PowerManagement TexasInstrumentsIncorporated IQ:Whatitis,whatitisn’t,andhowtouseit ByChrisGlaser ApplicationsEngineer Introduction Adevice’squiescentcurrent,orIQ,isanimportantyetoftenmisusedparameterforlow-power,energy-efficientdesigns.Inmanybattery-poweredapplications,suchasmetering,wearables,buildingsecurityandtheofThings(IoT),thecurrentdrawnfromthebatteryinastandbyconditionwithlightornoloaddefinesthetotalruntimeofthesystem.Inintegratedswitchconverters,theIQisonlyoneportionofthisbatterycurrent.This­articledefinesIQandhowitismeasured,explainswhatIQisnotandhowitshouldnotbeused,andgivesdesignconsiderationsonhowtouseIQwhilemonmeasurementerrors.ThisarticleappliestoanyoftheTexasInstruments(TI)TPS61xxx,TPS62xxx,TPS63xxx,orTPS650xxdevices. WhatIQis Unlessotherwisenotedinthedatasheetforapart,IQisdefinedasthecurrentdrawnbytheICinano-loadandnonswitchingbutenabledcondition.“Noload”meansthatnocurrentleavestheICtotheoutput.Typically,thiswouldbecurrentleavingviatheSWpinonbuckconvertersorviatheVOUTpinonboostconverters.AlloftheIQsimplytravelsinsidetheICtoground.“Nonswitching”meansthatnopowerswitchintheICison(closed).ThisincludesthemainorcontrolswitchaswellasthesynchronousrectifierifbothareintegratedintotheIC.Inotherwords,theICisinahigh-impedanceconditionwithapowerstagethatpletelydisconnectedfromtheoutput(exceptforintegratedMOSFETbodydiodesonsomedevicesthatcannotbeturnedoff).“Enabled”meansthattheICisturnedonviaitsENpinandisnotinaUVLOorothershutdowncondition.IQmeasuresoperatingcurrent,notshutdowncurrent,sothedevicemustbeon.Lastly,IQismeaningfulonlyinpower-savemode,soifthismodeisanoptionfortheparticulardevice,itmustbeenabled.Ifthedevicerunsinpulse-width-modulation(PWM)mode,thentheinputcurrenttothepowerstageandswitchinglosses morethandwarfstheminisculeamountofcurrent,theIQ,requiredtorunthedevice. IQesfromtwoinputs:VINandVOUT.Thedatasheetlistswhethertheesfromeitherorbothpins.Figure1showstheIQspecificationfromthedatasheetfortheTITPS61220/21/22,1whichareboost convertersthatdrawtheirIQfrombothVINandVOUT.Typically,abuckconverterdrawsIQonlyfromitsinput,whileaboostconverterorbuck-boostconverterdrawsIQfromboththeinputandtheoutput. IQmeasuresthecurrentrequiredtooperatethedevice’sbasicfunctionality,whichincludespoweringthingslikethe internalprecisionreferencevoltage,anoscillator,athermal shutdownorUVLOcircuit,thedevice’sstatemachineor otherlogicgates,etc.IQdoesnotincludeanyinputcurrenttothepowerstageorgatedrivers,asitismeasuredina nonswitchingconditionwherethesecurrentsarezero.The reasonformeasuringIQinthisconditionisthatitissolelydependentontheIC,whereasthepower-stageinputcur- rentandgate-drivecurrentaredependentontheselected ponents,whichinmostcasesdictatehow oftentheICswitchesinitspower-savemode.Thus,IQisanICmeasurement,whereasincludingtheothertwocur- rentsisasystemmeasurement.TIdoesnotcontroland cannotguaranteesuchasystemmeasurementbutdoes controlandcanspecifyanICmeasurement.Infact,TI guaranteestheIQspecificationand,fordeviceswhosedatasheetsspecifyamaximumvaluefortheIQ,testsitoneachandeverydevicethatisproduced.Thisisdoneby enablingthedevice,settingittothetestconditionsspeci- fiedinitsdatasheet,andthenartificiallyraising(with externallyappliedvoltages)theoutputvoltage,FBpin, andanyotherpinvoltageshighenoughtocausetheIC nottoswitch.Withnoloadandpower-savemodeenabled (ifavailable),theinputcurrenttotheICestheIQ. WhatIQisn’t IQisnottheno-loadinputcurrent.Aspreviouslymentioned,theIQissimplythe“overhead”currentrequiredtooperatetheIC’sbasicfunctionality.Itdoesnotincludethe Figure1.IQspecificationfromTPS61220/21/22datasheet DC/DCSTAGEPARAMETER IQ Quiescentcurrent VINVOUT TESTCONDITIONSIO=0mA,VEN=VIN=1.2V,VOUT=3.3V MINTYPMAXUNIT 0.5 0.9µ
