Managing Inrush Current (Rev. A) - Texas Instruments

1 ApplicationReportSLVA670A August2014 RevisedMay 2015 ManagingInrushCurrentAlek Kakneviciusand LoadSwitchesABSTRACTIn mostsystems,capacitorsare placedthroughouta designto ensurethereare no voltagedropson initiallyappliedto the system,chargingthesecapacitorscan resultin aninrushcurrentwhichcan exceedthe nominalload left unaddressed,this can causevoltagerailsto fall out of regulation,resultingin the systementeringan ,the inrushcurrentcan exceedthe currentcarryingcapabilityof boardconnectorsas wellas PCBtraces,resultingindamagingthe connectorsand be mitigatedby loadswitchesin theTPS229xxfamilyare slewrate controlledto applicationnoteexplorestypicalcausesof inrushcurrent,problemscausedby inrushcurrent,and solutionsfor inrushcurrentfeaturingintegratedload InrushCurrent?.. "Soft-start or Disadvantagesof Usinga August2014 RevisedMay 2015 ManagingInrushCurrentSubmitDocumentation FeedbackCopyright 2014 2015,TexasInstrumentsIncorporatedINRUSHL OADdVICdt= LoadLoadLoadPower SupplyWhatis InrushCurrent?

2 InrushCurrent?An examplesystem,shownin Figure1, usesa powersupply DC/DC,LDO,or externalsupply tosupplyvoltageto a TypicalPowerDistributionUponsystemstartu p,the powersupplywill rampup to the the voltageincreases,an inrushof currentflowsinto the also be generatedwhenacapacitiveload is switchedontoa powerrail and mustbe chargedto that amountofinrushcurrentinto the capacitorsis determinedby the slopeof the voltagerampas describedinEquation1:(1)WhereIINRUSH= amountof inrushcurrentcausedby a capacitanceC = total capacitancedV = changein voltageduringrampupdt = rise time (duringvoltagerampup)2 ManagingInrushCurrentSLVA670A August2014 RevisedMay 2015 SubmitDocumentationFeedbackCopyright 2014 2015, InrushCurrent? LoadCapacitanceIncreasingthe systemcapacitanceto reducetransientvoltagedips comesat the cost of increasedinrushcurrentgeneratedfromcharg ingthe followingtwo figuresdisplayinrushcurrentby showinga powersupplystartingup into , below,showsascopeshot of a V powersupplystartingup into a 47 F a 47 F CapacitorIn Figure2, as the powersupplyturnson and the capacitorcharges, of , below,showsthe samepowersupplyturningon with a V Appliedto a 22 F CapacitorWitha reducedcapacitanceof 22 F, Figure3 showsthat the inrushcurrentis reducedto load capacitancedecreasesinrushcurrent,but it can also decreasevoltagerail requirespecificoutputcapacitanceto operate.

3 Andreducingthis outputcapacitanceis not an this scenarioare August2014 RevisedMay 2015 ManagingInrushCurrentSubmitDocumentation FeedbackCopyright 2014 2015,TexasInstrumentsIncorporatedProblem sCausedby InrushCurrentThereare two key concernsassociatedwith first is exceedingthe absolutemaximumcurrentratingsof the tracesand componentson a connectorsand terminalblockshavea specificcurrentratingwhich,if exceeded,couldcausedamageto ,all PCBtracesaredesignedwith a certaincurrentcarryingcapabilityin mindand are also at risk to PCBtracesand selectingconnectors,not takingthe inrushcurrentpeakinto accountcan damagethepowerpathand lead to systemfailure;however,appropriatelydesig ningfor a largeinrushcurrentpeakwill lead to thickerPCBtracesand moredurableconnectorswhichcan increasethe size and cost of secondproblemoccurswhena capacitiveload switchesontoan alreadystablevoltagerail.

