LM5010资料

LM5010HighVoltage1AStepDownSwitchingRegulatorOctober2004

LM5010

HighVoltage1AStepDownSwitchingRegulator

GeneralDescription

TheLM5010StepDownSwitchingRegulatorfeaturesallthefunctionsneededtoimplementalowcost,efficient,buckbiasregulatorcapableofsupplyinginexcessof1Aloadcurrent.ThishighvoltageregulatorcontainsanN-ChannelBuckSwitch,andisavailableinthermallyenhancedLLP-10andTSSOP-14EPpackages.Thehystereticregulationschemerequiresnoloopcompensation,resultsinfastloadtransientresponse,andsimplifiescircuitimplementation.Theoperatingfrequencyremainsconstantwithlineandloadvariationsduetotheinverserelationshipbetweentheinputvoltageandtheon-time.Thevalleycurrentlimitdetectionissetat1.25A.Additionalfeaturesinclude:VCCunder-voltagelockout,thermalshutdown,gatedriveunder-voltagelockout,andmaximumdutycyclelimiter.

Features

nnnnnnnnnnnn

InputVoltageRange:8Vto75VValleyCurrentLimitAt1.25A

SwitchingFrequencyCanExceed1MHzIntegratedN-ChannelBuckSwitchIntegratedStartupRegulator

NoLoopCompensationRequiredUltra-FastTransientResponse

OperatingFrequencyRemainsConstantWithLoadandLineVariations

MaximumDutyCycleLimitedDuringStartupAdjustableOutputVoltage

Precision2.5VFeedbackReferenceThermalshutdown

TypicalApplications

nnnn

HighEfficiencyPoint-Of-Load(POL)RegulatorNon-IsolatedTelecommunicationsBuckRegulatorSecondaryHighVoltagePostRegulatorAutomotiveSystems

Package

nLLP-10(4mmx4mm)nTSSOP-14EP

nBothPackagesHaveExposedThermalPadForImprovedHeatDissipation

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BasicStepdownRegulator

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LM5010ConnectionDiagrams

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OrderingInformation

OrderNumberLM5010SDLM5010SDXLM5010MHLM5010MHX

PackageTypeLLP-10(4x4)LLP-10(4x4)TSSOP-14EPTSSOP-14EP

NSCPackageDrawingSDC10ASDC10AMXA14AMXA14A

SuppliedAs

1000UnitsonTapeandReel3500UnitsonTapeandReel1000UnitsonTapeandReel3500UnitsonTapeandReel

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LM5010PinDescription

PINNUMBERLLP-101

TSSOP-14

2

NAMESW

DESCRIPTION

SwitchingNode

APPLICATIONINFORMATION

Internallyconnectedtothebuckswitchsource.Connecttotheinductor,free-wheelingdiode,andbootstrapcapacitor.

Connecta0.022µFcapacitorfromSWtothispin.ThecapacitorischargedfromVCCviaaninternaldiodeduringeachoff-time.

There-circulatingcurrentflowsthroughtheinternalsenseresistor,andoutofthispintothe

free-wheelingdiode.Currentlimitisnominallysetat1.25A.

Re-circulatingcurrentflowsintothispintothecurrentsenseresistor.

Groundforallinternalcircuitryotherthanthecurrentlimitdetection.

Internallyconnectedtotheregulationand

over-voltagecomparators.Theregulationlevelis2.5V.

Aninternal11.5µAcurrentsourcechargesanexternalcapacitorto2.5V,providingthesoftstartfunction.

AnexternalresistorfromVINtothispinsetsthebuckswitchon-time.Groundingthispinshutsdowntheregulator.

Nominallyregulatesat7.0V.Anexternalvoltage(7.5V-14V)canbeappliedtothispintoreduceinternaldissipation.AninternaldiodeconnectsVCCtoVIN.

Nominalinputrangeis8.0Vto75V.Nointernalconnection.

23BSTBoostpinforbootstrapcapacitor

34

ISENCurrentsense

456

569

SGNDRTNFB

SenseGroundCircuitGround

FeedbackinputfromtheregulatedoutputSoftstart

710SS

811

RON/SD

On-timecontrolandshutdown

912

VCCOutputfromthestartupregulator

10131,7,8,14

VINNC

InputsupplyvoltageNoconnection.