A 5 7.5µ
A 18High-PerformanceAnalogProducts /aaj 2Q2011AnalogApplicationsJournal TexasInstrumentsIncorporated PowerManagement inputcurrentintothepowerstage(currentthatisactuallytransferredtotheoutput)or Figure2.No-loadoperationofTPS61220 currentrequiredtooperatethegatedrivers. Evenatnoload,thedevicestillswitchestokeeptheoutputregulated.Somelossesalwaysexistattheoutput,suchaslossfromthevoltagedividerusedtosettheoutputvoltage;leakagecurrentintotheloador VOUT(AC-Coupled,10mV/div)
2 VIN=1.2VVOUT=3.3V throughtheoutputcapacitor;pull-upresis- tors;etc.Becausetheselossescausevoltage decayattheoutputcapacitor,theICmustswitcheverysooftentoreplenishthepower Phase#
1 lost.So,ano-loadinput-currentmeasurementviolatestherequirementsthattheIC Phase#
2 mustbeinanonswitchingconditionandthat SwitchNode nocurrentmayleavetheICtorecharge VOUT.Asanexample,Figure2showsno-loadoperationfortheTPS61220boostconverter, 14IL(100mA/div) (1V/div) withaninputvoltageof1.2Vandanoutputvoltageof3.3V.TheICswitchesapproxi- Time(500µs/div) matelyevery1.75mstoregulatetheoutput voltage.ThisperioddependsonVIN,VOUT,andtheponentsandaffects howmuchaverageinputcurrentisdrawn.Duringphase#1,theICisswitching—either Figure3.SwitchingpulseofTPS61220duringno-loadoperation thehigh-sideMOSFETorthesynchronous rectifyingMOSFETison.Theinputcurrent isdominatedbythecurrentintothepowerstage,whichaveragesabout70mA(halfofthepeakcurrentintheinductor). VOUT(AC-Coupled,10mV/div) VIN=1.2VVOUT=3.3V Figure3showsanenlargedviewof
2 phase#
1.Oncetheoutputvoltagedrops belowthethreshold,theTPS61220beginsa switchingpulsebyturningthecontrolMOSFETon.TheSWpingoeslow,causingtheinductorcurrenttorampup.ItthenturnsoffthecontrolMOSFETandturnson SwitchNode(1V/div) Phase#
2 therectifyingMOSFET,allowingcurrentto flowtotheoutput.Theoutputvoltage increasesasthisenergyistransferredinto 14 theoutputcapacitor.Whentheinductorcur- IL(100mA/div) Phase#
1 rentreacheszero,alltheenergyhasbeendeliveredtotheoutput;sotherectifying Time(500ns/div) MOSFETturnsoff,andtheICgoesintoa sleepmode(phase#2).Atthispoint,both MOSFETsareoff(open),sotheSWpinisin astateofhighimpedance.Theinductorandparasitic switchingtime(phase#1),theaverageinputcurrentover capacitancesonthatpinringuntilitreachesitsDCvalue,whichequalstheinputvoltage. thistimemustbehigherthantheIC’sIQ.However,becausethedurationofphase#1isveryshort,theaverageinput Duringphase#2,theICishighimpedance,andtheout- currentisusuallyonlyslightlygreaterthantheinputcur- putvoltagedropsduetoleakageattheoutput.Because theICisnotswitching,thecurrentconsumedbytheIC duringthistimeistheIQ.Phases#1and#2defineaswitch­ingperiodoverwhichtheaverageinputcurrentis rentthatisduetotheIQ.ToaddressthisdifferencebetweentheIQandtheno- loadinputcurrent,thedatasheetsofsomeICshavetypical specificationsfortheno-loadinputcurrentintheelectri- calculated.Duetothehighinputcurrentduringthe calcharacteristicstable.Othershavegraphsthatshowthe AnalogApplicationsJournal2Q2011 /aaj 19High-PerformanceAnalogProducts PowerManagement TexasInstrumentsIncorporated Figure4.Graphofno-loadinputcurrentfromTPS61220/21/22datasheet InputCurrent,IIN(µA) 80DeviceEnabled 70 60 TPS61222,VOUT=5V50 TPS61221,VOUT=3.3V40 TPS61220,VOUT=1.8V30 20 10
