Managing Inrush Current (Rev. A)

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?

2 "Soft-start or Disadvantagesof Usinga August2014 RevisedMay 2015 ManagingInrushCurrentSubmitDocumentation FeedbackCopyright 2014 2015,TexasInstrumentsIncorporatedINRUSHL OADdVICdt= LoadLoadLoadPower SupplyWhatis InrushCurrent? 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.

3 (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.

4 Figure3 showsthat the inrushcurrentis reducedto load capacitancedecreasesinrushcurrent,but it can also decreasevoltagerail requirespecificoutputcapacitanceto operate,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.

5 However,appropriatelydesigningfor a largeinrushcurrentpeakwill lead to thickerPCBtracesand moredurableconnectorswhichcan increasethe size and cost of secondproblemoccurswhena capacitiveload switchesontoan alreadystablevoltagerail. 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.

6 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.

7 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)

8 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. 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.

9 TexasInstrumentsIncorporatedLOADINRUSHCd V22 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:(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.

10 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).