LM25010资料

LM2501042V,1.0AStep-DownSwitchingRegulatorFebruary2006

LM25010

42V,1.0AStep-DownSwitchingRegulator

GeneralDescription

TheLM25010featuresallthefunctionsneededtoimplementalowcost,efficient,buckregulatorcapableofsupplyinginexcessof1Aloadcurrent.Thishighvoltageregulatorinte-gratesanN-ChannelBuckSwitch,andisavailableinther-mallyenhancedLLP-10andTSSOP-14EPpackages.Theconstanton-timeregulationschemerequiresnoloopcom-pensationresultinginfastloadtransientresponseandsim-plifiedcircuitimplementation.Theoperatingfrequencyre-mainsconstantwithlineandloadvariationsduetotheinverserelationshipbetweentheinputvoltageandtheon-time.Thevalleycurrentlimitdetectionissetat1.25A.Addi-tionalfeaturesinclude:VCCunder-voltagelock-out,thermalshutdown,gatedriveunder-voltagelock-out,andmaximumdutycyclelimiter.

Features

nnnnnnnnnnnn

Wide6Vto42VInputVoltageRangeValleyCurrentLimitingAt1.25A

ProgrammableSwitchingFrequencyUpTo1MHzIntegratedN-ChannelBuckSwitch

IntegratedHighVoltageBiasRegulatorNoLoopCompensationRequiredUltra-FastTransientResponse

NearlyConstantOperatingFrequencyWithLineandLoadVariations

AdjustableOutputVoltage

2.5V,±2%FeedbackReferenceProgrammableSoft-StartThermalshutdown

TypicalApplications

nNon-IsolatedTelecommunicationsRegulatornSecondarySidePostRegulator

nPowerSUpplyforAutomotiveElectronics

Package

nLLP-10(4mmx4mm)nTSSOP-14EP

nBothPackagesHaveExposedThermalPadForImprovedHeatDissipation

BasicStep-DownRegulator

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LM25010ConnectionDiagrams

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OrderingInformation

OrderNumberLM25010SDLM25010SDXLM25010MHLM25010MHX

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

NSCPackageDrawingSDC10ASDC10AMXA14AMXA14A

JunctionTemperature

Range−40˚Cto+125˚C−40˚Cto+125˚C−40˚Cto+125˚C−40˚Cto+125˚C

SuppliedAs

1000UnitsonTapeandReel4500UnitsonTapeandReel

94UnitsinRail

2500UnitsonTapeandReel

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LM25010PinDescriptions

PinNumberLLP-101

TSSOP-14

2

NameSW

Description

SwitchingNode

ApplicationInformation

Internallyconnectedtothebuckswitchsource.Connecttotheinductor,free-wheelingdiode,andbootstrapcapacitor.

ConnectacapacitorfromSWtotheBSTpin.ThecapacitorischargedfromVCCviaaninternaldiodeduringthebuckswitchoff-time.

Duringthebuckswitchoff-time,theinductorcurrentflowsthroughtheinternalsenseresistor,andoutoftheISENpintothefree-wheelingdiode.ThecurrentlimitcomparatorkeepsthebuckswitchoffiftheISENcurrentexceeds1.25A(typical).Re-circulatingcurrentflowsintothispintothecurrentsenseresistor.

Groundreturnforallinternalcircuitryotherthanthecurrentsenseresistor.

Inputtoboththeregulationandover-voltage

comparators.TheFBpinregulationlevelis2.5V.Aninternal11.5µAcurrentsourcechargestheSSpincapacitorto2.5Vtosoft-startthereferenceinputoftheregulationcomparator.

AnexternalresistorfromVINtotheRON/SDpinsetsthebuckswitchon-time.Groundingthispinshutsdowntheregulator.

ThevoltageatVCCisnominallyequaltoVINforVIN<8.9V,andregulatedat7VforVIN>8.9V.

Connecta0.47µF,orlargercapacitorfromVCCtoground,ascloseaspossibletothepins.AnexternalvoltagecanbeappliedtothispintoreduceinternaldissipationifVINisgreaterthan8.9V.MOSFETbodydiodesclampVCCtoVINifVCC>VIN.Nominalinputrangeis6Vto42V.Inputbypass

capacitorsshouldbelocatedascloseaspossibletotheVINpinandRTNpins.

Nointernalconnection.Canbeconnectedtogroundplanetoimproveheatdissipation.

Exposedmetalpadontheundersideofthedevice.ItisrecommendedtoconnectthispadtothePCboardgroundplanetoaidinheatdissipation.