4 If thepowersupplycannothandlethe amountof inrushcurrentneededto chargethat capacitor,thenthevoltageon that rail will be is an exampleof a 100 F capacitancebeingappliedto avoltagesupplywithoutany slewrate of inrushcurrentandforcesthe voltagerail to V downto 960 PowerSupplyDip due to InrushCurrentIf othermodulesare connectedto this powerrail and the voltagedrops,thenthesemodulesmay resetthemselvesand put the rest of the systeminto an the voltageregulatoris unabletosupplyenoughcurrentat turn-on,the voltagerail couldcollapsecompletelyleadingto August2014 RevisedMay 2015 SubmitDocumentationFeedbackCopyright 2014 2015, ReducingInrushCurrent3 Methodsof ReducingInrushCurrentInrushcurrentcan be reducedby increasingthe voltagerise time on the load capacitanceand slowingdownthe rate at whichthe reduceinrushcurrentare shownbelow:voltageregulators,discretecom ponents,and integratedload threeof thesesolutionscenteraroundincreasingthe voltagerise time which,as shownin Equation1, leadsto "Soft-start or VoltageRegulatorsVoltageregulators,DC/DC converters,and LDOsmay havean ,the rise time can be increased,therebyreducingthe properlyselectedDC/DCconverteror LDO,the inrushcan be effectivelymanagedto a controlledrise time can be accomplishedby usingdiscretecircuitryand can bedonein examplecircuitof one solutionis shownin Figure5.

5 This particularsolutionrequiresa minimumof 4 components(2 MOSFETS,2 resistors)and the slewrate of VOUTcan becontrolledby usingthe resistorRSR. However,RSRneedsto be very large(in the rangeof M ) to haveaneffecton the rise time of be able to reducethe valueof RSR, an additionalcapacitorwouldneedto be DiscreteLoadSwitchImplementation5 SLVA670A August2014 RevisedMay 2015 ManagingInrushCurrentSubmitDocumentation FeedbackCopyright 2014 2015,TexasInstrumentsIncorporatedMethods of switchescan be usedin placeof the All TexasInstrumentsload switches(TPS229xxproducts) featurea controlledoutputslewrate to belowshowsthe typicalapplicationcircuitfor a load Disadvantagesof theseSolutionsWhileall of thesesolutionscan help to manageinrushcurrent,they all comewith leastintegratedof all the abovesolutionsis the its integratedcounterpart,the load switch,it requiresmorecomponentsand a contrast,the mostintegratedsolutionis the DC/DCconverteror voltageregulatorwithsoft-startalreadybui lt in.

6 Despiteits integration,addingload switchesmay be morebeneficialfor a voltagerail requiresmultiplecapacitiveloadswhichneed to be switchedindividually,thenmultipleload switchescan be will reduceoverallcostand ,if the chosenvoltageregulatordoesnot comewith an integratedslewrate control,thena load switchcan be usedbeforeor afterto providethat load switchto a systemfor inrushcurrentcontrolmay requirean additionalcomponent,but it can reducethe overalldesignsizeand August2014 RevisedMay 2015 SubmitDocumentationFeedbackCopyright 2014 2015,TexasInstrumentsIncorporatedLOADINR USHCdV22 F Vdt1 21 sI600 mA === INRUSHLOADdVICdt= followingapplicationexampleswill use the designparametersshownin Table1:Table1. ApplicationExample1 DesignParameterExampleValueLoadSwitchinp utvoltage(VIN) VCapacitiveload (CLOAD)22 FMaximumacceptableinrushcurrent600 mAUsinga VINof V, a CLOADof 22 F, and a maximumacceptableinrushcurrentof 600 mA, the requiredrise time for the outputcan be Equation2,(2)The rise time can be calculatedas.

7 (3)This meansthat the load switchwhichis chosenfor this applicationmusthavea rise time of 121 s , all availableTexasInstrumentsload switchescan be sortedby rise time usingthe method,an appropriateload switchcan VIN= V, the TPS22902 Bhas a typicalrise time of 146 s and can be usedto ensurean inrushcurrentlowerthan600 controlledrise time of the load switchand resultinginrushcurrentareshownin TPS22902 BInrushCurrentThe peakinrushcurrentmeasuredis 392 is well belowthe 600 mA designrequirementandmuchlowerthanthe A seenin Figure3 withoutany load selectingthecorrectload switch,the inrushcurrentis August2014 RevisedMay 2015 ManagingInrushCurrentSubmitDocumentation FeedbackCopyright 2014 2015, switcheswith a fixedrise time haveA, B, C, or D usedat the end of the part numberto denotedifferentrise A versionload switchhas thefastestrise time (typicallybelow10 s) and a D versionload switchhas the slowest(severalmilliseconds).

