开关电源布线设计指南-

Maxim>DesignSupport>TechnicalDocuments>ApplicationNotes>Automotive>APP4944Maxim>DesignSupport>TechnicalDocuments>ApplicationNotes>Power-SupplyCircuits>APP4944Keywords:automotive,DCDC,powersupply,DC-DC,automotiveswitchingpowersupply,automotiveswitchingpower,highfrequencyautomotivepowersupply,high-frequencyautomotivepower,2MHzpowersupply,2MHzDCDC,2MHzDC-DCAPPLICATIONNOTE4944LayoutGuidelinesMaximizeAutomotivePower-SupplyPerformanceandMinimizeEmissionsBy:JonWallaceJan24,2012Abstract:Whenusingahigh-frequencyswitchingregulator,agoodPCBlayoutforanautomotivepowersupplywillprovideacleanoutputsupplyandsavetimedebuggingemissionsissuesintheelectromagneticinterference(EMIchamber.ThisapplicationnoteexplainshowtolayouttheMAX16903/MAX16904switchingregulatorstomaximizeperformanceandminimizeemissions.AsimilarversionofthisarticleappearedonJan.19,2012inJohnDay'sAutomotiveElectronicsNews.IntroductionWhenusingahigh-frequencyswitchingregulatorsuchastheMAX16903/MAX16904,agoodPCBlayoutforanautomotivepowersupplywillprovideacleanoutputsupplyandsavetimedebuggingemissionsissuesintheelectromagneticinterference(EMIchamber.Thisapplicationnoteoutlinessomecriticaldesignissuesofthecircuitwhereoptimizingthelayoutprovidesthemostbenefit,usingtheMAX16903andMAX16904asexamples.GeneralLayoutGuidelines1.Minimizethetraceloopareafortheinputcapacitor(C3,inductor(L1,andoutputcapacitor(C2.2.PlacetheBIASoutputcapacitor(C4asclosetopin13(BIASandpin14(GNDaspossiblewithoutanyviasbetweenthepinsandthecapacitor.ThisistheanalogsupplyfortheIC;anyinductanceonthisconnectionwillincreasenoiseontheBIASsupplywhichcan,inturn,increasejitterontheLXoutput.3.Ashortertraceisbetterthanawidertrace.OptimizingtheAC-DCCurrentPathTominimizeemissions,thelayoutonthepassivecomponentsoftheMAX16903/MAX16904iscritical.Thepathswheretherearecurrent-stepchangesareconsideredtheAC-currentpathsandtheycanbeseenbyeliminatingthepathswherecurrentflowsonboththeONandOFFpartsoftheswitchingcycle.ThepathsthathavecurrentflowingthroughthemduringtheONandOFFcyclesareconsideredtheDC-currentpaths.AC-CurrentPathTheMAX16903synchronousDC-DCconverterhasthreepassivecomponents(C2,C3,andL1directlyintheswitchingcurrentpath.Thesethreecomponentshavethemostimpactonemissionsanddeviceperformance.Figures1and2showtheswitchingcurrentpathduringtheONandOFFcycles;Figure3showsthedifferencebetweenthesetwocurrentpathswherethehighestdi/dtoccurs.OptimizingthelayoutofcomponentC3isthehighestpriority,followedbyoptimizingforL1andC2.Figure1.OUT2currentflowwithPMOSon.Figure2.OUT2currentflowwithDMOSon.Figure3.OUT2AC-currentflowshowingdifference.BoostAC-CurrentPathTheMAX16903/MAX16904DC-DCconverterusesahigh-sideDMOSdevicewhichrequiresa5VsupplyvoltageabovetheLXpin(thesourceoftheDMOS.TogeneratethisvoltageaboostcapacitorisconnectedbetweentheLXandBSTpins(Figure4.DuringtheOFFcycleoftheDMOS,theboostcapacitor(C1ischargedfromthe5VBIASregulator.TheBIASoutputisalsousedtosupplytheerroramplifiers.Itis,therefore,importantthatBIASremainasquietaspossibletoremoveexcessnoisenegativelyinfluencingtheerror-amplifiercircuitry.ThebestwaytoaccomplishthisistominimizetheinductancebetweentheconnectiontoC4andtheMAX16903/MAX16904.Consequently,placeC4ascloseaspossibletopin14(GNDandpin13(BIASwithoutaddinganyvias.Figure4.BoostcapacitorAC-currentflow.SpreadSpectrumWhenagoodlayoutisnotenoughtopasscustomer-requiredemissionstests,theMAX16903/MAX16904canbeorderedwithspread-spectrumclockenabled.Thespread-spectrum-enableddevicecanreducetheFM-bandnoiseby15dBoverthestandardversion.Seethedatasheetforinformationonhowtoorderthespread-spectrum-enabledversions.Example:Two-LayerPCBLayoutUsingTSSOPPackageFigures5and6showanexampleofatwo-layerlayoutusingtheguidelinespresentedabove.Figure5.Exampleofatwo-layerPCBlayoutusingaTSSOPpackage—toplayer.Figure6.Exampleofatwo-layerPCBlayoutusingaTSSOPpackage—bottomlayer.Example:Two-LayerPCBLayoutUsingTDFNPackageFigures7and8showanexampleofatwo-layerlayoutusingtheguidelinespresentedabove.Figure7.Exampleofatwo-layerPCBlayoutusingaTDFNpackage—toplayer.Figure8.Exampleofatwo-layerPCBlayoutusingaTDFNpackage—bottomlayer.MainSupplyFilteringThefilteringonthemainsupplyisalsoveryimportant,asthisisthelastpointatwhichconductedemissionscanbereducedbeforeexitingthemodule.Forhigh-frequencyswitchingregulatorssuchastheMAX16903,conductedemissionsissuesusuallyoccurintheFMradioband(76MHzto108MHz.Toreducetheseemissions,addaferritebeadwithhighimpedanceinthisfrequencyrangeand/oraninductorwithaself-resonantfrequencyabove108MHz.ConclusionProperlayoutofthecriticalpassivecomponentsfortheswitchingregulatorsoftheMAX16903(Figure9willhelptominimizethenoiseandemissionsatthesource.Thiswillsavevaluabletimeandeffortduringthequalificationphaseoftheproject.Table1.ComponentListDesignationC1C2C3C4R1,R2L1U1Qty1111111Description0.1µF,10V±10%X7R0402ceramiccapacitor10µF,10V±10%X7R1206ceramiccapacitor4.7µF,50V±10%X7R1210ceramiccapacitor2.2µF,10V±10%X7