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LM5010AbsoluteMaximumRatings(Note1)

IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.VINtoGNDBSTtoGND

SWtoGND(SteadyState)BSTtoVCCBSTtoSWVCCtoGNDSGNDtoRTNSStoRTN

76V90V-1.5V76V14V14V

-0.3Vto+0.3V-0.3Vto4V

VINtoSW

CurrentOutofISENAllOtherInputstoGNDESDRating(Note2)HumanBodyModelStorageTemperatureRange

LeadTemperature(Soldering4sec)(Note4)

76VSeeText-0.3to7V

2kV

-55˚Cto+150˚C

260˚C

OperatingRatings(Note1)

VINOperatingJunctionTemperature

8Vto75V

−40˚Cto+125˚C

ElectricalCharateristics

SpecificationswithstandardtypefaceareforTJ=25˚C,andthosewithboldfacetypeapplyoverfullOperatingJunctionTemperaturerange.VIN=48V,RON=200kΩ,unlessotherwisestated(Note5)and(Note6).

SymbolVCCRegulatorVCCReg

VCCregulatedoutputVIN-VCCVCCoutputimpedance(0mA≤ICC≤5mA)VCCcurrentlimit(Note3)

UVLOVCCVCCunder-voltagelockoutthreshold

UVLOVCChysteresisUVLOVCCfilterdelayIINoperatingcurrentIINshutdowncurrent

SwitchCharacteristicsRds(on)UVLOGDSoftstartPin

Pull-upvoltageInternalcurrentsource

CurrentLimitILIMThreshold

ResistancefromISENtoSGNDResponsetime

OnTimer,RON/SDPintON-1tON-2

On-timeOn-time

ShutdownthresholdThresholdhysteresis

OffTimertOFFOff-time

265

ns

VIN=10V,RON=200kΩVIN=75V,RON=200kΩVoltageatRON/SDrisingVoltageatRON/SDfalling

2.12900.35

2.753900.60

3.44901.1

µsnsVmV

CurrentoutofISEN1

1.25130150

1.5

AmΩns

2.511.5

VµA

BuckSwitchRds(on)GateDriveUVLOUVLOGDhysteresis

ITEST=200mAVBST-VSWIncreasing

3.0

0.3.3440

0.805.0

ΩVmV

ICC=0mA,FS<200kHz7.5V≤VIN≤8.0VVIN=8.0VVIN=48VVCC=0VVCCincreasingVCCdecreasing100mVoverdriveNon-switching,FB=3VRON/SD=0V

6.6

71.31402.5105.8145365095

8502007.4

VoltsVΩmAVmVµsµAµA

Parameter

Conditions

Min

Typ

Max

Units

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LM5010ElectricalCharateristics

SymbolVREFParameter

(Continued)

SpecificationswithstandardtypefaceareforTJ=25˚C,andthosewithboldfacetypeapplyoverfullOperatingJunctionTemperaturerange.VIN=48V,RON=200kΩ,unlessotherwisestated(Note5)and(Note6).

Conditions

SSpin=steadystate

Min2.445

Typ2.52.9117520

SDCPackageMXAPackage

4040

Max2.550

UnitsVVnA˚C˚C

RegulationandOver-VoltageComparators(FBPin)

FBregulationthresholdFBover-voltagethresholdFBbiascurrent

ThermalShutdownTSDThermalshutdowntemperature

Thermalshutdownhysteresis

ThermalResistanceθJAJunctiontoAmbient

˚C/W

Note1:AbsoluteMaximumRatingsarelimitsbeyondwhichdamagetothedevicemayoccur.OperatingRatingsareconditionsunderwhichoperationofthedeviceisintendedtobefunctional.Forguaranteedspecificationsandtestconditions,seetheElectricalCharacteristics.Note2:Thehumanbodymodelisa100pFcapacitordischargedthrougha1.5kΩresistorintoeachpin.

Note3:VCCprovidesbiasfortheinternalgatedriveandcontrolcircuits.Devicethermallimitationslimitexternalloading.

Note4:FordetailedinformationonsolderingplasticTSSOPandLLPpackagesrefertothePackagingDataBookavailablefromNationalSemiconductorCorporation.

Note5:Typicalspecificationsrepresentthemostlikelyparametricnormat25˚Coperation.

Note6:Alllimitsareguaranteed.AllelectricalcharacteristicshavingroomtemperaturelimitsaretestedduringproductionwithTA=25˚C.Allhotandcoldlimitsareguaranteedbycorrelatingtheelectricalcharacteristicstoprocessandtemperaturevariationsandapplyingstatisticalprocesscontrol.