23BSTBoostpinforbootstrapcapacitor

34ISENCurrentsense

4567

56910

SGNDRTNFBSS

CurrentSenseGroundCircuitGround

VoltagefeedbackinputfromtheregulatedoutputSoftstart

811RON/SDOn-timecontrolandshutdown

912VCCOutputofthebiasregulator

1013VINInputsupplyvoltage

1,7,8,14NCEP

Noconnection.ExposedPad

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LM25010AbsoluteMaximumRatings(Note1)

IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.VINtoRTNBSTtoRTN

SWtoRTN(SteadyState)BSTtoVCCBSTtoSWVCCtoRTNSGNDtoRTNSStoRTN

-0.3Vto45V-0.3Vto59V

-1.5V45V14V

-0.3Vto14V-0.3Vto+0.3V-0.3Vto4V

VINtoSW

AllOtherInputstoRTNESDRating(Note2)HumanBodyModelStorageTemperatureRange

LeadTemperature(Soldering4sec)(Note4)

45V

-0.3Vto7V

2kV

-65˚Cto+150˚C

260˚C

OperatingRatings(Note1)

VINVoltage

JunctionTemperatureLM25010

−40˚Cto+125˚C

6.0Vto42V

SpecificationswithstandardtypeareforTJ=25˚Conly;limitsinboldfacetypeap-plyoverthefullOperatingJunctionTemperature(TJ)range.MinimumandMaximumlimitsareguaranteedthroughtest,de-sign,orstatisticalcorrelation.TypicalvaluesrepresentthemostlikelyparametricnormatTJ=25˚C,andareprovidedforref-erencepurposesonly.Unlessotherwisestatedthefollowingconditionsapply:VIN=24V,RON=200kΩ.See(Note5).SymbolVCCRegulatorVCCReg

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

UVLOVcc

VCCunder-voltagelock-outthreshold

UVLOVCChysteresisUVLOVCCfilterdelayIINoperatingcurrentIINshutdowncurrent

SwitchCharacteristicsRDS(on)UVLOGDBuckSwitchRDS(on)GateDriveUVLOUVLOGDhysteresis

SOFT-STARTPin

ISSCurrentLimit

ILIMThreshold

ResistancefromISENtoSGNDResponsetime

OnTimer,RON/SDPintON-1tON-2

On-timeOn-time

ShutdownthresholdThresholdhysteresis

VIN=10V,RON=200kΩVIN=42V,RON=200kΩVoltageatRON/SDrising

2.15000.30

2.756950.740

3.401.05

µsnsVmV

CurrentoutofISEN1

1.25130150

1.5

AmΩns

Internalcurrentsource

8.0

11.5

15

µA

ISW=200mA

VBST-VSWIncreasing

1.7

0.353.0400

0.804.0

ΩVmV

ICC=0mA,FS≤200kHz,6.0V≤VIN≤8.5V

VINIncreasingVINDecreasingVIN=6.0VVIN=8.0VVIN=24V

VIN=24V,VCC=0VVCCIncreasingVCCDecreasing100mVoverdriveNon-switching,FB=3VRON/SD=0V

6.6

71008.926055500.21155.251803590

920170

mAVmVµsµAµA

7.4

VoltsmVVmVΩ

Parameter

Conditions

Min

Typ

Max

Units

ElectricalCharateristics

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LM25010ElectricalCharateristicsSpecificationswithstandardtypeareforTJ=25˚Conly;limitsinboldfacetypeapply

overthefullOperatingJunctionTemperature(TJ)range.MinimumandMaximumlimitsareguaranteedthroughtest,design,orstatisticalcorrelation.TypicalvaluesrepresentthemostlikelyparametricnormatTJ=25˚C,andareprovidedforreferencepurposesonly.Unlessotherwisestatedthefollowingconditionsapply:VIN=24V,RON=200kΩ.See(Note5).(Continued)SymbolOffTimer

tOFFVREFMinimumOff-timeFBregulationthresholdFBover-voltagethresholdFBbiascurrent

ThermalShutdown

TSDThermalshutdowntemperatureThermalshutdownhysteresis

ThermalResistance

θJAθJCJunctiontoAmbient,0LFPMAirFlowJunctiontoCase

SDCPackageMXAPackageSDCPackageMXAPackage

40405.25.2

˚C/W˚C/W

17520

˚C˚C

2.445

2602.502.91

2.550

nsVVnA

RegulationandOver-VoltageComparators(FBPin)

Parameter

Conditions

Min

Typ

Max

Units

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

Note3:VCCprovidesbiasfortheinternalgatedriveandcontrolcircuits.Devicethermallimitationslimitexternalloading.

Note4:FordetailedinformationonsolderingplasticTSSOPandLLPpackagesrefertothePackagingDataBookavailablefromNationalSemiconductorCorporation.

Note5:Typicalspecificationsrepresentthemostlikelyparametricnormat25˚Coperation.

