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
Nationaldoesnotassumeanyresponsibilityforuseofanycircuitrydescribed,nocircuitpatentlicensesareimpliedandNationalreservestherightatanytimewithoutnoticetochangesaidcircuitryandspecifications.Forthemostcurrentproductinformationvisitusatwww.national.com.LIFESUPPORTPOLICY
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