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LM5010TypicalApplicationCircuitandBlockDiagram(pinnumbersarefortheLLP-10package)

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FIGURE1.

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LM5010TypicalPerformanceCharacteristics

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FIGURE2.VCCvsVINFIGURE4.ICCvsExternallyAppliedVCC2011990520119907

FIGURE3.VCCvsICCFIGURE5.On-TimevsVINandRON7www.national.com

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LM5010TypicalPerformanceCharacteristics(Continued)

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FIGURE7.IINvsVIN20119908

FIGURE6.VoltageatRON/SDPin

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LM5010TypicalPerformanceCharacteristics

(Continued)

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FIGURE8.StartupSequence

FunctionalDescription

TheLM5010StepDownSwitchingRegulatorfeaturesallthefunctionsneededtoimplementalowcost,efficientbuckbiaspowerconvertercapableofsupplyinginexcessof1Atotheload.ThishighvoltageregulatorcontainsanN-Channelbuckswitch,iseasytoimplement,andisavailableinthethermallyenhancedLLP-10andTSSOP-14EPpackages.Theregulator’soperationisbasedonahystereticcontrolscheme,andusesanon-timewhichvariesinverselywithVIN.Thisfeatureresultsintheoperatingfrequencyremain-ingrelativelyconstantwithloadandinputvoltagevariations.Theswitchingfrequencycanrangefrom100kHzto>1.0MHz.Thehystereticcontrolrequiresnoloopcompensationresultinginveryfastloadtransientresponse.Thevalleycurrentlimitdetectioncircuit,internallysetat1.25A,holdsthebuckswitchoffuntilthehighcurrentlevelsubsides.Figure1showsthefunctionalblockdiagram.TheLM5010canbeappliedinnumerousapplicationstoefficientlyregu-latedownhighervoltages.Thisregulatoriswellsuitedfor

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48Vtelecomapplications,aswellasthenew42Vautomo-tivepowerbus.ImplementedasaPoint-of-Loadregulatorfollowingahighlyefficientintermediatebusconvertercanresultinhighoverallsystemefficiency.Featuresinclude:Thermalshutdown,VCCunder-voltagelockout,gatedriveunder-voltagelockout,andmaximumdutycyclelimit.

HystereticControlCircuitOverview

TheLM5010buckDC-DCregulatoremploysacontrolschemebasedonacomparatorandaone-shoton-timer,withtheoutputvoltagefeedback(FB)comparedtoaninter-nalreference(2.5V).IftheFBvoltageisbelowthereferencethebuckswitchisturnedonforatimeperioddeterminedbytheinputvoltageandaprogrammingresistor(RON).Follow-ingtheon-timetheswitchremainsofffor265ns,oruntiltheFBvoltagefallsbelowthereference,whicheverislonger.Thebuckswitchthenturnsonforanotheron-timeperiod.

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LM5010HystereticControlCircuitOverview(Continued)

Typicallywhentheloadcurrentincreasessuddenly,theoff-timesaretemporarilyattheminimumof265ns.Onceregu-lationisestablished,theoff-timeresumesitsnormalvalue.Theoutputvoltageissetbytwoexternalresistors(R1,R2).Theregulatedoutputvoltageiscalculatedasfollows:

(1)VOUT=2.5Vx(R1+R2)/R2Outputvoltageregulationisbasedonripplevoltageatthe

feedbackinput,requiringaminimumamountofESRfortheoutputcapacitorC2.TheLM5010requiresaminimumof25mVofripplevoltageattheFBpin.Incaseswherethecapacitor’sESRisinsufficientadditionalseriesresistancemayberequired(R3inFigure1).

Wheninregulation,theLM5010operatesincontinuousconductionmodeatheavyloadcurrentsanddiscontinuousconductionmodeatlightloadcurrents.Incontinuouscon-ductionmodecurrentalwaysflowsthroughtheinductor,neverreachingzeroduringtheoff-time.Inthismodetheoperatingfrequencyremainsrelativelyconstantwithloadandlinevariations.Theminimumloadcurrentforcontinuousconductionmodeisone-halftheinductor’sripplecurrentamplitude.Theapproximateoperatingfrequencyiscalcu-latedasfollows:

(3)

Atlowloadcurrent,thecircuitoperatesindiscontinuousconductionmode,duringwhichtheinductorcurrentrampsupfromzerotoapeakduringtheon-time,thenrampsbacktozerobeforetheendoftheoff-time.Thenexton-timeperiodstartswhenthevoltageatFBfallsbelowtherefer-ence-untilthentheinductorcurrentremainszero,andtheloadcurrentissuppliedbytheoutputcapacitor(C2).Inthismodetheoperatingfrequencyislowerthanincontinuousconductionmode,andvarieswithloadcurrent.Conversionefficiencyismaintainedatlightloadssincetheswitchinglossesreducewiththereductioninloadandfrequency.Theapproximatediscontinuousoperatingfrequencycanbecal-culatedasfollows:

(4)

whereRL=theloadresistance.