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LM25010TypicalPerformanceCharacteristics

VCCvsVINVCCvsICC20172705

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ICCvsExternallyAppliedVCCOn-TimevsVINandRON2017270620172707

VoltageatRON/SDPin

IINvsVIN20172710

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LM25010BlockDiagram

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LM2501020172711

FIGURE1.StartupSequence

FunctionalDescription

TheLM25010Step-DownSwitchingRegulatorfeaturesallthefunctionsneededtoimplementalowcost,efficientbuckDC-DCconvertercapableofsupplyinginexcessof1Atotheload.ThishighvoltageregulatorintegratesanN-Channelbuckswitch,withaneasytoimplementconstanton-timecontroller.ItisavailableinthethermallyenhancedLLP-10andTSSOP-14EPpackages.Theregulatorcomparesthefeedbackvoltagetoa2.5Vreferencetocontrolthebuckswitch,andprovidesaswitchon-timewhichvariesinverselywithVIN.Thisfeatureresultsintheoperatingfrequencyremainingrelativelyconstantwithloadandinputvoltagevariations.Theswitchingfrequencycanrangefromlessthan100kHzto1.0MHz.Theregulatorrequiresnoloopcom-pensationresultinginveryfastloadtransientresponse.Thevalleycurrentlimitcircuitholdsthebuckswitchoffuntilthefree-wheelinginductorcurrentfallsbelowthecurrentlimitthreshold,nominallysetat1.25A.

TheLM25010canbeappliedinnumerousapplicationstoefficientlystep-downhigherDCvoltages.Featuresinclude:Thermalshutdown,VCCunder-voltagelock-out,gatedriveunder-voltagelock-out,andmaximumdutycyclelimit.

ControlCircuitOverview

TheLM25010employsacontrolschemebasedonacom-paratorandaone-shoton-timer,withtheoutputvoltagefeedback(FB)comparedtoaninternalreference(2.5V).IftheFBvoltageisbelowthereferencethebuckswitchisturnedonforatimeperioddeterminedbytheinputvoltageandaprogrammingresistor(RON).Followingtheon-timetheswitchremainsoffforafixed260nsoff-time,oruntiltheFBvoltagefallsbelowthereference,whicheverislonger.Thebuckswitchthenturnsonforanotheron-timeperiod.Refer-ringtotheBlockDiagram,theoutputvoltageissetbyR1andR2.Theregulatedoutputvoltageiscalculatedasfol-lows:

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

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LM25010ControlCircuitOverview

(Continued)

whereRL=theloadresistance.

TheLM25010requiresaminimumof25mVofripplevoltageattheFBpinforstablefixed-frequencyoperation.Iftheoutputcapacitor’sESRisinsufficientadditionalseriesresis-tancemayberequired(R3intheBlockDiagram).

TheLM25010operatesincontinuousconductionmodeatheavyloadcurrents,anddiscontinuousconductionmodeatlightloadcurrents.Incontinuousconductionmodecurrentalwaysflowsthroughtheinductor,neverdecayingtozeroduringtheoff-time.Inthismodetheoperatingfrequencyremainsrelativelyconstantwithloadandlinevariations.Theminimumloadcurrentforcontinuousconductionmodeisone-halftheinductor’sripplecurrentamplitude.Theoperat-ingfrequencyinthecontinuousconductionmodeiscalcu-latedasfollows:

Start-UpBiasRegulator(VCC)

AhighvoltagebiasregulatorisintegratedwithintheLM25010.Theinputpin(VIN)canbeconnecteddirectlytolinevoltagesbetween6Vand42V.ReferringtotheblockdiagramandthegraphofVCCvs.VIN,whenVINisbetween6Vandthebypassthreshold(nominally8.9V),thebypassswitch(Q2)ison,andVCCtracksVINwithin100mVto150mV.Thebypassswitchon-resistanceisapproximately50Ω,withinherentcurrentlimitingatapproximately100mA.WhenVINisabovethebypassthreshold,Q2isturnedoff,andVCCisregulatedat7V.TheVCCregulatoroutputcurrentislimitedatapproximately15mA.WhentheLM25010isshutdownusingtheRON/SDpin,theVCCbypassswitchisshutoff,regardlessofthevoltageatVIN.

WhenVINexceedsthebypassthreshold,thetimerequiredforQ2toshutoffisapproximately2-3µs.ThecapacitoratVCC(C3)mustbeaminimumof0.47µFtopreventthevoltageatVCCfromrisingaboveitsabsolutemaximumratinginresponsetoastepinputappliedatVIN.C3mustbelocatedascloseaspossibletotheLM25010pins.

Inapplicationswitharelativelyhighinputvoltage,powerdissipationinthebiasregulatorisaconcern.Anauxiliaryvoltageofbetween7.5Vand14VcanbediodeconnectedtotheVCCpin(D2inFigure2)toshutofftheVCCregulator,reducinginternalpowerdissipation.ThecurrentrequiredintotheVCCpinisshownintheTypicalPerformanceCharacter-istics.InternallyadiodeconnectsVCCtoVINrequiringthattheauxiliaryvoltagebelessthanVIN.

