AutomotiveElectronicSystemsDrivetheDemandfor
Multi-ChannelSynchronousBuckConverters
ByJeffGruetter
ProductMarketingManager
LinearTechnologyCorporation
Introduction
Asautomobilescontinuetoaddahigherdensityofelectronicsystemstoenhancesafety,
comfort,efficiencyandperformance,itisnotsurprisingthattheyrequirebothsmallerand
higherperformancepowerconversionsolutions.Thesenewmulti-channelpower
managementsolutionsneedtodeliveruptofourindependentpowerrailsusuallytopower
VCORE,VI/O,VMEMrequiredformicroprocessorsandgenerallyafourth5VrailtopoweraCAN
transceiver.AccordingtoStrategyAnalytics,“Thedemandforenablingsemiconductor
devicesisexpectedtogrowatacompoundaverageannualgrowthrate(CAGR)offive
percentoverthenextsevenyears,withthetotalmarketworthover$41billionby2021,
comparedto$30billionin2014.”Theyalsoidentifiedthatdemandformicrocontrollerand
powersemiconductorswilldriveover40percentofrevenues.
P354, EN
StrategyAnalyticsprovidesaveryquantitativedescriptionofforecastingthegrowthof
electronicscontentinautomobiles,butmoreinterestingistheprevalentrolethatpowerICs
playinthisgrowth.ThesenewpowerICsmustprovide:
1) MultiplevoltagerailsfromasinglepowermanagementIC
2) Robustperformanceacrossawiderangeofvoltages,includingcold-crankandstopstartscenariosfrom<4Vtoloaddumptransientsinexcessof36V
3) Outputsrangingfrom5Vdowntolessthan1V
4) Ultralowelectromagneticinterference(EMI)emissions
5) Thehighestefficiencypossibletobothminimizethermalissuesandoptimizebattery
run-time
6) Ultralowquiescentcurrent(<10uAperchannel)toenablealways-onsystemssuchas
security,environmentalcontrolandinfotainmentsystemstostayengagedwithout
drainingthevehiclesbatterywhenitsengine(alternator)isnotrunning.
7) Thesmallestsolutionfootprints,oftenrequiringmultiplevoltagerailstominimize
spacerequiredforpowerconversioncircuits
8) Switchingfrequenciesof2MHzorgreatertokeeptheswitchingnoiseoutoftheAM
Radiobandandtokeepsolutionfootprintsverysmall
ThegoaloftheincreasedperformancelevelsofpowerICsistoallowthedesignof
increasinglynumerousandcomplexandelectronicsystemsfoundwithinautomobiles.
Specificapplicationswhicharefuelingthegrowthforelectroniccontentincarsarefoundin
everyaspectofthevehicle.Forexample,newsafetysystemsincludinglanemonitoring,
adaptivesafetycontrol,automaticturninganddimmingheadlights.Infotainmentsystems
(telematics)whichcontinuetoevolveandpackmorefunctionalityintoanalreadytight
spacemustsupportanevergrowingnumberofcloudbasedapplications.Advancedengine
managementsystemswiththeimplementationofstop/startsystems,electronically
controlledtransmissionsandenginecontrol.Drivetrainandchassismanagementaimedat
simultaneouslyimprovingperformance,safetyandcomfort.Afewyearsagothesesystems
wereonlyfoundin“high-end”luxurycarsbutnowtheyarecommonplaceinautomobiles
fromeverymanufacturer,furtheracceleratingtheautomotivepowerICgrowth.
SmallerPowerConversionCircuits
Thereareafewwaystomakepowerconversioncircuitssmaller.Ingeneral,thelargest
componentsinacircuitisnotthepowerIC,buttheexternalinductorandcapacitors.By
increasingtheswitchingfrequencyofthepowerICfrom400kHzto2MHz,thesizeofthese
externalscanbedramaticallyreduced.However,inordertodothiseffectively,thepowerIC
mustbecapableofdeliveringhighefficiencyatthesehigherfrequencies,whichhistorically
hasnotbeenfeasible.Nevertheless,usingnewprocessanddesigntechniques,synchronous
powerICshaverecentlybeendevelopedthatcandeliverefficienciesinexcessof90%while
switchingat2MHz.Thehighefficiencyoperationminimizespowerloss,eliminatingtheneed
forheatsinks.ItalsohastheaddedbenefitofkeepingswitchingnoiseoutoftheAM
frequencybandwhichiscriticalinmanynoisesensitiveelectronicsystems.