Forapplicationswhereloweroutputvoltagerippleisre-quiredtheoutputcanbetakendirectlyfromalowESRoutputcapacitorasshowninFigure9.However,R3slightlydegradestheloadregulation.

(2)

Thebuckswitchdutycycleisapproximatelyequalto:

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FIGURE9.LowRippleOutputConfiguration

Start-upRegulator(VCC)

ThestartupregulatorisintegraltotheLM5010.Theinputpin(VIN)canbeconnecteddirectlytolinevoltagesupto75V.TheVCCoutputisregulatedat7.0V,±6%,andiscurrentlimitedto10mA.UponpoweruptheregulatorsourcescurrentintotheexternalcapacitoratVCC(C3).Witha0.1µFcapacitoratVCC,approximately58µsarerequiredfortheVCCvoltagetoreachtheunder-voltagelockoutthreshold(UVLO)of5.8V(t1inFigure8),atwhichtimethebuckswitchisenabled,andthesoftstartpinisreleasedtoallowthesoftstartcapacitor(C6)tochargeup.VOUTthenincreasestoitsregulatedvalueasthesoftstartvoltageincreases(t2inFigure8).

Theminimuminputoperatingvoltageisdeterminedbytheregulator’sdropoutvoltage,theVCCUVLOfallingthreshold()5.65V),andthefrequency.WhenVCCfallsbelowthefallingthresholdtheVCCUVLOactivatestoshutoffthebuckswitchandgroundthesoftstartpin.IfVCCisexternallyloaded,theminimuminputvoltageincreasessincetheout-putimpedanceatVCCis)140ΩatlowVIN.SeeFigures2and3.InapplicationsinvolvingahighvalueforVINwherepowerdissipationinthestartupregulatorisaconcern,anauxiliaryvoltagecanbediodeconnectedtotheVCCpin(Figure10).Settingtheauxiliaryvoltagetobetween7.5Vand14Vshutsofftheinternalregulator,reducinginternalpowerdissipation.ThecurrentrequiredintotheVCCpinisshowninFigure4.InternallyadiodeconnectsVCCtoVIN.

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LM5010Start-upRegulator(VCC)

(Continued)

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FIGURE10.SelfBiasedConfiguration

RegulationComparator

ThefeedbackvoltageatFBiscomparedtothevoltageattheSoftstartpin(2.5V,±2%).Innormaloperation(theoutputvoltageisregulated)anon-timeperiodisinitiatedwhenthevoltageatFBfallsbelow2.5V.Thebuckswitchstaysonfortheon-timecausingtheFBvoltagetoriseabove2.5V.Aftertheon-timeperiodthebuckswitchstaysoffuntiltheFBvoltagefallsbelow2.5V.BiascurrentattheFBpinislessthan5nAovertemperature.

(5)

TheinverserelationshipoftONvs.VINresultsinanearlyconstantfrequencyasVINisvaried.Iftheapplicationre-quiresahighfrequencytheminimumvaluefortON,andconsequentlyRON,islimitedbytheoff-time(265ns,±15%)whichlimitsthemaximumdutycycleatminimumVIN.ThetoleranceforEquation5is±25%.Frequenciesinexcessof1MHzarepossiblewiththeLM5010.

Over-VoltageComparator

ThefeedbackvoltageatFBiscomparedtoaninternal2.9Vreference.IfthevoltageatFBrisesabove2.9Vtheon-timeisimmediatelyterminated.Thisconditioncanoccuriftheinputvoltage,ortheoutputload,changesuddenly.ThebuckswitchwillnotturnonagainuntilthevoltageatFBfallsbelow2.5V.

Shutdown

TheLM5010canberemotelyshutdownbytakingtheRON/SDpinbelow0.65V.SeeFigure11.Inthismodethesoftstartpinisinternallygrounded,theon-timerisdisabled,andtheinputcurrentatVINisreduced(Figure7).ReleasingtheRON/SDpinallowsnormaloperationtoresume.Whentheswitchisopen,thenominalvoltageatRON/SDisshowninFigure6.

