1: SUMIDA CDEP105-3R2M L2: SUMIDA CDEP105-7R2M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP 38681 F12 Efficiency vs Output Current 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 VIN = 12V Burst Mode OPERATION 1 10 38681 F12b Start-Up SW Node Waveforms VOUT = 8.5V VOUT = 3.3V VOUT2 2V/DIV SW1 5V/DIV VOUT1 2V/DIV SW2 5V/DIV 0 0.00001 0.0001 0.001 0.01 0.1 OUTPUT CURRENT (A) 20ms/DIV 38681 F12c 1s/DIV 38681 F12d Figure 12. High Efficiency Dual 8.5V/3.3V Step-Down Converter 38681fb 0 LTC3868-1 Typical applicaTions High Efficiency Dual 2.5V/3.3V Step-Down Converter RB1 143k CF1 22pF RA1 68.1k CITH1A 100pF RITH1 22k CITH1 820pF C1 1nF LTC3868-1 SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 TG1 INTVCC 100k MBOT1 CB1 0.47F MTOP1 D1 VIN CIN 22F VIN 4V TO 24V L1 2.4H RSENSE1 7m COUT1 150F VOUT1 2.5V 5A CSS1 0.01F SS1 INTVCC PGND CINT 4.7F D2 MTOP2 CB2 0.47F CSS2 0.01F CITH2 820pF PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 SW2 BG2 L2 3.2H RSENSE2 7m 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 10TPD150M L1: SUMIDA CDEP105-2R5 L2: SUMIDA CDEP105-3R2M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP 38681 F13 38681fb LTC3868-1 Typical applicaTions High Efficiency Dual 12V/5V Step-Down Converter RB1 422k CF1 33pF RA1 34k CITH1A 100pF RITH1 33k CITH1 680pF CSS1 0.01F C1 1nF SENSE1+ SENSE1- VFB1 PGOOD1 BG1 SW1 BOOST1 ITH1 LTC3868-1 SS1 VIN INTVCC PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 RITH2 17k ITH2 SW2 BG2 MBOT2 CINT 4.7F D2 MTOP2 CB2 0.47F L2 4.3H RSENSE2 7m VOUT2 5V COUT2 5.5A 150F TG1 D1 CIN 22F VIN 12.5V TO 24V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 8.8H RSENSE1 10m COUT1 47F VOUT1 12V 3A RFREQ 60k CSS2 0.01F CITH2 680pF CITH2A 100pF RA2 75k CF2 15pF RB2 393k VFB2 SENSE2- SENSE2+ C2 1nF COUT1: KEMET T525D476M016E035 COUT2: SANYO 10TPD150M L1: SUMIDA CDEP105-8R8M L2: SUMIDA CDEP105-4R3M MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP 38681 TA02a 38681fb LTC3868-1 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 LTC3868-1 VIN SS1 INTVCC CINT 4.7F D2 MTOP2 CB2 0.47F L2 0.47H RSENSE2 4m VOUT2 1.2V COUT2 8A 220F x2 TG1 D1 CIN 22F VIN 12V CB1 0.47F MTOP1 INTVCC 100k L1 MBOT1 0.47H RSENSE1 4m COUT1 220F x2 VOUT1 1V 8A CSS1 0.01F RFREQ 60k CSS2 0.01F CITH2 1000pF PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ 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 2RSTPE220M L1: SUMIDA CDEP105-3R2M L2: SUMIDA CDEP105-7R2M MTOP1, MTOP2: RENESAS RJK0305 MBOT1, MBOT2: RENESAS RJK0328 38681 TA03a 38681fb LTC3868-1 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 LTC3868-1 VIN SS1 INTVCC CINT 4.7F D2 MTOP2 CB2 0.47F L2 0.47H VOUT2 1.2V COUT2 8A 220F x2 TG1 D1 CIN 22F VIN 12V CB1 0.47F MTOP1 INTVCC 100k MBOT1 L1 0.47H COUT1 220F x2 VOUT1 1V 8A CSS1 0.01F RFREQ 65k CSS2 0.01F CITH2 1000pF PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 SW2 BG2 RITH2 3.93k ITH2 MBOT2 CITH2A 220pF RA2 115k CF2 56pF RB2 57.6k VFB2 SENSE2- SENSE2+ RS2 1.18k 38681 TA05 C2 0.1F COUT1, COUT2: SANYO 2R5TPE220M L1, L2: SUMIDA IHL P2525CZERR47M06 MTOP1, MTOP2: RENESAS RJK0305 MBOT1, MBOT2: RENESAS RJK0328 38681fb LTC3868-1 package DescripTion (Reference LTC DWG # 05-08-1712 Rev B) UFD Package 28-Lead Plastic QFN (4mm x 5mm) 0.70 0.05 4.50 0.05 3.10 0.05 2.50 REF 2.65 0.05 3.65 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 3.50 REF 4.10 0.05 5.50 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 0.10 (2 SIDES) PIN 1 TOP MARK (NOTE 6) 0.75 0.05 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.35 45 CHAMFER 27 28 0.40 1 2 5.00 0.10 (2 SIDES) 0.10 2.50 REF R = 0.115 TYP 3.50 REF 3.65 0.10 2.65 0.10 (UFD28) QFN 0506 REV B 0.200 REF 0.00 - 0.05 0.25 BOTTOM VIEW--EXPOSED PAD 0.05 0.50 BSC NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X). 