P365, EN

PlaceDC/DCRegulatorsonBacksideofPCBtoClearRoomontheTopsideforDigitalICs

By Afshin Odabaee, Power Module Business Manager Sam Young, Applications Engineer

Linear Technology

FasterIC-to-ICdatacommunicationbenefitsusallwithmorecapablesystemsfromhigherqualityvideostreamingtomorecapablenetworkingequipment.ThisflowofmoredatainshortertimeperiodsexertspressureonsystemdesignerstotackletheeverlastingchallengeofdataqualitybyminimizingunintendedinducederrorsespeciallythosestemmingfromPCBlayouts,traceimped-ance,crosstalk,EMIandotherfactors.So,itmakessensetoshortendistancesbetweenICthatareinconstantcommunicationwitheachotherandaddmorelayerstothePCBwhichconstituteadensercircuitboards.Doingsostartsanotherbattle:thefighttofitasmanyICsandconnectorsontopofPCBwithoutincreasingthecostoftheend-product.Thecostisinlargepercentageassoci-atedwiththesizeandthicknessofthePCB.ForsmallersizeboardsandfewerPCBlayers,some-timesthequestionbecomeswhichcomponentsneedtoberemovedorwhichfunctionsmustbeeliminatedtoreducethemanufacturingcostalbeit,theresalevalueofthedevicemaybeinjeop-ardy.It’safighttofitperformanceandfeatures.

ThetopsideofsystemboardssuchasPCIecardsaredenselypopulatedwithFPGA/ASIC/µP,transceivers,connectors,memoryICsandDC/DCregulatorcircuitsofvariousheightsuptoafewcentimeters.However,thebacksideoftenhasaheightrestrictionwhereonlycomponentswithlessthan2.3mmtallpackagessuchascapacitorsareallowedandmuchofrealestateisunoccu-pied.Whatifthisempty PCBareacouldbeusedbyDC/DCregulatorcircuitsunclutteringthetopsideallowinguseofmorememoryICs,forexample,addingtothesystem’scapability?ThetopsideofsystemboardssuchasPCIecardsaredenselypopulatedwithFPGA/ASIC/uP,transceivers,connectors,memoryICsandDC/DCregulatorcircuitsofvariousheightsuptoafewcentimeters.However,thebacksideoftenhasaheightrestrictionwhereonlycomponentswithlessthan2.3mmtallpackagessuchascapacitorsareallowedandmuchofrealestateisunoccupuied.WhatifthisemptyPCBareacouldbeusedbyDC/DCregulatorcircuitsunclutteringthetopsideallowinguseofmorememoryICs,forexample,addingtothesystem’scapability?

TheLTM4622isadual2.5Aorbytyingtheoutputsincurrentsharing,single5A,step-downµModule®(powermodule)regulatorin6.25mm×6.25mm×1.82mmultrathinLGApackage.Atnearlytheheightofasoldereddown0805casesizecapacitor,itsultralowheightallowsittobemountedonthebacksideofaPCB,freeingspaceonthetopsidefordigitalICs.Thethinprofileallowsittomeetdemandingheightre-strictionssuchasthoserequiredbyPCIeandAdvancedMezzanineCardsinembeddedcomputingsystems.

Figure1.LTM4622TinyDualSupplyinUltrathinPackage

MirroredLayoutforHigherPower,SmallerPCB

TheLTM4622’spinconfigurationisorganizedsymmetrically,soforhighercurrentapplicationswheretwoLTM4622scanbeparallel,onedevicecanbeonthetopsideofPCBandtheothermirroredonthebottomside,minimizingPCBareawhileincreasingoutputpowerandpowerdensity.

Flexible Dual Supply in a Simple < 0.5cm2 Footprint

TheLTM4622hasawideinputvoltagerangeof3.6Vto20V,idealanditcanalsobeconfiguredtooperatedownto3.1Vforoperationfroma3.3Vinputsupply.Itregulatestwovoltagesforacompactmulti-railsolu-tionwhereeachoutputcansupplyupto2.5A(3Apeak)andiscapableofpreciselyregulatingbetween0.6Vto5.5Vtowith1.5%maximumtotalDCoutputvoltageerroroverline,loadandtemperature.Forhigheroutputcurrentupto5A,simplytietheoutputsforcurrentsharing.

TheLTM4622requiresonly3ceramiccapacitorsand2resistorstocompleteasolutionoccupyinglessthan1cm2

single-sidedor0.5cm2onadouble-sidedPCB.

Figure2showstheLTM4622circuitinatypicaldualoutputapplication,alsoillustratingitscompactsolutionsize.Efficiencyandpowerlossforthecircuitoperatingata12VinputareshowninFigure3.

Figure2.TypicalApplication:1.5V/2.5A,1.2V/2.5ADual

Figure3.Efficiency,Lossat12VInput(Fig.2)

Reliable High Performance Regulation

TheLTM4622hasacontrolledon-timecurrent-modearchitectureforafasttransientwithgoodloopstabil-ityoverawidevoltagerange.Itprovidessystemprotectionsuchasshort-circuit,overvoltageandover-temperatureaswellasensuresmonotonicoutputvoltagerampingwithtracking,soft-startandtheabilitytostartintoapre-biasedoutput.Italsohasnolimitationoninputsupplyslewrate.

Figures4and5demonstratethefasttransientandpre-biasstart-upperformancesforthe1.5VoutputrailofFigure2circuit.

Figure4.12VIN,1.5VOUT1.25A-2.5ALoadStep(Fig.2)

Figure5.12VIN,1.8VOUTStart-upintoPre-BiasedOutput

ParallelOperationforHigherCurrentApplicationsTheLTM4622’scurrent-modearchitecturegivesitgoodandreliablecycle-by-cyclecurrentmonitoringal-lowingforitsoutputstobeparalleledtogethertosupportloadcurrentsofupto5A.

Figures6and7demonstratethegoodthermalandcurrentsharingperformanceoftheLTM4622whileconfiguredforatwooutputscurrentsharing,generatinga3.3Vat5Afroma5Vinput.

Figure6.VIN=5V,VOUT=3.3V/5A,TA=25CThermals

Figure7.VIN=5V,VOUT=3.3V/5ACurrentSharing

ConclusionTheultrathinLTM4622providesahighperformanceregulatorforsingleandmulti-railapplications.Itswideoperatingrange,featuresandcompactsolutionsizemakeitahighlyflexibleandrobustsolutioncapableoffittingintothetightestspacesontopsideandbacksideofaPCB.