Theturn-onsequenceisshowninFigure1.WhenVCCexceedstheunder-voltagelock-outthreshold(UVLO)of5.25V(t1inFigure1),thebuckswitchisenabled,andtheSSpinisreleasedtoallowthesoft-startcapacitor(C6)tochargeup.TheoutputvoltageVOUTisregulatedatareducedlevelwhichincreasestothedesiredvalueasthesoft-startvoltageincreases(t2inFigure1).

(2)

Thebuckswitchdutycycleisequalto:

(3)

Underlightloadconditions,theLM25010operatesindiscon-tinuousconductionmode,withzerocurrentflowingthroughtheinductorforaportionoftheoff-time.Theoperatingfrequencyisalwayslowerthanthatofthecontinuouscon-ductionmode,andtheswitchingfrequencyvarieswithloadcurrent.Conversionefficiencyismaintainedatarelativelyhighlevelatlightloadssincetheswitchinglossesdiminishasthepowerdeliveredtotheloadisreduced.Thediscon-tinuousmodeoperatingfrequencyisapproximately:

(4)

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FIGURE2.SelfBiasedConfiguration

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LM25010RegulationComparator

ThefeedbackvoltageattheFBpiniscomparedtothevoltageattheSSpin(2.5V,±2%).Innormaloperationanon-timeperiodisinitiatedwhenthevoltageatFBfallsbelow2.5V.Thebuckswitchconductsfortheon-timeprogrammedbyRON,causingtheFBvoltagetoriseabove2.5V.Aftertheon-timeperiodthebuckswitchremainsoffuntiltheFBvoltagefallsbelow2.5V.InputbiascurrentattheFBpinislessthan5nAovertemperature.

Tosetaspecificcontinuousconductionmodeswitchingfrequency(Fs),theRONresistorisdeterminedfromthefollowing:

Over-VoltageComparator

ThefeedbackvoltageatFBiscomparedtoaninternal2.9Vreference.IfthevoltageatFBrisesabove2.9Vtheon-timeisimmediatelyterminated.Thisconditioncanoccuriftheinputvoltage,ortheoutputload,changessuddenly.ThebuckswitchremainsoffuntilthevoltageatFBfallsbelow2.5V.

(7)

InhighfrequencyapplicationstheminimumvaluefortONislimitedbythemaximumdutycyclerequiredforregulationandtheminimumoff-timeoftheLM25010(260ns,±15%).Thefixedoff-timelimitsthemaximumdutycycleachievablewithalowvoltageatVIN.Theminimumallowedon-timetoregulatethedesiredVOUTattheminimumVINisdeterminedfromthefollowing:

ON-TimeControl

Theon-timeoftheinternalbuckswitchisdeterminedbytheRONresistorandtheinputvoltage(VIN),andiscalculatedasfollows:

(8)

Shutdown

TheLM25010canberemotelyshutdownbyforcingtheRON/SDpinbelow0.7Vwithaswitchoropendraindevice.SeeFigure3.IntheshutdownmodetheSSpinisinternallygrounded,theon-timeone-shotisdisabled,theinputcurrentatVINisreduced,andtheVCCbypassswitchisturnedoff.TheVCCregulatorisnotdisabledintheshutdownmode.ReleasingtheRON/SDpinallowsnormaloperationtore-sume.ThenominalvoltageatRON/SDisshownintheTypicalPerformanceCharacteristics.WhenswitchingtheRON/SDpin,thetransitiontimeshouldbefasterthanonetotwocyclesoftheregulator’snominalswitchingfrequency.

(5)

TheRONresistorcanbedeterminedfromthedesiredon-timebyre-arrangingEquation5tothefollowing:

(6)

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FIGURE3.ShutdownImplementation

CurrentLimit

Currentlimitdetectionoccursduringtheoff-timebymonitor-ingtherecirculatingcurrentthroughtheinternalcurrentsenseresistor(RSENSE).Thedetectionthresholdis1.25A,±0.25A.ReferringtotheBlockDiagram,ifthecurrentintoSGNDduringtheoff-timeexceedsthethresholdlevelthecurrentlimitcomparatordelaysthestartofthenexton-timeperiod.Thenexton-timestartswhenthecurrentintoSGNDisbelowthethresholdandthevoltageatFBisbelow2.5V.Figure4illustratestheinductorcurrentwaveformduringnormaloperationandduringcurrentlimit.TheoutputcurrentIOistheaverageoftheinductorripplecurrentwaveform.TheLowLoadCurrentwaveformillustratescontinuouscon-ductionmodeoperationwithpeakandvalleyinductorcur-www.national.com