ON-TimeControl

Theon-timeoftheinternalswitch(seeFigure5)isdeter-minedbytheRONresistorandtheinputvoltage(VIN),cal-culatedfromthefollowing:

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FIGURE11.ShutdownImplementation

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LM5010CurrentLimit

Currentlimitdetectionoccursduringtheoff-timebymonitor-ingtherecirculatingcurrentthroughthefree-wheelingdiode(D1).Thedetectionthresholdis1.25A,±0.25A.ReferringtoFigure1,whenthebuckswitchisofftheinductorcurrentflowsthroughtheload,intoSGND,throughthesenseresistor,outofISENandthroughD1.Ifthatcurrentexceedsthethresholdthecurrentlimitcomparatoroutputswitchestodelaythestartofthenexton-timeperiod.Thenexton-timestartswhenthecurrentoutofISENisbelowthethresholdandthevoltageatFBisbelow2.5V.Iftheoverloadconditionpersistscausingtheinductorcurrenttoexceedthethresholdduringeachon-time,thatisdetectedatthebeginningofeachoff-time.Theoperatingfrequencyislowerduetolonger-than-normaloff-times.

Figure12illustratestheinductorcurrentwaveform.DuringnormaloperationtheloadcurrentisIO,theaverageoftheripplewaveform.Whentheloadresistancedecreasesthe

currentratchetsupuntilthelowerpeakattemptstoexceedthethreshold.DuringtheCurrentLimitedportionofFigure12,thecurrentrampsdowntothethresholdduringeachoff-time,initiatingthenexton-time(assumingthevoltageatFBis<2.5V).Duringeachon-timethecurrentrampsupanamountequalto:

(6)

DuringthistimetheLM5010isinaconstantcurrentmode,withanaverageloadcurrent(IOCL)equaltothethreshold+∆I/2.

The“valleycurrentlimit”techniqueallowstheloadcurrenttoexceedthecurrentlimitthresholdaslongasthelowerpeakoftheinductorcurrentislessthanthethreshold.

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FIGURE12.InductorCurrent-CurrentLimitOperation

Thecurrentlimitthresholdcanbeincreasedbyconnectinganexternalresistor(RCL)betweenSGNDandISEN.Theexternalresistortypicallyislessthan1Ω,anditscalculationisexplainedintheApplicationsInformationsection.

ThepeakcurrentoutofSWandISENmustnotexceed3.5A.TheaveragecurrentoutofSWmustbelessthan3A,andtheaveragecurrentoutofISENmustbelessthan2A.

Softstart

Thesoftstartfeatureallowstheconvertertograduallyreachasteadystateoperatingpoint,therebyreducingstartupstressesandcurrentsurges.Uponturn-on,afterVCCreachestheunder-voltagethreshold(t1inFigure8),aninternal11.5µAcurrentsourcechargestheexternalcapaci-torattheSoftstartpinto2.5V(t2inFigure8).TherampingvoltageatSS(andatthenon-invertinginputoftheregulationcomparator)rampsuptheoutputvoltageinacontrolledmanner.Thisfeaturekeepstheloadcurrentfromgoingtocurrentlimitduringstartup,therebyreducinginrushcurrents.AninternalswitchgroundstheSoftstartpinifVCCisbelowtheunder-voltagelockoutthreshold,ifathermalshutdownoccurs,orifthecircuitisshutdownusingtheRON/SDpin.

N-ChannelBuckSwitchandDriver

TheLM5010integratesanN-Channelbuckswitchandas-sociatedfloatinghighvoltagegatedriver.Thepeakcurrentthroughthebuckswitchmustnotbeallowedtoexceed3.5A,andtheaveragecurrentmustbelessthan3A.ThegatedrivercircuitispoweredbytheexternalbootstrapcapacitorbetweenBSTandSW(C4).Duringeachoff-time,theSWpinisatapproximately-1V,andC4isre-chargedfromVCCthroughtheinternalhighvoltagediode.Theminimumoff-timeof265nsensuresaminimumtimeeachcycletorechargethebootstrapcapacitor.A0.022µFceramicca-pacitorisrecommendedforC4.

