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LM3485.pdf

LM3485.pdf

上传者: cumtboy2004 2012-05-08 评分 0 0 0 0 0 0 暂无简介 简介 举报

简介:本文档为《LM3485pdf》,可适用于IT/计算机领域,主题内容包含LMHystereticPFETBuckControllerGeneralDescriptionTheLMisahighefficiencyPFET符等。

LMHystereticPFETBuckControllerGeneralDescriptionTheLMisahighefficiencyPFETswitchingregulatorcontrollerthatasystemdesignercanusetoquicklyandeasilydevelopasmall,lowcost,switchingbuckregulatorforawiderangeofapplicationsTheuseofahystereticcontrolschemeprovidesforsimpledesignwithoutanycontrolloopstabilityconcernsusingawidevarietyofexternalcomponentsThePFETarchitecturealsoallowsforlowcomponentcountaswellasultralowdropoutoperationAnotherbenefitishighefficiencyoperationatlightloadswithoutanincreaseinoutputrippleCurrentlimitprotectioncircuitisprovidedbymeasuringthevoltageacrossthePFET’sRDSONthuseliminatingacostlysenseresistorThecurrentlimitcanbeadjustedallowingfordesignsatvariousoutputcurrentsandcostsFeaturesnEasytousecontrolmethodologynNocontrolloopcompensationrequirednVtoVwideinputrangenVtoVINadjustableoutputrangenHighEfficiencyn(overtemp)internalreferencendutycyclenMaximumoperatingfrequency>MHznCurrentlimitprotectionnMSOPApplicationsnSetTopBoxnDSLCableModemnPCIAnAutoPCnTFTMonitornBatteryPoweredPortableApplicationsnDistributedPowerSystemsnAlwaysOnPowerTypicalApplicationCircuitMayLMHystereticPFETBuckControllerNationalSemiconductorCorporationDSwwwnationalcom查询LMMM供应商ConnectionDiagramTopViewLeadPlasticMSOPNSpackageNumberMUAAPackageMarkingandOrderingInformationOrderNumberPackageTypePackageMarkingSuppliedAs:LMMMMSOPSBunitsonTapeandReelLMMMXMSOPSBunitsonTapeandReelPinDescriptionPinNamePinNumberDescriptionISENSEThecurrentsenseinputpinThispinshouldbeconnectedtoDrainnodeoftheexternalPFETGNDSignalgroundNCNoconnectionFBThefeedbackinputConnecttheFBtoaresistorvoltagedividerbetweentheoutputandGNDforanadjustableoutputvoltageADJCurrentlimitthresholdadjustmentItconnectstoaninternalµAcurrentsourceAresistorisconnectedbetweenthispinandtheinputPowerSupplyThevoltageacrossthisresistoriscomparedwiththeVDSoftheexternalPFETtodetermineifanovercurrentconditionhasoccurredPWRGNDPowergroundPGATEGateDriveoutputfortheexternalPFETPGATEswingsbetweenVINandVINVVINPowersupplyinputpinLMwwwnationalcomAbsoluteMaximumRatings(Note)IfMilitaryAerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOfficeDistributorsforavailabilityandspecificationsVINVoltageVtoVPGATEVoltageVtoVFBVoltageVtoVISENSEVoltageVtoVADJVoltageVtoVMaximumJunctionTempCPowerDissipationmWTA=CESDSusceptibiltyHumanBodyModel(Note)kVLeadTemperatureVaporPhase(sec)Infared(sec)CCStorageTemperatureCtoCOperatingRatings(Note)SupplyVoltageVtoVOperatingJunctionTemperatureCtoCElectricalCharacteristicsSpecificationsinStandardtypefaceareforTJ=C,andinboldtypefaceapplyoverthefullOperatingTemperatureRange(TJ=CtoC)Unlessotherwisespecified,VIN=V,VISNS=VINV,andVADJ=VINVDatasheetminmaxspecificationlimitsareguaranteedbydesign,test,orstatisticalanalysisSymbolParameterConditionsMin(Note)Typ(Note)Max(Note)UnitIQQuiescentCurrentatgroundpinFB=V(NotSwitching)µAVFBFeedbackVoltage(Note)VVHYSTComparatorHysteresismVVCL(Note)CurrentlimitcomparatortripvoltageRADJ=kΩmVRADJ=kΩVCLOFFSETCurrentlimitcomparatoroffsetVFB=VmVICLADJCurrentlimitADJcurrentsourceVFB=VµATCLCurrentlimitoneshotofftimeVADJ=VVISNS=VVFB=VµsRPGATEDriverresistanceSourceISOURCE=mAΩSinkISink=mAIPGATEDriverOutputcurrentSourceVIN=V,PGATE=VASinkVIN=V,PGATE=VVPGATEMINMinimumdrivervoltageVIN=VVFB=VIGATE=µAsinkVIFBFBpinBiasCurrent(Note)VFB=VnATONMINNORMinimumontimeinnormaloperationVISNS=VADJVCloadonOUT=pF(Note)nsLMwwwnationalcomElectricalCharacteristics(Continued)SpecificationsinStandardtypefaceareforTJ=C,andinboldtypefaceapplyoverthefullOperatingTemperatureRange(TJ=CtoC)Unlessotherwisespecified,VIN=V,VISNS=VINV,andVADJ=VINVDatasheetminmaxspecificationlimitsareguaranteedbydesign,test,orstatisticalanalysisSymbolParameterConditionsMin(Note)Typ(Note)Max(Note)UnitTONMINCLMinimumontimeincurrentlimitVISNS=VADJVVFB=VCloadonOUT=pF(Note)nsVFBVINFeedbackVoltageLineRegulationVINVVNote:AbsolutemaximumratingsarelimitsbeyondwhichdamagetothedevicemayoccurOperatingRatingsareconditionsforwhichthedeviceisintendedtobefunctional,butdeviceparameterspecificationsmaynotbeguaranteedForguaranteedspecificationsandtestconditions,seetheElectricalCharacteristicsNote:Themaximumallowablepowerdissipationisafunctionofthemaximumjunctiontemperature,TJMAX,thejunctiontoambientthermalresistance,θJA=CW,andtheambienttemperature,TAThemaximumallowablepowerdissipationatanyambienttemperatureiscalculatedusing:PDMAX=(TJMAXTA)θJAExceedingthemaximumallowablepowerdissipationwillcauseexcessivedietemperatureNote:ThehumanbodymodelisapFcapacitordischargedthroughakΩresistorintoeachpinNote:Alllimitsareguaranteedatroomtemperature(standardtypeface)andattemperatureextremes(boldtypeface)AllroomtemperaturelimitsaretestedAlllimitsattemperatureextremesareguaranteedviacorrelationusingstandardStatisticalQualityControl(SQC)methodsAlllimitsareusedtocalculateAverageOutgoingQualityLevel(AOQL)Note:TypicalnumbersareatCandrepresentthemostlikelynormNote:TheVFBisthetripvoltageattheFBpinwhenPGATEswitchesfromhightolowNote:VCL=ICLADJ*RADJNote:BiascurrentflowsoutfromtheFBpinNote:ApFcapacitorisconnectedbetweenVINandPGATELMwwwnationalcomTypicalPerformanceCharacteristicsUnlessotherwisespecified,TJ=CQuiescentCurrentvsInputVoltage(FB=V)FeedbackVoltagevsTemperatureHysteresisVoltagevsInputVoltageHysteresisVoltagevsTemperatureCurrentLimitADJCurrentvsTemperatureCurrentLimitOneShotOFFTimevsTemperatureLMwwwnationalcomTypicalPerformanceCharacteristicsUnlessotherwisespecified,TJ=C(Continued)PGATEVoltagevsInputVoltageMinimumONTimevsTemperatureOperatingONTimevsOutputLoadCurrent(VIN=V)OperatingONTimevsOutputLoadCurrent(VIN=V)EfficiencyvsLoadCurrent(VOUT=V,L=µH)EfficiencyvsLoadCurrent(VOUT=V,L=µH)LMwwwnationalcomTypicalPerformanceCharacteristicsUnlessotherwisespecified,TJ=C(Continued)EfficiencyvsLoadCurrent(VOUT=V,L=µH)StartUpContinuousModeOperation(VIN=V,VOUT=V,IOUT=mA,L=µH)DiscontinuousModeOperation(VIN=V,VOUT=V,IOUT=mA,L=µH)OperatingFrequencyvsInputVoltage(VOUT=V,IOUT=A,COUT(ESR)=mΩ,Cff=pF)OutputRippleVoltagevsInputVoltage(VOUT=V,IOUT=A,COUT(ESR)=mΩ,Cff=pF)LMwwwnationalcomTypicalPerformanceCharacteristicsUnlessotherwisespecified,TJ=C(Continued)OperatingFrequencyvsOutputLoadCurrent(L=µH,COUT(ESR)=mΩ,Cff=pF)FeedForwardCapacitor(Cff)Effect(VOUT=V,L=µH,IOUT=mA)LMwwwnationalcomBlockDiagramFunctionalDescriptionOverviewTheLMisbuck(stepdown)DCDCcontrollerthatusesahystereticcontrolschemeThecomparatorisdesignedwithapproximatelymVofhysteresisInresponsetothevoltageattheFBpin,thegatedrive(PGATEpin)turnstheexternalPFETonoroffWhentheinductorcurrentistoohigh,thecurrentlimitprotectioncircuitengagesandturnsthePFEToffforapproximatelyµsThehystereticcontroldoesnotprovideaninternaloscillatorSwitchingfrequencydependsontheexternalcomponentsandoperatingconditionsOperatingfrequencyreducesatlightloadsresultinginexcellentefficiencycomparedtootherarchitecturesexternalresistorscaneasilyprogramtheoutputvoltageTheoutputcanbesetinawiderangefromVtoVINHystereticControlCircuitTheLMusesacomparatorbasedvoltagecontrolloopThefeedbackiscomparedtoaVreferenceandamVhysteresisisdesignedintothecomparatortoensurenoisefreeoperationWhentheFBinputtothecomparatorfallsbelowthereferencevoltage,theoutputofthecomparatormovestoalowstateThisresultsinthedriveroutput,PGATE,pullingthegateofthePFETlowandturningonthePFETWiththePFETon,theinputsupplychargesCoutandsuppliescurrenttotheloadviatheseriespaththroughthePFETandtheinductorCurrentthroughtheInductorrampsuplinearlyandtheoutputvoltageincreasesAstheFBvoltagereachestheupperthreshold,whichistheinternalreferencevoltageplusmV,theoutputofthecomparatorchangesfromlowtohigh,andthePGATErespondsbyturningthePFEToffAsthePFETturnsoff,theinductorvoltagereverses,thecatchdiodeturnson,andthecurrentthroughtheinductorrampsdownThen,astheoutputvoltagereachestheinternalreferencevoltageagain,thenextcyclestartsTheLMoperatesindiscontinuousconductionmodeatlightloadcurrentorcontinuousconductionmodeatheavyloadcurrentIndiscontinuousconductionmode,currentthroughtheinductorstartsatzeroandrampsuptothepeak,thenrampsdowntozeroNextcyclestartswhentheFBvoltagereachestheinternalvoltageUntilthen,theinductorcurrentremainszeroOperatingfrequencyislowerandswitchinglossesreduceIncontinuousconductionmode,currentalwaysflowsthroughtheinductorandneverrampsdowntozeroTheoutputvoltage(VOUT)canbeprogrammedbyexternalresistorsItcanbecalculatedasfollowingVOUT=*(RR)RFIGUREHystereticWindowLMwwwnationalcomFunctionalDescription(Continued)Theminimumoutputvoltageripple(VOUTPP)canbecalculatedinthesamewayVOUTPP=VHYST(RR)RForexample,withVOUTsettoV,VOUTPPismVVOUTPP=*(KK)K=VOperatingfrequency(F)isdeterminedbyknowingtheinputvoltage,outputvoltage,inductor,VHYST,ESR(EquivalentSeriesResistance)ofoutputcapacitor,andthedelayItcanbeapproximatelycalculatedusingtheformula:where:α:(RR)Rdelay:ItincludestheLMpropagationdelaytimeandthePFETdelaytimeThepropagationdelayisnstypically(SeethePropagationDelaycurvebelow)Theoperatingfrequencyandoutputripplevoltagecanalsobesignificantlyinfluencedbythespeedupcapacitor(Cff)Cffisconnectedinparallelwiththehighsidefeedbackresistor,RThelocationofthiscapacitorissimilartowhereafeedforwardcapacitorwouldbelocatedinaPWMcontrolschemeHoweverit’seffectonhystereticoperationismuchdifferentTheoutputripplecausesacurrenttobesourcedorsunkthroughthiscapacitorThiscurrentisessentiallyasquarewaveSincetheinputtothefeedbackpin,FB,isahighimpedancenode,thecurrentflowsthroughRTheendresultisareductioninoutputrippleandanincreaseinoperatingfrequencyWhenaddingCff,calculatetheformulaabovewithα=ThevalueofCffdependonthedesiredoperatingfrequencyandthevalueofRAgoodstartingpointispFceramicatkHzdecreasinglinearlywithincreasedoperatingfrequencyAlsonotethatastheoutputvoltageisprogrammedbelowV,theeffectofCffwilldecreasesignificantlyCurrentLimitOperationTheLMhasacyclebycyclecurrentlimitCurrentlimitissensedacrosstheVDSofthePFEToracrossanadditionalsenseresistorWhencurrentlimitisreached,theLMturnsofftheexternalPFETforaperiodofµsThecurrentlimitisadjustedbyanexternalresistor,RADJThecurrentlimitcircuitiscomposedoftheISENSEcomparatorandtheoneshotpulsegeneratorThepositiveinputoftheISENSEcomparatoristheADJpinAninternalµAcurrentsinkcreatesavoltageacrosstheexternalRADJresisterThisvoltageiscomparedtothevoltageacrossthePFETorsenseresistorTheADJvoltagecanbecalculatedasfollowsVADJ=VIN(RADJ*µA)ThenegativeinputoftheISENSEcomparatoristheISENSEpinthatshouldbeconnectedtothedrainoftheexternalPFETTheinductorcurrentisdeterminedbysensingtheVDSItcanbecalculatedasfollowsVISENSE=VIN(RDSON*IINDPEAK)=VINVDSThecurrentlimitisactivatedwhenthevoltageattheISENSEpinexceedsthevoltageattheADJpinTheISENSEcomparatortriggerstheµsoneshotpulsegeneratorforcingthedrivertoturnthePFEToffThedriverturnsthePFETbackonafterµsIfthecurrenthasnotreducedbelowthesetthreshold,thecyclewillrepeatcontinuouslyDuringcurrentlimitoperation,theoutputvoltagewilldropsignificantlyaswilloperatingfrequencyAstheloadcurrentisreduced,theoutputwillreturntotheprogrammedvoltageHowever,thereisacurrentlimitfoldbackphenomenoninherentinthiscurrentlimitarchitectureSeeFigureFIGUREPropagationDelayFIGURECurrentSensingbyVDSFIGURECurrentLimitFoldBackPhenomenonLMwwwnationalcomFunctionalDescription(Continued)StartUpThecurrentlimitcircuitisactiveduringstartupDuringstartupthePFETwillstayonuntileitherthecurrentlimitorthefeedbackcomparatoristrippedIfthecurrentlimitcomparatoristrippedfirstthenthefoldbackcharacteristicshouldbetakenintoaccountStartupintofullloadmayrequireahighercurrentlimitsetpointortheloadmustbeappliedafterstartupOneproblemwithselectingahighercurrentlimitisinrushcurrentduringstartupAddingacapacitance(CADJ)inparallelwithRADJresultsinsoftstartCADJandRADJcreateanRCtimeconstantforcingcurrentlimittoactivateatalowercurrentTheoutputvoltagewillrampmoreslowlywhenusingthesoftstartfunctionalityTheCADJalsofiltersunwantednoisesothattheISENSEcomparatorwillnotbeaccidentallytriggeredAvalueofpFtonFisrecommendedinmostapplicationsTheselowvaluesforCADJwillhavelittletonoeffectonsoftstartThereareexamplestartupplotsforCADJequaltonFandnFintheTypicalPerformanceCharacteristicsExternalSenseResistorTheVDSofaPFETwilltendtovarysignificantlyovertemperatureThiswillresultanequivalentvariationincurrentlimitToimprovecurrentlimitaccuracyanexternalsenseresistorcanbeconnectedfromVINtothesourceofthePFET,asshowninFigureFIGURECurrentSensingbyExternalResistorLMwwwnationalcomDesignInformationHystereticcontrolisasimplecontrolschemeHowevertheoperatingfrequencyandotherperformancecharacteristicshighlydependonexternalconditionsandcomponentsIfeithertheinductance,outputcapacitance,ESR,VIN,orCffischanged,therewillbeachangeintheoperatingfrequencyandoutputrippleThebestapproachistodeterminewhatoperatingfrequencyisdesirableintheapplicationandthenbeginwiththeselectionoftheinductorandCOUTESRInductorSelection(L)TheimportantparametersfortheinductoraretheinductanceandthecurrentratingTheLMoperatesoverawidefrequencyrangeandcanuseawiderangeofinductancevaluesAgoodruleofthumbistousetheequationsusedforNational’sSimpleSwitchersTheequationforinductorripple(i)asafunctionofoutputcurrent(IOUT)is:forIout<AmpsiIout**IoutforIout>AmpsiIout*Theinductancecanbecalculatedbaseduponthedesiredoperatingfrequencywhere:AndwhereDisthedutycycleandVDisthediodeforwardvoltageTheinductorshouldberatedtothefollowing:Ipk=(Iouti)*TheinductancevalueandtheresultingrippleisoneofthekeyparameterscontrollingoperatingfrequencyThesecondistheESROutputCapacitorSelection(COUT)TheESRoftheoutputcapacitortimestheinductorripplecurrentisequaltotheoutputrippleoftheregulatorHowever,theVHYSTsetsthefirstordervalueofthisrippleAsESRisincreasedwithagiveninductance,thenoperatingfrequencyincreasesaswellIfESRisreducedthentheoperatingfrequencyreducesTheuseofceramiccapacitorshasbecomeacommondesireofmanypowersupplydesignersHowever,ceramiccapacitorshaveaverylowESRresultinginaphaseshiftoftheoutputvoltagerippleThisresultsinlowoperatingfrequencyandincreasedoutputrippleTofixthisproblemalowvalueresistorshouldbeaddedinserieswiththeceramicoutputcapacitorAlthoughcounterintuitive,thiscombinationofaceramiccapacitorandexternalseriesresistanceprovidehighlyaccuratecontrolovertheoutputvoltagerippleTheothertypescapacitor,suchasSanyoPOSCAPandOSCON,PanasonicSPCAP,Nichicon’NA’series,arealsorecommendedandmaybeusedwithoutadditionalseriesresistanceForallpracticalpurposes,anytypeofoutputcapacitormaybeusedwithpropercircuitverificationInputCapacitorSelection(CIN)AbypasscapacitorisrequiredbetweentheinputsourceandgroundItmustbelocatednearthesourcepinoftheexternalPFETTheinputcapacitorpreventslargevoltagetransientsattheinputandprovidestheinstantaneouscurrentwhenthePFETturnsonTheimportantparametersfortheinputcapacitorarethevoltageratingandtheRMScurrentratingFollowthemanufacturer’srecommendedvoltagederatingForhighinputvoltageapplication,lowESRelectrolyticcapacitor,theNichicon’UD’seriesorthePanasonic’FK’series,isavailableTheRMScurrentintheinputcapacitorcanbecalculatedTheinputcapacitorpowerdissipationcanbecalculatedasfollowsPD(CIN)=IRMSCIN*ESRCINTheinputcapacitormustbeabletohandletheRMScurrentandthePDSeveralinputcapacitorsmaybeconnectedinparalleltohandlelargeRMScurrentsInsomecasesitmaybemuchcheapertousemultipleelectrolyticcapacitorsthanasinglelowESR,highperformancecapacitorsuchasOSCONorTantalumThecapacitancevalueshouldbeselectedsuchthattheripplevoltagecreatedbythechargeanddischargeofthecapacitanceislessthanofthetotalrippleacrossthecapacitorProgrammingtheCurrentLimit(RADJ)Thecurrentlimitisdeterminedbyconnectingaresistor(RADJ)betweeninputvoltageandtheADJpi

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