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rentsbelowthecurrentlimitthreshold.Whentheloadcur-rentisincreased(HighLoadCurrent),theripplewaveformmaintainsthesameamplitudeandfrequencysincethecur-rentfallsbelowthecurrentlimitthresholdatthevalleyoftheripplewaveform.NotetheaveragecurrentintheHighLoadCurrentportionofFigure4isabovethecurrentlimitthresh-old.Sincethecurrentreducesbelowthethresholdinthenormaloff-timeeachcycle,thestartofeachon-timeisnotdelayed,andthecircuit’soutputvoltageisregulatedatthecorrectvalue.Whentheloadcurrentisfurtherincreasedsuchthatthelowerpeakwouldbeabovethethreshold,theoff-timeislengthenedtoallowthecurrenttodecreasetothethresholdbeforethenexton-timebegins(CurrentLimitedportionofFigure4).BothVOUTandtheswitchingfrequencyarereducedasthecircuitoperatesinaconstantcurrent

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LM25010CurrentLimit

(Continued)

mode.Theloadcurrent(IOCL)isequaltothecurrentlimitthresholdplushalftheripplecurrent(∆I/2).Therippleampli-tude(∆I)iscalculatedfrom:

Thecurrentlimitthresholdcanbeincreasedbyconnectinganexternalresistor(RCL)betweenSGNDandISEN.RCLtypicallyislessthan1Ω,andthecalculationofitsvalueisexplainedintheApplicationsInformationsection.Ifthecur-rentlimitthresholdisincreasedbyaddingRCL,themaximumcontinuousloadcurrentshouldnotexceed1.5A,andthepeakcurrentoutoftheSWpinshouldnotexceed2A.

(9)

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FIGURE4.InductorCurrent-CurrentLimitOperation

N-ChannelBuckSwitchandDriver

TheLM25010integratesanN-Channelbuckswitchandassociatedfloatinghighvoltagegatedriver.Thepeakcur-rentthroughthebuckswitchshouldnotexceed2A,andtheloadcurrentshouldnotexceed1.5A.ThegatedrivercircuitispoweredbytheexternalbootstrapcapacitorbetweenBSTandSW(C4),whichisrechargedeachoff-timefromVCCthroughtheinternalhighvoltagediode.Theminimumoff-time,nominally260ns,ensuressufficienttimeduringeachcycletorechargethebootstrapcapacitor.A0.022µFce-ramiccapacitorisrecommendedforC4.

tureincreasesduringafaultorabnormaloperatingcondi-tion,theinternalThermalShutdowncircuitactivatestypicallyat175˚C.TheThermalShutdowncircuitreducespowerdissipationbydisablingthebuckswitchandtheon-timer,andgroundingtheSSpin.Thisfeaturehelpspreventcata-strophicfailuresfromaccidentaldeviceoverheating.Whenthejunctiontemperaturereducesbelowapproximately155˚C(20˚Ctypicalhysteresis),theSSpinisreleasedandnormaloperationresumes.

ApplicationsInformation

EXTERNALCOMPONENTS

Theprocedureforcalculatingtheexternalcomponentsisillustratedwithadesignexample.ReferringtotheBlockDiagram,thecircuitistobeconfiguredforthefollowingspecifications:•VOUT=5V

•VIN=6Vto40V•FS=175kHz

•Minimumloadcurrent=200mA

Soft-Start

Thesoft-startfeatureallowstheregulatortograduallyreachasteadystateoperatingpoint,therebyreducingstartupstressesandcurrentsurges.Atturn-on,whileVCCisbelowtheunder-voltagethreshold(t1inFigure1),theSSpinisinternallygrounded,andVOUTisheldat0V.WhenVCCexceedstheunder-voltagethreshold(UVLO)aninternal11.5µAcurrentsourcechargestheexternalcapacitor(C6)attheSSpinto2.5V(t2inFigure1).TheincreasingSSvoltageatthenon-invertinginputoftheregulationcompara-torgraduallyincreasestheoutputvoltagefromzerotothedesiredvalue.Thesoft-startfeaturekeepstheloadinductorcurrentfromreachingthecurrentlimitthresholdduringstart-up,therebyreducinginrushcurrents.

AninternalswitchgroundstheSSpinifVCCisbelowtheunder-voltagelock-outthreshold,ifathermalshutdownoc-curs,orifthecircuitisshutdownusingtheRON/SDpin.

ThermalShutdown

TheLM25010shouldbeoperatedbelowtheMaximumOp-eratingJunctionTemperaturerating.Ifthejunctiontempera-11

•Maximumloadcurrent=1.0A•Softstarttime=5ms.

R1andR2:Theseresistorssettheoutputvoltage,andtheirratioiscalculatedfrom:

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

R1/R2calculatesto1.0.Theresistorsshouldbechosenfromstandardvalueresistorsintherangeof1.0kΩ-10kΩ.Avalueof1.0kΩwillbeusedforR1andforR2.