ThermalShutdown

TheLM5010shouldbeoperatedsothejunctiontemperaturedoesnotexceed125˚C.Ifthejunctiontemperaturein-creasesabovethat,aninternalThermalShutdowncircuitactivates(typically)at175˚C,takingthecontrollertoalowpowerresetstatebydisablingthebuckswitchandtheon-timer,andgroundingtheSoftstartpin.Thisfeaturehelpspreventcatastrophicfailuresfromaccidentaldeviceover-12

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LM5010ThermalShutdown

(Continued)

heating.Whenthejunctiontemperaturereducesbelow155˚C(typicalhysteresis=20˚C),theSoftstartpinisre-leasedandnormaloperationresumes.

ApplicationsInformation

EXTERNALCOMPONENTS

Theprocedureforcalculatingtheexternalcomponentsisillustratedwithadesignexample.ThecircuitinFigure1istobeconfiguredforthefollowingspecifications:•VOUT=10V

•VIN=15Vto75V

RON,FS:RONsetstheon-time,andcanbechosenusingEquation2tosetanominalfrequency,orfromEquation5iftheon-timeataparticularVINisimportant.Ahigherfre-quencygenerallymeansasmallerinductorandcapacitors(value,sizeandcost),buthigherswitchinglosses.Alowerfrequencymeansahigherefficiency,butwithlargercompo-nents.IfPCboardspaceistight,ahigherfrequencyisbetter.Theresultingon-timeandfrequencyhavea±25%toler-ance.Re-arrangingEquation2,

•FS=625kHz

•Minimumloadcurrent=150mA•Maximumloadcurrent=1.0A•Softstarttime=5ms.

R1andR2:Theratiooftheseresistorsiscalculatedfrom:

R1/R2=(VOUT/2.5V)-1(7)

R1/R2calculatesto3.0.Theresistorsshouldbechosenfromstandardvalueresistorsintherangeof1.0kΩ-10kΩ.Valuesof3.0kΩforR1,and1.0kΩforR2willbeused.

Thenextlargerstandardvalue(137kΩ)ischosenforRON,yieldinganominalfrequencyof618kHz.

L1:Theinductorvalueisdeterminedbasedontheloadcurrent,ripplecurrent,andtheminimumandmaximuminputvoltage(VIN(min),VIN(max)).RefertoFigure13.

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FIGURE13.InductorCurrent

Tokeepthecircuitincontinuousconductionmode,themaxi-mumallowedripplecurrentistwicetheminimumloadcur-rent,or300mAp-p.Usingthisvalueofripplecurrent,theinductor(L1)iscalculatedusingthefollowing:

(9)

(8)

whereFS(min)istheminimumfrequency(FS-25%).

IPK+=1.0A+0.234A/2=1.117A

RCL:Sinceitisobviousthatthelowerpeakoftheinductorcurrentwaveformdoesnotexceed1.0Aatmaximumloadcurrent(seeFigure13),itisnotnecessarytoincreasethecurrentlimitthreshold.ThereforeRCLisnotneededforthisexercise.Forapplicationswherethelowerpeakexceeds1.0A,seethesectionbelowonincreasingthecurrentlimitthreshold.

C2andR3:SincetheLM5010requiresaminimumof25mVp-pofrippleattheFBpinforproperoperation,therequiredrippleatVOUT1isincreasedbyR1andR2.ThisnecessaryrippleiscreatedbytheinductorripplecurrentactingonC2’sESR+R3.First,theminimumripplecurrentisdetermined.

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ThisprovidesaminimumvalueforL1-thenexthigherstandardvalue(100µH)willbeused.L1mustberatedforthepeakcurrent(IPK+)topreventsaturation.Thepeakcur-rentoccursatmaximumloadcurrentwithmaximumripple.Themaximumrippleiscalculatedbyre-arrangingEquation8usingVIN(max),FS(min),andtheminimuminductorvalue,basedonthemanufacturer’stolerance.Assume,forthisexercise,theinductor’stoleranceis±20%.

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LM5010ApplicationsInformation

(Continued)

peakoftheoutputcurrentwaveform,rampsuptothepeakvalue,thendropstozeroatturn-off.TheaveragecurrentintoVINduringthison-timeistheloadcurrent.Foraworstcasecalculation,C1mustsupplythisaverageloadcurrentduringthemaximumon-time.Themaximumon-timeiscalculatedusingEquation5,witha25%toleranceadded:

(10)

TheminimumESRforC2isthenequalto:

C1iscalculatedfrom:

IfthecapacitorusedforC2doesnothavesufficientESR,R3isaddedinseriesasshowninFigure1.C2shouldgenerallybenosmallerthan3.3µF,althoughthatisdependentonthefrequencyandtheallowablerippleamplitudeatVOUT1.Ex-perimentationisusuallynecessarytodeterminethemini-mumvalueforC2,asthenatureoftheloadmayrequirealargervalue.Aloadwhichcreatessignificanttransientsre-quiresalargervalueforC2thananon-varyingload.