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 38681fb LTC3868-1 package DescripTion GN Package 28-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 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 38681fb LTC3868-1 revision hisTory REV B DATE 12/09 DESCRIPTION ChangetoAbsoluteMaximumRatings ChangetoElectricalCharacteristics ChangetoTypicalPerformanceCharacteristics ChangetoPinFunctions TextChangestoOperationSection TextChangestoApplicationsInformationSection ChangetoFigure10 ChangestoRelatedParts (Revision history begins at Rev B) PAGE NUMBER 2 3,4 6 8,9 11,12,13 21,22,23,24,26 28 38 38681fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However,noresponsibilityisassumedforitsuse.LinearTechnologyCorporationmakesnorepresentationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights. LTC3868-1 relaTeD parTs PART NUMBER LTC3857/LTC3857-1 LTC3858/LTC3858-1 LTC3834/LTC3834-1 LTC3835/LTC3835-1 LT3845 LT3800 LTC3824 DESCRIPTION LowIQ,DualOutput2-PhaseSynchronousStep-Down DC/DCControllerswith99%DutyCycle LowIQ,DualOutput2-PhaseSynchronousStep-Down DC/DCControllerswith99%DutyCycle LowIQ,SynchronousStep-DownDC/DCControllers LowIQ,SynchronousStep-DownDC/DCControllers LowIQ,HighVoltageSynchronousStep-Down DC/DCController LowIQ,HighVoltageSynchronousStep-Down DC/DCController LowIQ,HighVoltageDC/DCController,100%DutyCycle COMMENTS Phase-LockableFixedOperatingFrequency50kHzto900kHz, 4VVIN38V,0.8VVOUT24V,IQ=50A, 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 SelectableFixed200kHzto600kHzOperatingFrequency, 4VVIN60V,0.8VVOUTVIN,IQ=40A,MSOP-10E Phase-LockableFixedOperatingFrequency250kHzto780kHz, 4VVIN30V,0.8VVOUT5.25V Phase-LockableFixedFrequency250kHzto770kHz, 4.5VVIN38V,0.8VVOUT12.5V LTC3850/LTC3850-1 Dual2-Phase,HighEfficiencySynchronousStep-Down LTC3850-2 DC/DCControllers,RSENSEorDCRCurrentSensingand Tracking LTC3855 Dual,Multiphase,SynchronousDC/DCStep-Down ControllerwithDiffampandDCRTemperature Compensation LTC3853 LTC3854 LTC3775 LTC3851A/ LTC3851A-1 LTC3878/LTC3879 TripleOutput,MultiphaseSynchronousStep-DownDC/DC Phase-LockableFixedOperatingFrequency250kHzto750kHz, Controller,RSENSEorDCRCurrentSensingandTracking 4VVIN24V,VOUTUpto13.5V SmallFootprintWideVINRangeSynchronousStep-Down Fixed400kHzOperatingFrequency,4.5VVIN38V, DC/DCController 0.8VVOUT5.25V,2mmx3mmQFN-12,MSOP-12 HighFrequencySynchronousVoltageModeStep-Down DC/DCController NoRSENSE TMWideVINRangeSynchronousStep-Down DC/DCControllers NoRSENSEConstantOn-TimeSynchronousStep-Down DC/DCControllers FastTransientResponse,tON(MIN)=30ns,4VVIN38V, 0.6VVOUT0.8VIN,MSOP-16E,3mmx3mmQFN-16 Phase-LockableFixedOperatingFrequency250kHzto750kHz, 4VVIN38V,0.8VVOUT5.25V,MSOP-16E,3mmx3mm QFN-16,SSOP-16 VeryFastTransientResponse,tON(MIN)=43ns,4VVIN38V, VOUTUp90%ofVIN,MSOP-16E,3mmx3mmQFN-16,SSOP-16 38681fb Linear Technology Corporation (408)432-1900 FAX: (408) 434-0507 www.linear.com LT 0110 REV B * PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 LINEAR TECHNOLOGY CORPORATION 2009 |
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