RON,FS:RONcanbechosenusingEquation7tosetthenominalfrequency,orfromEquation6iftheon-timeataparticularVINisimportant.Ahigherfrequencygenerallymeansasmallerinductorandcapacitors(value,sizeandcost),buthigherswitchinglosses.Alowerfrequencymeans

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LM25010ApplicationsInformation

(Continued)

ahigherefficiency,butwithlargercomponents.Generally,ifPCboardspaceistight,ahigherfrequencyisbetter.Theresultingon-timeandfrequencyhavea±25%tolerance.Usingequation7atanominalVINof8V,

Avalueof200kΩwillbeusedforRON,yieldinganominalfrequencyof161kHzatVIN=6V,and203kHzatVIN=40V.L1:Theguidelineforchoosingtheinductorvalueinthisexampleisthatitmustkeepthecircuit’soperationincon-tinuousconductionmodeatminimumloadcurrent.ThisisnotastrictrequirementsincetheLM25010regulatescor-rectlywhenindiscontinuousconductionmode,althoughatalowerfrequency.However,toprovideaninitialvalueforL1theaboveguidelinewillbeused.

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FIGURE5.InductorCurrent

Tokeepthecircuitincontinuousconductionmode,themaxi-mumallowedripplecurrentistwicetheminimumloadcur-rent,or400mAp-p.Usingthisvalueofripplecurrent,theinductor(L1)iscalculatedusingthefollowing:

RCL:Sinceitisobviousthatthelowerpeakoftheinductorcurrentwaveformdoesnotexceed1.0Aatmaximumloadcurrent(seeFigure5),itisnotnecessarytoincreasethecurrentlimitthreshold.ThereforeRCLisnotneededforthisexercise.Forapplicationswherethelowerpeakexceeds1.0A,seethesectionentitledIncreasingTheCurrentLimitThreshold.

C1:ThiscapacitorlimitstheripplevoltageatVINresultingfromthesourceimpedanceofthesupplyfeedingthiscircuit,andtheon/offnatureoftheswitchcurrentintoVIN.Atmaximumloadcurrent,whenthebuckswitchturnson,thecurrentintoVINstepsupfromzerotothelowerpeakoftheinductorcurrentwaveform(IPK-inFigure5),rampsuptothepeakvalue(IPK+),thendropstozeroatturn-off.TheaveragecurrentintoVINduringthison-timeistheloadcurrent.Foraworstcasecalculation,C1mustsupplythisaveragecurrentduringthemaximumon-time.Themaximumon-timeiscal-culatedatVIN=6VusingEquation5,witha25%toleranceadded:

(11)

whereFS(min)istheminimumfrequencyof152kHz(203kHz-25%)atVIN(max).

ThisprovidesaminimumvalueforL1-thenexthigherstandardvalue(100µH)willbeused.Topreventsaturation,andpossibledestructivecurrentlevels,L1mustberatedforthepeakcurrentwhichoccursifthecurrentlimitandmaxi-mumripplecurrentarereachedsimultaneously(IPKinFigure4).Themaximumrippleamplitudeiscalculatedbyre-arrangingEquation11usingVIN(max),FS(min),andthemini-muminductorvalue,basedonthemanufacturer’stolerance.Assume,forthisexercise,theinductor’stoleranceis±20%.

ThevoltageatVINshouldnotbeallowedtodropbelow5.5VinordertomaintainVCCaboveitsUVLO.

(12)

IPK=ILIM+IOR(max)=1.5A+0.36A=1.86A

whereILIMisthemaximumguaranteedcurrentlimitthresh-old.Atthenominalmaximumloadcurrentof1.0A,thepeakinductorcurrentis1.18A.

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NormallyalowervaluecanbeusedforC1sincetheabovecalculationisaworstcasecalculationwhichassumesthepowersourcehasahighsourceimpedance.Aqualityce-ramiccapacitorwithalowESRshouldbeusedforC1.C2andR3:SincetheLM25010requiresaminimumof25mVp-pofrippleattheFBpinforproperoperation,therequiredrippleatVOUTisincreasedbyR1andR2,andisequalto:

VRIPPLE=25mVp-px(R1+R2)/R2=50mVp-p

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LM25010ApplicationsInformation

(Continued)

ThisnecessaryripplevoltageiscreatedbytheinductorripplecurrentactingonC2’sESR+R3.First,theminimumripplecurrent,whichoccursatminimumVIN,maximumin-ductorvalue,andmaximumfrequency,isdetermined.

C4suppliesthesurgecurrenttochargethebuckswitchgateateachturn-on.AlowESRalsoensuresacompletere-chargeduringeachoff-time.

C5:ThiscapacitorsuppressestransientsandringingduetoleadinductanceatVIN.AlowESR,0.1µFceramicchipcapacitorisrecommended,locatedphysicallyclosetotheLM25010.

