Load Switches: What Are They, Why Do You Need …

1 ApplicationReportSLVA652 April2014 LoadSwitches:WhatAre They, WhyDo You NeedThemAndHowDo You ChooseThe RightOne? LoadSwitchesABSTRACTI ntegratedload switchesare electronicrelaysthat can be usedto turn on and turn off powersupplyrails the systemand can includeprotectionfeaturesthatare oftendifficultto manydifferentapplicationswhereload switchescan be usedincluding,but not limitedto: PowerDistribution PowerSequencingand PowerStateTransition ReducedLeakageCurrentin StandbyMode InrushCurrentControl ControlledPowerdownThisapplicationnotewi ll providethe fundamentalbasicsof whatloadswitchesare, whentheyshouldbeused,and how they can be implementedin a LoadSwitches?.. You Selectionand (RON).. (VIN) and Current(IMAX) (ISD) and QuiescentCurrent(IQ).. (tR).. (QOD).. April2014 LoadSwitches:WhatAre They, WhyDo You NeedThemAnd HowDo YouChooseThe RightOne?

2 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporatedControl LogicVINONVOUTGNDD riverChargePump*Quick OutputDischarge*PassFET*Not present on all load switches12345 ThermalShutdown*CurrentLimiting*ReverseC urrentProtection*666 VINONGNDVOUTOFFONCINRLCLWhatAre LoadSwitches? LoadSwitches?Integratedload switchesare integratedelectronicrelaysusedto turn on and turn off switchesconsistof four pins for inputvoltage,outputvoltage,enable,and enabledvia the ON pin, the passFET turnson, therebyallowingcurrentto flow fromthe inputpin to the outputpin, and poweris passedto the understandingof whatthe architectureof a load switchlookslike will be helpfulin determiningthespecificationsof a load Figure2 is a blockdiagramof a basicload switch,whichis madeup of five be includedto add functionalityto the load BlockDiagramof GeneralLoadSwitch2 LoadSwitches:WhatAre They, WhyDo You NeedThemAnd HowDo YouSLVA652 April2014 ChooseThe RightOne?

3 SubmitDocumentationFeedbackCopyright 2014, LoadSwitches?1. The passFET is the maincomponentof the load switch,whichdeterminesthe maximuminputvoltageand maximumload currentthe load switchcan on-resistanceof the load switchis acharacteristicof the passFET and will be usedin calculatingthe powerdissipatedby the load passFET can be eitheran N-channelor P-channelFET,whichwill determinethe architectureofthe load The gatedriverchargesand dischargesthe gateof the FET in a controlledmanner,therebycontrollingthe rise time of the The controllogicis drivenby an controlsthe turn-onand turn-offof the passFET and otherblocks,suchas quickoutputdischarge,the chargepump,and blockswith The chargepumpis not includedin all load is usedin load switcheswith an N-channelFET,sincea positivedifferentialvoltagebetweenthe gateand the source(VOUT)

4 Is neededin orderto turn on the FET Quickoutputdischargeis an on-chipresistorfromVOUTto GNDthat is turnedon whenthe deviceisdisabledvia the ON pin. This will dischargethe outputnode,preventingthe deviceswith quickoutputdischarge,this featureis only presentwhenVIN and VBIASare withinthe Additionalfeaturesare includedin differentload ,but are not limitedto, thermalshutdown,currentlimiting,and a list of commondatasheetparametersand definitionsfor load switches . Inputvoltagerange(VIN) This is the rangeof inputvoltagesthat the load switchcan support. Biasvoltagerange(VBIAS) This is the rangeof bias voltagesthat the load switchcan be requiredto powerthe internalblocksof the load switch,dependingon the architectureofthe load switch. Maximumcontinuouscurrent(IMAX) This is the maximumcontinuousDC currentthe load switchcan support.

