P354, EN

AutomotiveElectronicSystemsDrivetheDemandforMulti-ChannelSynchronousBuckConverters

ByJeffGruetter

ProductMarketingManagerLinearTechnologyCorporation

IntroductionAsautomobilescontinuetoaddahigherdensityofelectronicsystemstoenhancesafety,comfort,efficiencyandperformance,itisnotsurprisingthattheyrequirebothsmallerandhigherperformancepowerconversionsolutions.Thesenewmulti-channelpowermanagementsolutionsneedtodeliveruptofourindependentpowerrailsusuallytopowerVCORE,VI/O,VMEMrequiredformicroprocessorsandgenerallyafourth5VrailtopoweraCANtransceiver.AccordingtoStrategyAnalytics,“Thedemandforenablingsemiconductordevicesisexpectedtogrowatacompoundaverageannualgrowthrate(CAGR)offivepercentoverthenextsevenyears,withthetotalmarketworthover$41billionby2021,comparedto$30billionin2014.”Theyalsoidentifiedthatdemandformicrocontrollerandpowersemiconductorswilldriveover40percentofrevenues.

StrategyAnalyticsprovidesaveryquantitativedescriptionofforecastingthegrowthofelectronicscontentinautomobiles,butmoreinterestingistheprevalentrolethatpowerICsplayinthisgrowth.ThesenewpowerICsmustprovide:

1) MultiplevoltagerailsfromasinglepowermanagementIC2) Robustperformanceacrossawiderangeofvoltages,includingcold-crankandstop-

startscenariosfrom

independentoutputstopowerVCORE,VI/O,VMEMrequiredformicroprocessorsandafourth5VrailtopoweraCANtransceiverwhichenablesthemicroprocessortocommunicatewiththerestofthesystemelectronics.BecauseaproperlydesignedquadconverterICisonlymoderatelylargerthananequivalentsingleconverterwhereasitssolutionfootprintcanbelessthanhalfthesizeoffourseparatesingleconverters.Thissizeimprovementisespeciallycompellingwhencomparingaquadoutputregulatorusingaswitchingfrequencyof2MHzversusfoursingleswhichrunat500KHz.Additionally,quadoutputconvertersaredesignedtominimizeunwantedcross-talkbetweenthechannels,whereascrosstalkbetweenfouradjacentsingleconverterscanbeproblematic,unlesstheyareallsynchronizedtoacommonclock.Addinganexternalclockandsynchronizationaddsbothsize,complexityandcosttothecircuit.LowEMIOperationBecausetheautomotiveelectricalenvironmentisinherentlynoisy,withmanyapplicationsbeingsusceptibletoelectromagneticinterference(EMI),itisimperativethatswitchingregulatorsdon’texacerbatetheseEMIconcerns.Becauseaswitchingregulatoristypicallythefirstactivecomponentontheinputpowerbusline,andregardlessofdownstreamconverters,itsignificantlyimpactsoverallconverterEMIperformance.SominimizingEMIisimperative.Historically,thesolutionwastouseanEMIshieldingbox,butthisaddssignificantcostandsizetothesolutionfootprintwhilecomplicatingthermalmanagement,testingandmanufacturability.AnotherpotentialsolutionwithinthepowermanagementICistoslowdowntheswitchingedgesoftheinternalMOSFETs.However,thishastheundesiredeffectofreducingtheefficiencyandincreasingtheminimumon-timewhichcompromisestheIC’sabilitytodeliverlowdutycyclesatswitchingfrequenciesat,orabove,2MHz.Asbothhighefficiencyandsmallsolutionfootprintsaredesired,thisisn’taviablesolution.Fortunately,somerecentpowerICdesignshavebeenintroducedtoenablefastswitchingfrequencies,highefficiencyoperationandlowminimumon-timesconcurrently.ThesedesignscanofferlowEMIemissionsevenwith2MHzswitchingfrequenciesandover90%efficiency.Theseperformancelevelsareaccomplishedwithnoadditionalcomponentsorshielding,offeringasignificantbreakthroughinswitchingregulatordesign. ANewAlternativeLinearTechnology’sLT8602isa42Vvoltinputcapable,highefficiency,quadoutputmonolithicsynchronousstep-downswitchingregulator.Its3Vto42Vinputvoltagerangemakeitidealforautomotiveapplicationswhichmustregulatethroughcold-crankandstop-startscenarioswithminimuminputvoltagesaslowas3Vandloaddumptransientsinexcessof40V.Ascanbeseeninfigure1,itsquadchanneldesigncombinestwohighvoltage2.5Aand1.5Achannelswithtwolowervoltage1.8Achannelstodeliverfourindependentoutputsdeliveringvoltagesaslowas0.8Venablingittodrivethelowestvoltagemicroprocessorcorescurrentlyavailable.Itssynchronousrectificationtopologydeliversupto94%efficiencywhileBurstMode®operationkeepsquiescentcurrentunder30µA(allchannelson)inno-loadstandbyconditionsmakingitidealforalways-onsystems.