C6:ThecapacitorattheSSpindeterminesthesoft-starttime,i.e.thetimeforthereferencevoltageattheregulationcomparator,andtheoutputvoltage,toreachtheirfinalvalue.Thecapacitorvalueisdeterminedfromthefollowing:

(13)

TheminimumESRforC2isthenequalto:

Fora5mssoftstarttime,C6calculatesto0.022µF.D1:ASchottkydiodeisrecommended.Ultra-fastrecoverydiodesarenotrecommendedasthehighspeedtransitionsattheSWpinmayinadvertentlyaffecttheIC’soperationthroughexternalorinternalEMI.ThediodeshouldberatedforthemaximumVIN(40V),themaximumloadcurrent(1A),andthepeakcurrentwhichoccurswhencurrentlimitandmaximumripplecurrentarereachedsimultaneously(IPKinFigure4),previouslycalculatedtobe1.86A.Thediode’sforwardvoltagedropaffectsefficiencyduetothepowerdissipatedduringtheoff-time.Theaveragepowerdissipa-tioninD1iscalculatedfrom:

PD1=VFxIOx(1-D)

whereIOistheloadcurrent,andDisthedutycycle.FINALCIRCUIT

ThefinalcircuitisshowninFigure6,anditsperformanceisshowninFigures7&8.Currentlimitmeasuredapproxi-mately1.3A.

IfthecapacitorusedforC2doesnothavesufficientESR,R3isaddedinseriesasshownintheBlockDiagram.ThevaluechosenforC2isapplicationdependent,anditisrecom-mendedthatitbenosmallerthan3.3µF.C2affectstherippleatVOUT,andtransientresponse.ExperimentationisusuallynecessarytodeterminetheoptimumvalueforC2.C3:ThecapacitorattheVCCpinprovidesnoisefilteringandstability,preventsfalsetriggeringoftheVCCUVLOatthebuckswitchon/offtransitions,andlimitsthepeakvoltageatVCCwhenahighvoltagewithashortrisetimeisinitiallyappliedatVIN.C3shouldbenosmallerthan0.47µF,andshouldbeagoodquality,lowESR,ceramiccapacitor,physi-callyclosetotheICpins.

C4:TherecommendedvalueforC4is0.022µF.AhighqualityceramiccapacitorwithlowESRisrecommendedas

20172733

FIGURE6.ExampleCircuit

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LM25010ApplicationsInformation

ItemC1C2C3C4,C6C5D1L1R1R2R3RONU1

(Continued)

DescriptionCeramicCapacitorCeramicCapacitorCeramicCapacitorCeramicCapacitorCeramicCapacitorSchottkyDiode

InductorResistorResistorResistorResistor

NationalSemiLM25010

MINIMUMLOADCURRENT

TheLM25010requiresaminimumloadcurrentof500µA.Iftheloadcurrentfallsbelowthatlevel,thebootstrapcapacitor(C4)maydischargeduringthelongoff-time,andthecircuitwilleithershutdown,orcycleonandoffatalowfrequency.Iftheloadcurrentisexpectedtodropbelow500µAintheapplication,R1andR2shouldbechosenlowenoughinvaluesotheyprovidetheminimumrequiredcurrentatnomi-nalVOUT.

LOWOUTPUTRIPPLECONFIGURATIONS

ForapplicationswherelowoutputvoltagerippleisrequiredtheoutputcanbetakendirectlyfromthelowESRoutputcapacitor(C2)asshowninFigure9.However,R3slightlydegradestheloadregulation.Thespecificcomponentval-ues,andtheapplicationdetermineifthisissuitable.

Value(2)2.2µF,50V22µF,16V0.47µF,16V0.022µF,16V0.1µF,50V60V,2A100µH1.0kΩ1.0kΩ1.5Ω200kΩ

20172735

FIGURE7.EfficiencyvsLoadCurrentandVINCircuitofFigure6

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FIGURE9.LowRippleOutput

WherethecircuitofFigure9isnotsuitable,thecircuitsofFigure10orFigure11canbeused.

20172737

20172748

FIGURE8.FrequencyvsVINCircuitofFigure6

FIGURE10.LowOutputRippleUsingaFeedforward

Capacitor

14

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LM25010ApplicationsInformation

(Continued)

whenthecircuitisincurrentlimit,theupperpeakcurrentoutoftheSWpin(IPKinFigure4)canbeashighas:

InFigure10,CffisaddedacrossR1toAC-coupletherippleatVOUTdirectlytotheFBpin.ThisallowstherippleatVOUTtobereduced,insomecasesconsiderably,byreducingR3.InthecircuitofFigure6,therippleatVOUTrangedfrom50mVp-patVIN=6Vto285mVp-patVIN=40V.Byaddinga1000pFcapacitoratCffandreducingR3to0.75Ω,theVOUTripplewasreducedby50%,rangingfrom25mVp-pto142mVp-p.

whereIOR(max)iscalculatedusingEquation12.TheinductorL1anddiodeD1mustberatedforthiscurrent.IfIPKexceeds2A,theinductorvaluemustbeincreasedtoreducetherippleamplitude.ThiswillnecessitaterecalculationofIOR(min),IPK-,andRCL.

Increasingthecircuit’scurrentlimitwillincreasepowerdis-sipationandthejunctiontemperaturewithintheLM25010.Seethenextsectionforguidelinesonthisissue.