5 ON-stateresistance(RON) This is the resistancemeasuredfromthe VIN pin to the VOUTpin,whichtakesinto considerationthe resistanceof the packagingand the internalpassFET. QuiescentCurrent(IQ) This is the requiredamountof currentto powerthe internalblocksof thedevice,whichis measuredas the currentflowinginto the VIN pin withoutany load on VOUT. ShutdownCurrent(ISD) This is the amountof currentflowinginto VIN whenthe deviceis disabled. ON pin inputleakagecurrent(ION) This is the amountof currentthat is flowinginto the ON pinwhenthe ON pin has a high voltageappliedto it. Pull-downresistance(RPD) This is the valueof the pull-downresistorfromVOUTto GNDwhenthe deviceis You NeedLoadSwitchesThis sectionwill providea generaloverviewof someapplicationswhereusinga load switchis iillustratedin Figure3, loadswitchescan be usedto turn on and off sub-systemsof the sameinputvoltageinsteadof usingmultipleDC/DCconvertersor LDO s.

6 By usinga load switch,powercan be distributedacrossdifferentloadswithcontr olfor April2014 LoadSwitches:WhatAre They, WhyDo You NeedThemAnd HowDo YouChooseThe RightOne?SubmitDocumentationFeedbackCopy right 2014,TexasInstrumentsIncorporatedVLoad 1 TimeVLoad 2 VLoad 3 VSupplyLoad SwitchPower SupplyLoad 1 load 2 load 3 load SwitchLoad SwitchPower SupplyLoadLoadLoadLoad SwitchWhyDo You PowerStateTransitionIn somesystems,especiallythosewith a processor,thereis a strictpower-upsequencethat usinga GPIOor I2C interface, load switchesare a simplesolutionto implementpowersequencingto meetthe provideindependentcontrolof eachpowerpathto providesimplifiedpoint-of-loadcontrolfor powersequencing,as shownin PowerSequencingwithLoadSwitches4 LoadSwitches:WhatAre They, WhyDo You NeedThemAnd HowDo YouSLVA652 April2014 ChooseThe RightOne?SubmitDocumentationFeedbackCopy right 2014,TexasInstrumentsIncorporatedSlew Rate ControlVsupplyVLoadILoadSupply DipInrush CurrentVoltage DroopILoadVsupplyVLoadPower SupplyLoadLoad SwitchCurrent 2 Power SupplyLoadCurrent 1 ISUPPLYI nput Supply You manydesigns,thereare sub-systemsthat are only usedduringcertainmodesof be usedto limit the amountof leakagecurrentand powerconsumptionby turningoff powerto showsa comparisonof the leakagecurrentwith and withouta load the Inputand OutputCapacitancesectionfor Comparisonof LeakageCurrentwithand withoutthe LoadSwitchIn someapplications,the circuitry- suchas DC/DCconverters,LDOs,and modules- can be disabledandput into ,the leakagecurrentof thesemodulescan be relativelyhigh,evenin placinga load switchbeforethe load ,as shownabove,the leakagecan be.

7 Powerconsumptioncan be reducedsignificantlywith a load switchplacedin the a sub-systemwithoutany slewrate control,the inputrail may sag becauseof the inrushcurrentthat can happenfromquicklycharginga load can be problematicas this rail maybe supplyingpowerto othersub-systems(Figure6). Loadswitchessolvethis issueby controllingthe risetime of the outputvoltage,therebyeliminatingthe sag on the inputvoltage(Figure7). The inrushcurrentis proportionalto the load capacitanceas will be explainedlaterin the InrushCurrentCausingPowerSupplyDipFigure 7. SlewRateControlwithLoadSwitch5 SLVA652 April2014 LoadSwitches:WhatAre They, WhyDo You NeedThemAnd HowDo YouChooseThe RightOne?SubmitDocumentationFeedbackCopy right 2014,TexasInstrumentsIncorporatedPower SupplyLoad SwitchLoadSupply VoltageLoad VoltageVoltageTimeControlled power downdue to quick outputdischargeSupply VoltageVoltageTimeUncontrolled power downDC/DC or LDOLoadWhyDo You DC/DCconverteror LDOwithoutquickoutputdischargeturnsoff, the load voltageis left floatingand powerdownis dictatedby the load ,as shownin Figure8.