8 For example,the TPS22924load switchhas B, C, and D variationswith rise timesof 96 s, 800 s, and9 ms, this applicationexample,a rise time of greaterthan121 s was calculatedto limitthe inrushcurrentto 600 mA. The rise time of the B versionwouldbe too fast and eitherthe C or Dversioncouldbe TexasInstrumentsload switchesfeaturea controlledrise time,and for someload switchesthis risetime can be rise time of thesedevicescan be increasedby addingan externalcapacitorbetweenthe availableCT pin and TPS22965offersthis feature,and its typicalapplicationschematicshownin TPS22965 ApplicationCircuitUsingthe datasheetfor this device,the appropriateCT capacitorcan be chosento implementa desiredrise equationand tablein the AdjustableRiseTimesectionof the TPS22965datasheetcan be usedto this belowshowsthe datasheettablewhichallowsthe userto determinethe appropriateCT capacitorneededfor a desiredrise TPS22965 RiseTimevs CT Capacitor8 ManagingInrushCurrentSLVA670A August2014 RevisedMay 2015 SubmitDocumentationFeedbackCopyright 2014 2015, no CT capacitoris used,thenthe rise time of the load switchmay be too fast to limit the inrushcurrentto the showsthe TPS22965poweringup into a 22 F load withoutany TPS22965 ScopeCapture(CT cap = 0 pF)Withno CT capacitor,the rise time of the TPS22965is fasterthanthe calculated121 s and resultsin aninrushcurrentof about670 mA, largerthanthe designgoal of 600 mA.

9 The belowscreenshotsshowthedevicepoweringup into the 22 F load with differentCT TPS22965 ScopeCapture(CT cap = 150 pF)9 SLVA670A August2014 RevisedMay 2015 ManagingInrushCurrentSubmitDocumentation FeedbackCopyright 2014 2015,TexasInstrumentsIncorporated66 F100 FVOUTINRUSH5V TPS22965 ScopeCapture(CT cap = 2200pF)Figure11 was takenwith a CT capacitorof 150 pF and Figure12 with 2200pF. As the CT capacitorincreases,the rise time of the devicealso increasesand the inrushcurrentis reducedto well belowthedesigngoal of 600 CT pin increasesthe amountof flexibilityin design,it doesrequireanadditionalcomponentto ,this allowsfor a singleload switchto be usedacrossmultipledesignswith Usinga SlewRateControlledLoadSwitchThe followingexampleusesa 5 V powersupplywhichis broughtdownto V usinga V rail has poweredup, a 100 F capacitanceis appliedto the system,as shownin SystemBlockDiagramwithoutSlewRateControl 10 ManagingInrushCurrentSLVA670A August2014 RevisedMay 2015 SubmitDocumentationFeedbackCopyright 2014 2015,TexasInstrumentsIncorporated66 F100 BuckConverter5V controlledrise time,the switchdoesnot provideany inrushcurrentmanagementand the followingresultscan be observed:Figure14.

10 InrushCurrentand VoltageDropwithoutSlewRateControlThe inrushcurrentgeneratedby the 100 F capacitorpeaksat and bringsthe V rail downto320 82%voltagereductionon the powerrail can causethe systemto resetor LoadSwitchFigure15 showsthe samesystemas before,exceptthe TPS22965load switchfromTexasInstrumentswith controlledrise time is usedto switchthe 100 F SystemBlockDiagramutilizingthe TPS2296511 SLVA670A August2014 RevisedMay 2015 ManagingInrushCurrentSubmitDocumentation FeedbackCopyright 2014 2015, 150 pF capacitoron the CT pin of the load switch,the followingresultscan be observed:Figure16. InrushCurrentand VoltageDropwithSlewRateControlWiththe controlledslewrate of the TPS22965,the to 960 V outputof the buckconverteralso showsno lead to inrushcurrentresultingin devicedamage,systeminstabilityor TI load switchis a size and cost efficientsolutionfor TI load switchportfoliohas a widevarietyof partswith differentslewratesto addressthe inrushcurrentsof usingEquation1 and the , inrushcurrentcan be effectivelymanagedby usinga TI IntegratedLoadSwitchfromthe , ,6-A, 16-m On-ResistanceLoadSwitch(SLVSBJ0) , ,500-mA,78-m ON-ResistanceLoadSwitchWithControlledTur non(SLVS803)12 ManagingInrushCurrentSLVA670A August2014 RevisedMay 2015 SubmitDocumentationFeedbackCopyright 2014 2015,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedand its subsidiaries(TI)