D1:Theimportantparametersarereverserecoverytimeandforwardvoltagedrop.Thereverserecoverytimedetermineshowlongthecurrentsurgelastseachtimethebuckswitchisturnedon.Theforwardvoltagedropissignificantintheeventtheoutputisshort-circuitedasitismainlythisdiode’svoltage(plusthevoltageacrossthecurrentlimitsensere-sistor)whichforcestheinductorcurrenttodecreaseduringtheoff-time.Forthisreason,ahighervoltageisbetter,althoughthataffectsefficiency.Areverserecoverytimeof)30ns,andaforwardvoltagedropof)0.75Varepreferred.Thereverseleakagespecificationisimportantasthatcansignificantlyaffectefficiency.Othertypesofdiodesmayhavealowerforwardvoltagedrop,butmayhavelongerrecoverytimes,orgreaterreverseleakage.D1shouldberatedforthemaximumVIN,andforthepeakcurrentwhenincurrentlimit(IPKinFigure11)whichisequalto:

IPK=1.5A+IOR(max)=1.734A

where1.5Aisthemaximumguaranteedcurrentlimitthresh-old,andthemaximumripplecurrentwaspreviouslycalcu-latedas234mAp-p.NotethatthiscalculationisvalidonlywhenRCLisnotrequired.

C1:AssumingthevoltagesupplyfeedingVINhasasourceimpedancegreaterthanzero,thiscapacitorlimitstheripplevoltageatVINwhilesupplyingmostoftheswitchcurrentduringtheon-time.Atmaximumloadcurrent,whenthebuckswitchturnson,thecurrentintoVINincreasestothelower

whereIOistheloadcurrent,and∆VistheallowableripplevoltageatVIN(1Vforthisexample).Qualityceramiccapaci-torswithalowESRshouldbeusedforC1.Toallowforcapacitortolerancesandvoltageeffects,a2.2µFcapacitorwillbeused

C3:ThecapacitorattheVCCpinprovidesnotonlynoisefilteringandstability,butalsopreventsfalsetriggeringoftheVCCUVLOatthebuckswitchon/offtransitions.Forthisreason,C3shouldbenosmallerthan0.1µF,andshouldbeagoodquality,lowESR,ceramiccapacitor.Thiscapacitoralsodeterminestheinitialstartupdelay(t1inFigure8).C4:TherecommendedvalueforC4is0.022µF.AhighqualityceramiccapacitorwithlowESRisrecommendedasC4suppliesthesurgecurrenttochargethebuckswitchgateatturn-on.AlowESRalsoensuresacompleterechargeduringeachoff-time.

C5:ThiscapacitorsuppressestransientsandringingduetolongleadinductanceatVIN.AlowESR,0.1µFceramicchipcapacitorisrecommended,locatedphysicallyclosetotheLM5010.

C6:ThecapacitorattheSSpindeterminesthesoftstarttime,i.e.thetimeforthereferencevoltageattheregulationcomparator,andtheoutputvoltage,toreachtheirfinalvalue.Thetimeisdeterminedfromthefollowing:

Fora5mssoftstarttime,C6calculatesto0.022µF.FINALCIRCUIT

ThefinalcircuitisshowninFigure14,anditsperformanceisshowninFigures15-18.

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LM5010ApplicationsInformation

(Continued)

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FIGURE14.LM5010ExampleCircuit

ItemC1C2C3C4,C6C5D1L1R1R2R3RONU1

DescriptionCeramicCapacitorCeramicCapacitorCeramicCapacitorCeramicCapacitorCeramicCapacitorUltrafastdiode

InductorResistorResistorResistorResistorSwitchingregulator

PartNo.