8 This can causeunwantedactivityasmodulesdownstream are not powereddownto a UncontrolledpowerdownwithoutloadswitchUs inga load switchwith quickoutputdischargecan load will be powereddownquicklyin a controlledmannerand will be resetto a knowngoodstatefor the next powerup, asshownin Figure9. This will eliminateany floatingvoltagesat the load and ensurethat it is in a definedpowerstateat all the QuickOutputDischarge(QOD)sectionfor requirefault protectionfeaturesto be integratedinto the load reversecurrentprotection,ON pin hysteresis,currentlimiting,undervoltagel ock-outand implementingthesecomplexcircuitsthroughd iscretecomponents,it is possibleto reduceBOMcount,solutionsize,and developmenttime byusingan integratedload a list that brieflyexplainswhatsomeof thesefeaturescan be: Reversecurrentprotectionwill stop currentfromflowingfromthe VOUTpin to the VIN pin.

9 In theabsenceof this feature,currentmay flow fromthe VOUTpin to the VIN pin, if the voltageon VOUT isgreaterthanVIN by a ,reversecurrentblockingmay be beneficialin certainapplications,suchas in a powermux applicationwherecurrentshouldnot flow fromVOUTto manydifferentmethodsof somesituations,suchas the TPS22912,the devicewill monitorthe voltagelevelsat the VIN pin and VOUTpin. Whenthisdifferentialvoltageexceedsa certainthreshold,the switchis disabledand the bodydiodeisdisengagedto preventany reversecurrentflow to ,suchas the TPS22963C,onlyhavereversecurrentprotecti onwhenthe deviceis :WhatAre They, WhyDo You NeedThemAnd HowDo YouSLVA652 April2014 ChooseThe RightOne?SubmitDocumentationFeedbackCopy right 2014,TexasInstrumentsIncorporatedVwithON Pin HysteresisOUTV withoutON Pin HysteresisOUTVONE rratic Behavior on OutputOutput is immune to noiseNoise on ON You NeedLoadSwitches ON pin hysteresisallowsfor voltagedifferencebetweena logiclevelhigh and logiclevellow on the ON pin, the controlcircuitrywill operateas intendedwhenthereis noiseon the GPIO line.

10 Figure10 illustrateshow ON pin hysteresiscan providerobustnesson the GPIO enableline. Currentlimitingis a featurethat will limit the amountof currentthe load switchwill willensurethat thereis not an excessiveamountof currentbeingpulledby an currentis not limited,the externalcircuitrycan potentiallybringthe the currentlimitedmode, load switchworksas a constantcurrentuntil the switchcurrentfalls belowthe currentlimit. Undervoltagelock-out(UVLO)is usedto turn off the deviceif the VIN voltagedropsbelowa thresholdvalue,ensuringthat the downstreamcircuitryis not damagedby beingsuppliedby a voltagelowerthanintended. Overtemperatureprotectiondisablesthe switchif the temperatureof the deviceexceedsa feature,the devicecan operateas a safetyswitchthat turnsoff whena hightemperatureis ON Pin PCBAreaUsingan integratedload switchcan lowerthe BOMcountof a thereare discreteFETsthat areusedin conjunctionwith othercomponents,a load switchcouldbe consideredto reducethe numberoftotal componentsin the load switchis createddiscretely,thereare manyresistors,capacitorsand transistorsthat will be requiredto implementa gatedriver,controllogic,outputdischargea nd integratedload switch,this is all accomplishedwith only a singledeviceand the BOMcountis DesignConsiderationsThis sectionwill look at the specificationsthat needto be kept in mindin choosinga load PMOSIn an NMOS device,the passFET is turnedon by bringingthe gatevoltageabovethe.