0 0.7 1.7 2.7 3.7 4.7 InputVoltage,VIN(V) no-loadinputcurrentforaparticularcircuit.Figure4showssuchagraphfromtheTPS61220/21/22datasheet.1Alternatively,Figure5showstheIQspecificationinanelectricalcharacteristicstable.ThistableistakenfromthedatasheetfortheTITPS62120/22,2whicharehighefficiencybuckconverters.Thetypicalspecificationof13µAisvalidonlyforthespecifictestconditionsstated.ForboththeTPS61220andTPS62120,notethatthenoloadinputcurrentishigherthantheIC’sIQ.Figure4showsthattheno-loadinputcurrenttotheTPS61221boostconverteris20µAwithaVINof1.2VandaVOUTof3.3V.ThisismuchhigherthantheIQinFigure1of5µAatVOUTand0.5µAatVINwiththesametestconditions.Thisdifferenceisexplainedlaterinthisarticleunderitem#3of“Designconsidera­tions.” HowtouseIQ KnowingtheIQassiststhedesigner­paringthelowpowerperformanceofdifferentICs.However,anIC’sIQisonlypartofthesystem’sinputcurrent,whichisaffected bythreethings:eachIC’sinternaldesign(itsIQ),theponentsaroundeachIC,andtheoverallsystemconfiguration.Becausetheinputcurrentisbinationofthesethreeitems,IQlossesmayormaynotbethedominantlossforaparticularsystemandmayormaynotbethedeterminingfactorinthebattery’sruntime. IftheendapplicationtrulyoperatestheICatnooutputload,thenanICwithlowerIQtypicallyhaslowerno-loadinputcurrent,whichresultsinlongerbatteryruntime.ThisassumesthatbothICshaveapower-savemodeandthatitisenabled.However,power-savemodescanbehavedifferentlyamongdifferentICs,resultinginvastlydifferentno-loadinputcurrents. Iftheapplicationdoesnotrunatnoloadbutinsteadrunsina“standby”or“hibernate”modeinwhichtheproc­essororanotherloadstilldrawssomecurrent,thentheusefulnessofIQquicklydecreases.Todemonstrate,considertheTPS62120poweringTI’sMSP430™andothercircuitrythataltogetherconsume100µAat2V.Withan8-Vinput,theTPS62120isrunningat60%efficiency(see Figure5.No-loadinput-currentspecificationfromTPS62120/22datasheet SUPPLY PARAMETER IQ Quiescentcurrent TESTCONDITIONS IOUT=0mA,Devicenotswitching,EN=VIN,regulatorsleepsIOUT=0mA,Deviceswitching,VIN=8V,VOUT=1.8V MINTYPMAXUNIT 1118µ
A 13 µ
A 20High-PerformanceAnalogProducts /aaj 2Q2011AnalogApplicationsJournal TexasInstrumentsIncorporated Figure6.EfficiencygraphforTPS62120 100 VIN=4V90VIN=6V VIN=2.5V 80 VIN=12V 70 VIN=10V VIN=8V60 VIN=15V Efficiency(%) 50 40 300.1
1 10 OutputCurrent,IOUT(mA) VOUT=2VL=18µHLPS3015COUT=4.7µF 100 PowerManagement Figure62),resultinginaninputcurrentof 2V×100µA=42µA.0.6×8V ThisinputcurrentincludestheIQ(11µA),whichisaverysignificantportionofthetotalinputcurrent(about26%).If,however,thestandbyloadincreasesto1mA,theinputcurrentat8Vis 2V×1mA=313µA.0.8×8V Nowthe11µAofIQisnotsignificantatall(about3.5%).Touratelyestimatetheinputcurrentinasystem’sstandbymode,theloadcurrentdrawnmustbeknown.SimplyusingtheIQinplaceofthislight-loadinputcurrentdoesnoturatelyestimatethebatterycurrentdrawn. AnyefficiencygraphinadatasheetindicatesthetotalcircuitefficiencyandincludestheIQlosses.Therefore,theIQlossesshouldnotbeaddedtothelossesgiveninthegraphs. Designconsiderations NumerouserrorscanbemadewhenIQvaluesaremeasuredortakenfromadatasheet.Thefollowingfiveconsiderationswillhelpthedesigneravoidtheseerrors.
1.TheIQofanICcannotbechanged.NothingcanbedonefromoutsidetheICthataffectstheIQ.TheIQdoesvaryoverinputvoltageandtemperature,butthebehavioroftheIC’sinternalcircuitrysetsthisvariation.IftheICisoperatedinforcedPWMmodeoraloadisattachedtotheoutput,thentheIQisnolongerapplicabletothecircuit,andtheinputcurrentesapplicable instead.Manythingscanbedoneinanapplicationthataffecttheinputcurrent,butnottheIQ.2.Specifiedoperatingconditionsneedtobecon­sidered.IQisspecifiedonlyforanIC’smendedoperatingconditionsandforcertaintestconditions,specificallyaninputvoltageandanoutputvoltage.ForanyIC,thespecifiedIQisnotguaranteedwhentheinputvoltageisabovethemendedmaximum(butlessthantheabsolutemaximum)orwhentheinputvoltageisbelowthemendedminimum(butabovetheUVLOlevel).Forabuckconverter,IQisvalidonlywhentheinputvoltageisgreaterthantheoutputvoltageandwhenthedeviceisnotindropout(100%mode).Foraboostconverter,theinputvoltagemustbelessthantheoutputvoltagesothattheICisnotindownmode.