TDKC4532X7R2A225MTDKC4532X7R1E156MKemetC0805C104K4RACKemetC0805C223K4RACTDKC2012X7R2A104MCentralSemiCMR2U-01

TDKSLF10145VishayCRCW08053001FVishayCRCW08051001FVishayCRCW08052R80FVishayCRCW08051373FNationalSemiLM5010

Package18121812080508050805SMB10.1x10.10805080508050805

Value2.2µF,100V15µF,25V0.1µF,16V0.022µF,16V0.1µF,100V100V,2A100µH3.0kΩ1.0kΩ2.8Ω137kΩ

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LM5010ApplicationsInformation

(Continued)

20119936

20119934

FIGURE17.OutputVoltageRipplevsVINCircuitofFigure14

FIGURE15.EfficiencyvsVINCircuitofFigure14

20119937

20119935

FIGURE18.FrequencyvsVINCircuitofFigure14

FIGURE16.EfficiencyvsLoadCurrentandVINCircuitofFigure14

INCREASINGTHECURRENTLIMITTHRESHOLD

Thecurrentlimitthresholdisnominally1.25A,withamini-mumguaranteedvalueof1.0A.If,atmaximumloadcurrent,thelowerpeakoftheinductorcurrent(IPK-inFigure13)exceeds1.0A,resistorRCLmustbeaddedbetweenSGNDandISENtoincreasethecurrentlimitthresholdtoequalorexceedthatlowerpeakcurrent.Thisresistordivertssomeoftherecirculatingcurrentfromtheinternalsenseresistorsothatahighercurrentlevelisneededtoswitchtheinternalcurrentlimitcomparator.IPK-iscalculatedfrom:

(11)

whereIO(max)isthemaximumloadcurrent,andIOR(min)istheminimumripplecurrentcalculatedusingEquation10.RCLiscalculatedfrom:

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LM5010ApplicationsInformation

(Continued)

(12)

where0.11ΩistheminimumvalueoftheinternalresistancefromSGNDtoISEN.ThenextsmallerstandardvalueresistorshouldbeusedforRCL.WiththeadditionofRCLitisneces-sarytochecktheaverageandpeakcurrentvaluestoensuretheydonotexceedtheLM5010limits.Atmaximumloadcurrenttheaveragecurrentthroughtheinternalsenseresis-toris:

TheinductorL1anddiodeD1mustberatedforthiscurrent.PCBOARDLAYOUT

TheLM5010regulation,over-voltage,andcurrentlimitcom-paratorsareveryfast,andwillrespondtoshortdurationnoisepulses.Layoutconsiderationsarethereforecriticalforoptimumperformance.Thelayoutmustbeasneatandcompactaspossible,andallthecomponentsmustbeascloseaspossibletotheirassociatedpins.ThecurrentloopformedbyD1,L1,C2,andtheSGNDandISENpinsshouldbeassmallaspossible.ThegroundconnectionfromC2toC1shouldbeasshortanddirectaspossible.IfitisexpectedthattheinternaldissipationoftheLM5010willproducehighjunctiontemperaturesduringnormaloperation,gooduseofthePCboard’sgroundplanecanhelpconsiderablytodissi-pateheat.TheexposedpadontheICpackagebottomcanbesolderedtoagroundplane,andthatplaneshouldbothextendfrombeneaththeIC,andbeconnectedtoexposedgroundplaneontheboard’sothersideusingasmanyviasaspossible.TheexposedpadisinternallyconnectedtotheICsubstrate.

TheuseofwidePCboardtracesatthepins,wherepossible,canhelpconductheatawayfromtheIC.ThefourNoCon-nectpinsontheTSSOPpackagearenotelectricallycon-nectedtoanypartoftheIC,andmaybeconnectedtogroundplanetohelpdissipateheatfromthepackage.Judi-ciouspositioningofthePCboardwithintheendproduct,alongwiththeuseofanyavailableairflow(forcedornaturalconvection)canhelpreducethejunctiontemperature.

(13)

IfIAVEislessthan2.0Anochangesarenecessary.Ifitexceeds2.0A,RCLmustbereduced.Theupperpeakoftheinductorcurrent(IPK+),atmaximumloadcurrent,iscalcu-latedusingthefollowing:

(14)

whereIOR(max)iscalculatedusingEquation9.IfIPK+ex-ceeds3.5A,theinductorvaluemustbeincreasedtoreducetherippleamplitude.ThiswillnecessitaterecalculationofIOR(min),IPK-,andRCL.

Whenthecircuitisincurrentlimit,theupperpeakcurrentoutoftheSWpinis

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LM5010PhysicalDimensions

inches(millimeters)unlessotherwisenoted

14-LeadTSSOPPackageNSPackageNumberMXA14A

10-LeadLLPPackageNSPackageNumberSDC10A

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LM5010HighVoltage1AStepDownSwitchingRegulatorNotes

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