Figure1.LT8602SchematicDelivering5V,3.3V,1.8V&1.2VOutputs Fornoisesensitiveapplications,theLT8602,withasmallexternalfilter,canutilizeitspulse-skippingmodetominimizeswitchingnoiseandcanmeettheCISPR25,Class5EMIrequirementsasshowninFigure2.

Figure2.LT8602RadiatedEMIPerformance,

(CISPR25RadiatedEmissionTestswithClass5PeakLimit)

TheLT8602’sswitchingfrequencycanbeprogrammedfrom250kHzto2MHzandcanbesynchronizedthroughoutthisrange.Its60nsminimumon-timeenables16VINto2.0VOUTstep-downconversionsonthehighvoltagechannelswitha2MHzswitchingfrequency.AsthehighvoltageVOUT2channelfeedsthetwolowvoltagechannels(VOUT3andVOUT4),thesecandeliveroutputsaslowas0.8Vwhilealsoswitchingat2MHz,offeringaverycompact(~25mmx25mm)quadoutputsolutionasshowninFigure3.

Figure3.LT8602QuadOutputSolutionFootprint(2xactualsize)

Inadditiontheminimizingthesolutionfootprint,theLT8602’s2MHzswitchingfrequencyenablesdesignerstoavoidcriticalnoise-sensitivefrequencybands,suchasAMradio.EachchanneloftheLT8602maintainsaminimumdropoutvoltageofonly200mV(@1A)underallconditions,enablingittoexcelinscenariossuchasautomotivecold-crank.Programmablepower-onresetandpowergoodindicatorsforeachchannelhelpstoensureoverallsystemreliability.TheLT8602’s40-leadthermallyenhanced6mmx6mmQFNpackageandhighswitchingfrequencykeepsexternalinductorsandcapacitorssmall,providingacompact,thermallyefficientfootprint.TheLT8602utilizesquadinternaltopandbottomhighefficiencypowerswitcheswithallthenecessaryboostdiodes,oscillator,controlandlogiccircuitryintegratedontoasingledie.Channel1and3switch180degoutofphasewithchannel2and4toreduceoutputripple.Eachchannelhasaseparateinputforaddeddesignflexibility,butmostapplicationswillrunthetwolowvoltagechannelsdirectlyfromthetwohighvoltagechannelstoofferaverysimple,highfrequencyquadoutputdesign.LowrippleBurstMode®operationmaintainshighefficiencyatlowoutputcurrentswhilekeepingoutputripplebelow15mVPK-PK.UniquedesigntechniquesandanewhighspeedprocessenablehighefficiencyoverawideinputvoltagerangeandtheLT8602’scurrent-modetopologyenablesfasttransientresponseandexcellentloopstability.Otherfeaturesincludeinternalcompensation,apowergoodflags,outputsoftstart/tracking,short-circuitandthermalprotection.

ConclusionTherapidincreaseinthenumberandcomplexityofelectronicsystemsinautomobileshasnecessitatedevenhigherdemandsonpowermanagementICperformance.ByhavingquadoutputpowerICs,automotivedesignerscandramaticallyreducethespacerequiredbytheirpowerconversioncircuits.Combinedwithswitchingfrequenciesof2MHz,thesizeoftheexternalcomponents,namelytheinductorsandoutputcapacitors,canalsobedramaticallyreducedofferingaverycompactquadrailsolutionfootprint.Thesecompactdesignsarealsoveryrugged,capabletowithstandtherequiredtransientbehaviorfoundinstop-start,cold-crankandload-dumpconditionswhileaccuratelyregulatingalloutputs.Additionally,ultralowquiescentcurrentsmakethemidealforalwaysonsystems.Asmoreelectronicsystemsareaddedintoevershrinkingspaces,minimizingthesolutionfootprintwhilemaximizingefficiencyisalsocritical.Fortunatelyanewgenerationofmulti-outputpower ICswhichmeetthesedemandsarealreadyavailablepavingthewayforevenhigherelectroniccontentinfuturevehicles.