PCBOARDLAYOUTandTHERMALCONSIDERATIONS

20172749

FIGURE11.LowOutputRippleUsingRippleInjectionToreduceVOUTripplefurther,thecircuitofFigure11canbeused.R3hasbeenremoved,andtheoutputrippleamplitudeisdeterminedbyC2’sESRandtheinductorripplecurrent.RAandCAarechosentogeneratea40-50mVp-psawtoothattheirjunction,andthatvoltageisAC-coupledtotheFBpinviaCB.InselectingRAandCA,VOUTisconsideredavirtualgroundastheSWpinswitchesbetweenVINand-1V.Sincetheon-timeatSWvariesinverselywithVIN,thewaveformamplitudeattheRA/CAjunctionisrelativelyconstant.R1andR2musttypicallybeincreasedtomorethan5keachtonotsignificantlyattenuatethesignalprovidedtoFBthroughCB.TypicalvaluesfortheadditionalcomponentsareRA=200k,CA=680pF,andCB=0.01µF.

INCREASINGTHECURRENTLIMITTHRESHOLD

Thecurrentlimitthresholdisnominally1.25A,withamini-mumguaranteedvalueof1.0A.If,atmaximumloadcurrent,thelowerpeakoftheinductorcurrent(IPK-inFigure5)exceeds1.0A,resistorRCLmustbeaddedbetweenSGNDandISENtoincreasethecurrentlimitthresholdtoequalorexceedthatlowerpeakcurrent.Thisresistordivertssomeoftherecirculatingcurrentfromtheinternalsenseresistorsothatahighercurrentlevelisneededtoswitchtheinternalcurrentlimitcomparator.IPK-iscalculatedfrom:

(14)

whereIO(max)isthemaximumloadcurrent,andIOR(min)istheminimumripplecurrentcalculatedusingEquation13.RCLiscalculatedfrom:

(15)

where0.11ΩistheminimumvalueoftheinternalresistancefromSGNDtoISEN.Thenextsmallerstandardvalueresis-torshouldbeusedforRCL.WiththeadditionofRCL,and

TheLM25010regulation,over-voltage,andcurrentlimitcomparatorsareveryfast,andwillrespondtoshortdurationnoisepulses.Layoutconsiderationsarethereforecriticalforoptimumperformance.Thelayoutmustbeasneatandcompactaspossible,andallthecomponentsmustbeascloseaspossibletotheirassociatedpins.Thetwomajorcurrentloopshavecurrentswhichswitchveryfast,andsotheloopsshouldbeassmallaspossibletominimizecon-ductedandradiatedEMI.ThefirstloopisthatformedbyC1,throughtheVINtoSWpins,L1,C2,andbacktoC1.ThesecondloopisthatformedbyD1,L1,C2,andtheSGNDandISENpins.ThegroundconnectionfromC2toC1shouldbeasshortanddirectaspossible,preferablywithoutgoingthroughvias.DirectlyconnecttheSGNDandRTNpintoeachother,andtheyshouldbeconnectedasdirectlyaspossibletotheC1/C2groundlinewithoutgoingthroughvias.ThepowerdissipationwithintheICcanbeapproximatedbydeterminingthetotalconversionloss(PIN-POUT),andthensubtractingthepowerlossesinthefree-wheelingdiodeandtheinductor.Thepowerlossinthediodeisapproximately:

PD1=IOxVFx(1-D)

whereIoistheloadcurrent,VFisthediode’sforwardvoltagedrop,andDisthedutycycle.Thepowerlossintheinductorisapproximately:

PL1=IO2xRLx1.1

whereRListheinductor’sDCresistance,andthe1.1factorisanapproximationfortheAClosses.IfitisexpectedthattheinternaldissipationoftheLM25010willproducehighjunctiontemperaturesduringnormaloperation,gooduseofthePCboard’sgroundplanecanhelpconsiderablytodissi-pateheat.TheexposedpadontheICpackagebottomshouldbesolderedtoagroundplane,andthatplaneshouldbothextendfrombeneaththeIC,andbeconnectedtoexposedgroundplaneontheboard’sothersideusingasmanyviasaspossible.Theexposedpadisinternallycon-nectedtotheICsubstrate.TheuseofwidePCboardtracesatthepins,wherepossible,canhelpconductheatawayfromtheIC.ThefourNoConnectpinsontheTSSOPpackagearenotelectricallyconnectedtoanypartoftheIC,andmaybeconnectedtogroundplanetohelpdissipateheatfromthepackage.JudiciouspositioningofthePCboardwithintheendproduct,alongwiththeuseofanyavailableairflow(forcedornaturalconvection)canhelpreducethejunctiontemperature.

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LM25010PhysicalDimensions

inches(millimeters)unlessotherwisenoted

14-LeadTSSOPPackageNSPackageNumberMXA14A

10-LeadLLPPackageNSPackageNumberSDC10A

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LM2501042V,1.0AStep-DownSwitchingRegulatorNotes

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