Anotherwaytomakepowerconversioncircuitsdramaticallysmaller,istouseamultioutputconvertersinlieuofmultiplesingledevices,whenseveraluniqueoutputvoltagerails
arerequired.Forexamplewhenpoweringamicroprocessor,mostdesignsrequirethree
independentoutputstopowerVCORE,VI/O,VMEMrequiredformicroprocessorsandafourth
5VrailtopoweraCANtransceiverwhichenablesthemicroprocessortocommunicatewith
therestofthesystemelectronics.BecauseaproperlydesignedquadconverterICisonly
moderatelylargerthananequivalentsingleconverterwhereasitssolutionfootprintcanbe
lessthanhalfthesizeoffourseparatesingleconverters.Thissizeimprovementisespecially
compellingwhencomparingaquadoutputregulatorusingaswitchingfrequencyof2MHz
versusfoursingleswhichrunat500KHz.Additionally,quadoutputconvertersaredesigned
tominimizeunwantedcross-talkbetweenthechannels,whereascrosstalkbetweenfour
adjacentsingleconverterscanbeproblematic,unlesstheyareallsynchronizedtoa
commonclock.Addinganexternalclockandsynchronizationaddsbothsize,complexityand
costtothecircuit.
LowEMIOperation
Becausetheautomotiveelectricalenvironmentisinherentlynoisy,withmanyapplications
beingsusceptibletoelectromagneticinterference(EMI),itisimperativethatswitching
regulatorsdon’texacerbatetheseEMIconcerns.Becauseaswitchingregulatoristypically
thefirstactivecomponentontheinputpowerbusline,andregardlessofdownstream
converters,itsignificantlyimpactsoverallconverterEMIperformance.SominimizingEMIis
imperative.Historically,thesolutionwastouseanEMIshieldingbox,butthisadds
significantcostandsizetothesolutionfootprintwhilecomplicatingthermalmanagement,
testingandmanufacturability.AnotherpotentialsolutionwithinthepowermanagementIC
istoslowdowntheswitchingedgesoftheinternalMOSFETs.However,thishasthe
undesiredeffectofreducingtheefficiencyandincreasingtheminimumon-timewhich
compromisestheIC’sabilitytodeliverlowdutycyclesatswitchingfrequenciesat,orabove,
2MHz.Asbothhighefficiencyandsmallsolutionfootprintsaredesired,thisisn’taviable
solution.Fortunately,somerecentpowerICdesignshavebeenintroducedtoenablefast
switchingfrequencies,highefficiencyoperationandlowminimumon-timesconcurrently.
ThesedesignscanofferlowEMIemissionsevenwith2MHzswitchingfrequenciesandover
90%efficiency.Theseperformancelevelsareaccomplishedwithnoadditionalcomponents
orshielding,offeringasignificantbreakthroughinswitchingregulatordesign.
ANewAlternative
LinearTechnology’sLT8602isa42Vvoltinputcapable,highefficiency,quadoutput
monolithicsynchronousstep-downswitchingregulator.Its3Vto42Vinputvoltagerange
makeitidealforautomotiveapplicationswhichmustregulatethroughcold-crankandstopstartscenarioswithminimuminputvoltagesaslowas3Vandloaddumptransientsin
excessof40V.Ascanbeseeninfigure1,itsquadchanneldesigncombinestwohighvoltage
2.5Aand1.5Achannelswithtwolowervoltage1.8Achannelstodeliverfourindependent
outputsdeliveringvoltagesaslowas0.8Venablingittodrivethelowestvoltage
microprocessorcorescurrentlyavailable.Itssynchronousrectificationtopologydeliversup
to94%efficiencywhileBurstMode®operationkeepsquiescentcurrentunder30µA(all
channelson)inno-loadstandbyconditionsmakingitidealforalways-onsystems.