3.Inputcurrentisoftenlinkedtotheoutput.ThemajorityoftheIQforasynchronousboostesfromtheoutputvoltage.Sincethispowermustefromtheinput,theinputcurrentinanoloadconditionissubstantiallyhigherthantheIQbecausetheinputcurrentforaboostconvertermustbegreaterthanitsoutputcurrent.ConsidertheTPS61220boostingfrom1.2Vto3.3V.WithanIQof5µAatVOUTand0.5µAatVIN,andassuming100%conversionefficiency,theinputcurrentfromtheIQaloneis 3.3V×5µA+0.5µA=14.25µA.1.2V Thecircuitactuallydrawsabout20µAofinputcurrentatnoload(asshowninFigure4)simplybecauseofnon-IQlossessuchasswitchinglossesandgate-drive AnalogApplicationsJournal2Q2011 /aaj 21High-PerformanceAnalogProducts PowerManagement losses.Theimportantpointisthatthis20µAofinputcurrentismuchgreaterthantheIC’sIQof5.5µAbecausetheTPS61220isaboostconverterthatdrawsmostofitsIQfromtheoutputvoltage.4.Lookforallpossibleinput-currentpaths.WhenmeasuringtheIQonanevaluationmodule(EVM)orotherboard,thedesignershouldensurethattheinputcurrenttotheboardisgoingentirelyintotheICandnottootherplacesontheboard.Leakagesfromcapacitorsorotherdevices,evenifthedevicesaredisabled,maybesignificantduetothesmallIQvaluesandmayaffecttheinputcurrenttotheboard.Inaddition,onsomeEVMsandmostend-equipmentboards,theinputvoltageoroutputvoltageisroutedtopull-upresistors,indicatorLEDs,orotherdevicesthatmaysinkcurrentundersomeconditions.Obviously,thiscurrentdrawisnotpartoftheIC’sIQ.Finally,theIC’sIQisofnoimportanceasasystemparameter,sincetotalinputcurrentisactuallywhatisneeded;andthatiseasilymeasuredattherequiredtestconditions.
5.Measurementtechniquescanmakeabigdifference.Togeturatemeasurementsofthelowpowerinputcurrentortheefficiencyinpower-savemode,itiscriticaltofollowthetestsetupdetailedinReference3. Conclusion IQisanimportantICdesignparameterinmodernlowpowerDC/DCconvertersandpartiallydefinesthecurrentdrawnfromthebatteryinlight-loadconditions.TheIQis TexasInstrumentsIncorporated nottheIC’sno-loadinputcurrent,astheICconsumestheIQcurrentonlyinano-load,enabled,andnonswitchingcondition.Duetoleakageattheoutput,theICmustswitchtokeeptheoutputvoltageregulated.InsteadofusinganIC’sIQasanestimateofthebattery’scurrentdraw,thedesignershouldmeasureandusetheno-loadinputcurrenttothesystem.Anevenbetterwaytoestimatethebattery’scurrentdrawistodefinethesystem’sloadwhenthesystemisinlow-powermodeandthenmeasurethebattery’sactualcurrentdrawatthisoperatingpoint.DoingthisinsteadofsimplyusingIQallowsuratepredictionofbatteryruntimes. References Formoreinformationrelatedtothisarticle,youcandown­loadanAcrobat®Reader®fileat/lit/litnumberandreplace“litnumber”withtheTILit.#forthematerialslistedbelow. DocumentTitle TILit.#
1.“Lowinputvoltagestep-upconverterin6pin SC-70package,”TPS61220/21/22Datasheet...slvs776
2.“15V,75mAhighefficientbuckconverter,” TPS62120/22Datasheet....................slvsad5
3.JatanNaik,“PerforminguratePFMmode efficiencymeasurements,”ApplicationReport.slva236 RelatedWebsites /sc/device/TPS61220/sc/device/TPS62120 22High-PerformanceAnalogProducts /aaj 2Q2011AnalogApplicationsJournal TIWorldwideTechnicalSupport TISupport Thankyouforyourbusiness.Findtheanswertoyoursupportneedorgetintouchwithoursupportcenterat /supportChina:/docs/supporthome.tspJapan:http://www.tij.co.jp/guidedsupport/jp/docs/supporthome.tsp Technicalsupportforums SearchthroughmillionsoftechnicalquestionsandanswersatTI’sE2E™Community(engineer-to-engineer)at China:/Japan:/group/jp/ TITraining Fromtechnologyfundamentalstoadvancedimplementation,weofferon-demandandlivetrainingtohelpbringyournext-generationdesignstolife.Getstartednowat China:/docs/gencontent.tsp?
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