Download 42V High Power Density Buck Regulators in a Tiny QFN Package

42VHighPowerDensityBuckRegulatorsinaTinyQFNPackage
ByYingCheng,SeniorApplicationsEngineer,PowerProducts,LinearTechnologyCorp.
PowerdissipationisasignificantproblemfacingthedesignersofDC/DCconvertersinindustrialandautomotive
applications,wherehighcurrentsarerequired,butspaceislimited.Itispossibletoproduceahighlyefficientregulatorfrom
highperformancediscretecomponents,butexpenseandsolutionfootprintmakethisapproachprohibitive.The
LT8612/LT8613highefficiencybuckregulatorsintegrateallthenecessarycomponentsintoasingleIC,significantly
reducingtheDC/DCconvertersize,evenwhenfacedwithhighstep-downratios.Thesedevicescanalsobeparalleledto
increasetheoutputcurrentcapabilityandspreadtheloadandheat.
Otherdesirablefeaturesincludedintheseregulatorsareultralowquiescentcurrenttomaximizebatterylife,andhigh
switchingfrequencytominimizesolutionsizeandavoidnoise-sensitivefrequencybands.
HIGHLYEFFICIENT42V,6AREGULATORSIN3MM×6MMPACKAGE
TheLT8612/LT8613are42V,6Astep-downmonolithicregulators.Thelowpowerdissipationoftheintegratedhigh
efficiencypowerswitchesallowstheseswitches,theboostdiode,internalcompensationandallnecessarycircuitrytobe
encapsulatedinatiny3mm×6mmQFNpackagewithoutoverheating.Figure1showsatypical5V/30WLT8612converter;
itsefficiencyandpowerlossaredemonstratedinFigure2.Evenwithaninputvoltageof24V,theLT8612canproducea
30Woutputwithnomorethan50°Ctemperaturerise.Attheotherendoftheloadspectrum,theLT8612/LT8613useslow
rippleBurstMode®operationtoenhanceefficiencyinultralowloadsituations.
Figure1.A5V/30Wstep-downconverterusingtheLT8612
P394, EN
Figure2.Efficiencyandpowerlossofthe5V/30WLT8612step-downconverterinFigure1.
Theminimumon-timesofLT8612/LT8613areaslowas40ns,allowingforhighVIN/VOUTratiosevenathighswitching
frequencies.Highswitchingfrequencies,upto2.2MHz,minimizethesizeandvalueofthepowerinductorandoutput
capacitor.Furthermore,theinductorcanbeconfidentlysizedbasedontheoutputloadrequirement—noneedtousea
largerinductortoprovidedesignoverhead—aresultofthedevices’highspeedpeak-currentmodearchitectureandrobust
switchdesign.
MULTIPHASEDESIGNTOINCREASEOUTPUTCURRENTCAPABILITY
Loadsmorethan6Aarenotuncommoninautomotiveandindustrialapplications.Fortheserelativelyhighcurrent
conditions,amultiphasedesigncanextendtheoutputcapabilitiesoftheLT8612/LT8613regulators.TheLT8613hasa
built-inrail-to-railcurrentsenseamplifierwithmonitorandcontrolpins,enablingaccurateinputoroutputaveragecurrent
regulation.ThiscurrentloopmodulatestheinternalswitchcurrentlimitsuchthatthevoltageacrosstheISP/ISNpinsdoes
notexceedthevoltagesetbytheICTRLpin.AnotherIMONpinisusedtomonitortheaveragecurrentthatismeasured
throughtheISP/ISNpins.ThiscurrentcontrolfeaturemakesaccuratecurrentsharingamongseveralLT8613spossible
withoutanyadditionalcontrolcircuitry.
Figure3showsthreeLT8613sparalleledforupto16Aoutput.ThetopLT8613isthemasterregulatorwithitsoutput
voltagesetto4V,whiletheothertwoLT8613sareslaveregulatorswiththeiroutputvoltagessetalittlehigherthan4V.
Figure3.ThreeparallelLT8613sina3-phasedesigndeliver16Aat4VVOUT
InordertosharecurrentamongthethreeLT8613regulators,theIMONpinofthemasterLT8613isconnectedtotheICTRL
pinsoftheslaveLT8613s.ThethreeLT8613regulatorsaresynchronizedbythe3-phase700kHzclocksignals(120°phase
shift)generatedfromtheLTC6909oscillator.Figure4showsathermalimageofthis3-phaseLT8613design.Thesimilarity
intemperaturesoftheLT8613sindicatesevencurrentsharingamongthethreephases.Theefficiencyofthisdesignis
showninFigure5.
Figure4.Thermalimageof3-phaseLT8613designshowsbalancedcurrentsharing
Figure5.Typicalefficiencyof3-phaseLT8613designshowninFigure3
Ifitisdesirabletoweightthecurrentsharingamongthephases—thatistounbalancethecurrentsharing—simplyadjust
thevaluesofthesenseresistorsacrossISPandISNpins.
CONCLUSION
TheLT8612andLT8613arefullyintegratedhighpowerdensitymonolithicstep-downregulatorsthatsatisfychallenging
automotiveandindustrialrequirements.Theycanbeeasilyparalleledforefficient,highcurrentapplicationsandcompact
solutionfootprints.