Figure1.LT8602SchematicDelivering5V,3.3V,1.8V&1.2VOutputs
Fornoisesensitiveapplications,theLT8602,withasmallexternalfilter,canutilizeitspulseskippingmodetominimizeswitchingnoiseandcanmeettheCISPR25,Class5EMI
requirementsasshowninFigure2.
Figure2.LT8602RadiatedEMIPerformance,
(CISPR25RadiatedEmissionTestswithClass5PeakLimit)
TheLT8602’sswitchingfrequencycanbeprogrammedfrom250kHzto2MHzandcanbe
synchronizedthroughoutthisrange.Its60nsminimumon-timeenables16VINto2.0VOUT
step-downconversionsonthehighvoltagechannelswitha2MHzswitchingfrequency.As
thehighvoltageVOUT2channelfeedsthetwolowvoltagechannels(VOUT3andVOUT4),these
candeliveroutputsaslowas0.8Vwhilealsoswitchingat2MHz,offeringaverycompact
(~25mmx25mm)quadoutputsolutionasshowninFigure3.
Figure3.LT8602QuadOutputSolutionFootprint
(2xactualsize)
Inadditiontheminimizingthesolutionfootprint,theLT8602’s2MHzswitchingfrequency
enablesdesignerstoavoidcriticalnoise-sensitivefrequencybands,suchasAMradio.Each
channeloftheLT8602maintainsaminimumdropoutvoltageofonly200mV(@1A)underall
conditions,enablingittoexcelinscenariossuchasautomotivecold-crank.Programmable
power-onresetandpowergoodindicatorsforeachchannelhelpstoensureoverallsystem
reliability.TheLT8602’s40-leadthermallyenhanced6mmx6mmQFNpackageandhigh
switchingfrequencykeepsexternalinductorsandcapacitorssmall,providingacompact,
thermallyefficientfootprint.
TheLT8602utilizesquadinternaltopandbottomhighefficiencypowerswitcheswithallthe
necessaryboostdiodes,oscillator,controlandlogiccircuitryintegratedontoasingledie.
Channel1and3switch180degoutofphasewithchannel2and4toreduceoutputripple.
Eachchannelhasaseparateinputforaddeddesignflexibility,butmostapplicationswillrun
thetwolowvoltagechannelsdirectlyfromthetwohighvoltagechannelstoofferavery
simple,highfrequencyquadoutputdesign.LowrippleBurstMode®operationmaintains
highefficiencyatlowoutputcurrentswhilekeepingoutputripplebelow15mVPK-PK.Unique
designtechniquesandanewhighspeedprocessenablehighefficiencyoverawideinput
voltagerangeandtheLT8602’scurrent-modetopologyenablesfasttransientresponseand
excellentloopstability.Otherfeaturesincludeinternalcompensation,apowergoodflags,
outputsoftstart/tracking,short-circuitandthermalprotection.
Conclusion
Therapidincreaseinthenumberandcomplexityofelectronicsystemsinautomobileshas
necessitatedevenhigherdemandsonpowermanagementICperformance.Byhavingquad
outputpowerICs,automotivedesignerscandramaticallyreducethespacerequiredbytheir
powerconversioncircuits.Combinedwithswitchingfrequenciesof2MHz,thesizeofthe
externalcomponents,namelytheinductorsandoutputcapacitors,canalsobedramatically
reducedofferingaverycompactquadrailsolutionfootprint.Thesecompactdesignsare
alsoveryrugged,capabletowithstandtherequiredtransientbehaviorfoundinstop-start,
cold-crankandload-dumpconditionswhileaccuratelyregulatingalloutputs.Additionally,
ultralowquiescentcurrentsmakethemidealforalwaysonsystems.Asmoreelectronic
systemsareaddedintoevershrinkingspaces,minimizingthesolutionfootprintwhile
maximizingefficiencyisalsocritical.Fortunatelyanewgenerationofmulti-outputpower ICs
whichmeetthesedemandsarealreadyavailablepavingthewayforevenhigherelectronic
contentinfuturevehicles.