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LM3485

cumtboy2004
2012-05-08 0人阅读 0 0 0 暂无简介 举报

简介:本文档为《LM3485pdf》,可适用于IT/计算机领域

LMHystereticPFETBuckControllerGeneralDescriptionTheLMisahighefficiencyPFETswitchingregulatorcontrollerthatasystemdesignercanusetoquicklyandeasilydevelopasmall,lowcost,switchingbuckregulatorforawiderangeofapplicationsTheuseofahystereticcontrolschemeprovidesforsimpledesignwithoutanycontrolloopstabilityconcernsusingawidevarietyofexternalcomponentsThePFETarchitecturealsoallowsforlowcomponentcountaswellasultralowdropoutoperationAnotherbenefitishighefficiencyoperationatlightloadswithoutanincreaseinoutputrippleCurrentlimitprotectioncircuitisprovidedbymeasuringthevoltageacrossthePFET’sRDSONthuseliminatingacostlysenseresistorThecurrentlimitcanbeadjustedallowingfordesignsatvariousoutputcurrentsandcostsFeaturesnEasytousecontrolmethodologynNocontrolloopcompensationrequirednVtoVwideinputrangenVtoVINadjustableoutputrangenHighEfficiencyn±(±overtemp)internalreferencendutycyclenMaximumoperatingfrequency>MHznCurrentlimitprotectionnMSOPApplicationsnSetTopBoxnDSLCableModemnPCIAnAutoPCnTFTMonitornBatteryPoweredPortableApplicationsnDistributedPowerSystemsnAlwaysOnPowerTypicalApplicationCircuitMayLMHystereticPFETBuckController©NationalSemiconductorCorporationDSwwwnationalcom查询LMMM供应商ConnectionDiagramTopViewLeadPlasticMSOPNSpackageNumberMUAAPackageMarkingandOrderingInformationOrderNumberPackageTypePackageMarkingSuppliedAs:LMMMMSOPSBunitsonTapeandReelLMMMXMSOPSBunitsonTapeandReelPinDescriptionPinNamePinNumberDescriptionISENSEThecurrentsenseinputpinThispinshouldbeconnectedtoDrainnodeoftheexternalPFETGNDSignalgroundNCNoconnectionFBThefeedbackinputConnecttheFBtoaresistorvoltagedividerbetweentheoutputandGNDforanadjustableoutputvoltageADJCurrentlimitthresholdadjustmentItconnectstoaninternalµAcurrentsourceAresistorisconnectedbetweenthispinandtheinputPowerSupplyThevoltageacrossthisresistoriscomparedwiththeVDSoftheexternalPFETtodetermineifanovercurrentconditionhasoccurredPWRGNDPowergroundPGATEGateDriveoutputfortheexternalPFETPGATEswingsbetweenVINandVINVVINPowersupplyinputpinLMwwwnationalcomAbsoluteMaximumRatings(Note)IfMilitaryAerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOfficeDistributorsforavailabilityandspecificationsVINVoltage−VtoVPGATEVoltage−VtoVFBVoltage−VtoVISENSEVoltage−VtoVADJVoltage−VtoVMaximumJunctionTemp˚CPowerDissipationmWTA=˚CESDSusceptibiltyHumanBodyModel(Note)kVLeadTemperatureVaporPhase(sec)Infared(sec)˚C˚CStorageTemperature−˚Cto˚COperatingRatings(Note)SupplyVoltageVtoVOperatingJunctionTemperature−˚Cto˚CElectricalCharacteristicsSpecificationsinStandardtypefaceareforTJ=˚C,andinboldtypefaceapplyoverthefullOperatingTemperatureRange(TJ=−˚Cto˚C)Unlessotherwisespecified,VIN=V,VISNS=VIN−V,andVADJ=VIN−VDatasheetminmaxspecificationlimitsareguaranteedbydesign,test,orstatisticalanalysisSymbolParameterConditionsMin(Note)Typ(Note)Max(Note)UnitIQQuiescentCurrentatgroundpinFB=V(NotSwitching)µAVFBFeedbackVoltage(Note)VVHYSTComparatorHysteresismVVCL(Note)CurrentlimitcomparatortripvoltageRADJ=kΩmVRADJ=kΩVCLOFFSETCurrentlimitcomparatoroffsetVFB=V−mVICLADJCurrentlimitADJcurrentsourceVFB=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=−˚Cto˚C)Unlessotherwisespecified,VIN=V,VISNS=VIN−V,andVADJ=VIN−VDatasheetminmaxspecificationlimitsareguaranteedbydesign,test,orstatisticalanalysisSymbolParameterConditionsMin(Note)Typ(Note)Max(Note)UnitTONMINCLMinimumontimeincurrentlimitVISNS=VADJVVFB=VCloadonOUT=pF(Note)nsVFB∆VINFeedbackVoltageLineRegulation≤VIN≤VVNote:AbsolutemaximumratingsarelimitsbeyondwhichdamagetothedevicemayoccurOperatingRatingsareconditionsforwhichthedeviceisintendedtobefunctional,butdeviceparameterspecificationsmaynotbeguaranteedForguaranteedspecificationsandtestconditions,seetheElectricalCharacteristicsNote:Themaximumallowablepowerdissipationisafunctionofthemaximumjunctiontemperature,TJMAX,thejunctiontoambientthermalresistance,θJA=˚CW,andtheambienttemperature,TAThemaximumallowablepowerdissipationatanyambienttemperatureiscalculatedusing:PDMAX=(TJMAXTA)θJAExceedingthemaximumallowablepowerdissipationwillcauseexcessivedietemperatureNote:ThehumanbodymodelisapFcapacitordischargedthroughakΩresistorintoeachpinNote:Alllimitsareguaranteedatroomtemperature(standardtypeface)andattemperatureextremes(boldtypeface)AllroomtemperaturelimitsaretestedAlllimitsattemperatureextremesareguaranteedviacorrelationusingstandardStatisticalQualityControl(SQC)methodsAlllimitsareusedtocalculateAverageOutgoingQualityLevel(AOQL)Note:Typicalnumbersareat˚CandrepresentthemostlikelynormNote: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)=VIN−VDSThecurrentlimitisactivatedwhenthevoltageattheISENSEpinexceedsthevoltageattheADJpinTheISENSEcomparatortriggerstheµsoneshotpulsegeneratorforcingthedrivertoturnthePFEToffThedriverturnsthePFETbackonafterµsIfthecurrenthasnotreducedbelowthesetthreshold,thecyclewillrepeatcontinuouslyDuringcurrentlimitoperation,theoutputvoltagewilldropsignificantlyaswilloperatingfrequencyAstheloadcurrentisreduced,theoutputwillreturntotheprogrammedvoltageHowever,thereisacurrentlimitfoldbackphenomenoninherentinthiscurrentlimitarchitectureSeeFigureFIGUREPropagationDelayFIGURECurrentSensingbyVDSFIGURECurrentLimitFoldBackPhenomenonLMwwwnationalcomFunctionalDescription(Continued)StartUpThecurrentlimitcircuitisactiveduringstartupDuringstartupthePFETwillstayonuntileitherthecurrentlimitorthefeedbackcomparatoristrippedIfthecurrentlimitcomparatoristrippedfirstthenthefoldbackcharacteristicshouldbetakenintoaccountStartupintofullloadmayrequireahighercurrentlimitsetpointortheloadmustbeappliedafterstartupOneproblemwithselectingahighercurrentlimitisinrushcurrentduringstartupAddingacapacitance(CADJ)inparallelwithRADJresultsinsoftstartCADJandRADJcreateanRCtimeconstantforcingcurrentlimittoactivateatalowercurrentTheoutputvoltagewillrampmoreslowlywhenusingthesoftstartfunctionalityTheCADJalsofiltersunwantednoisesothattheISENSEcomparatorwillnotbeaccidentallytriggeredAvalueofpFtonFisrecommendedinmostapplicationsTheselowvaluesforCADJwillhavelittletonoeffectonsoftstartThereareexamplestartupplotsforCADJequaltonFandnFintheTypicalPerformanceCharacteristicsExternalSenseResistorTheVDSofaPFETwilltendtovarysignificantlyovertemperatureThiswillresultanequivalentvariationincurrentlimitToimprovecurrentlimitaccuracyanexternalsenseresistorcanbeconnectedfromVINtothesourceofthePFET,asshowninFigureFIGURECurrentSensingbyExternalResistorLMwwwnationalcomDesignInformationHystereticcontrolisasimplecontrolschemeHowevertheoperatingfrequencyandotherperformancecharacteristicshighlydependonexternalconditionsandcomponentsIfeithertheinductance,outputcapacitance,ESR,VIN,orCffischanged,therewillbeachangeintheoperatingfrequencyandoutputrippleThebestapproachistodeterminewhatoperatingfrequencyisdesirableintheapplicationandthenbeginwiththeselectionoftheinductorandCOUTESRInductorSelection(L)TheimportantparametersfortheinductoraretheinductanceandthecurrentratingTheLMoperatesoverawidefrequencyrangeandcanuseawiderangeofinductancevaluesAgoodruleofthumbistousetheequationsusedforNational’sSimpleSwitchers®Theequationforinductorripple(∆i)asafunctionofoutputcurrent(IOUT)is:forIout<Amps∆i≤Iout**Iout−forIout>Amps∆i≤Iout*Theinductancecanbecalculatedbaseduponthedesiredoperatingfrequencywhere:AndwhereDisthedutycycleandVDisthediodeforwardvoltageTheinductorshouldberatedtothefollowing:Ipk=(Iout∆i)*TheinductancevalueandtheresultingrippleisoneofthekeyparameterscontrollingoperatingfrequencyThesecondistheESROutputCapacitorSelection(COUT)TheESRoftheoutputcapacitortimestheinductorripplecurrentisequaltotheoutputrippleoftheregulatorHowever,theVHYSTsetsthefirstordervalueofthisrippleAsESRisincreasedwithagiveninductance,thenoperatingfrequencyincreasesaswellIfESRisreducedthentheoperatingfrequencyreducesTheuseofceramiccapacitorshasbecomeacommondesireofmanypowersupplydesignersHowever,ceramiccapacitorshaveaverylowESRresultingina˚phaseshiftoftheoutputvoltagerippleThisresultsinlowoperatingfrequencyandincreasedoutputrippleTofixthisproblemalowvalueresistorshouldbeaddedinserieswiththeceramicoutputcapacitorAlthoughcounterintuitive,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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