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CapacitorSelection
InputCapacitor
OutputCapacitor
C
最小值
I
输出
V
输出
V
BAT
ƒ
V
V
输出
(6)
V
等效串联电阻
I
输出
R
等效串联电阻
(7)
一个
REG
d
V
FB
5
(r3
r4)
R4
(1
i
2.3
s)
(8)
LAYOUTCONSIDERATIONS
TPS61030
tps61031,tps61032
SLUS534D–SEPTEMBER2002–REVISEDAPRIL2004
atleasta10-µfinputcapacitorisrecommendedtoimprovetransientbehavioroftheregulatorandemibehavior
ofthetotalpowersupplycircuit.aceramiccapacitororatantalumcapacitorwitha100-nfceramiccapacitorin
并行的,placedclosetotheic,isrecommended.
Themajorparameternecessarytodefinetheoutputcapacitoristhemaximumallowedoutputvoltagerippleof
theconverter.thisrippleisdeterminedbytwoparametersofthecapacitor,thecapacitanceandtheesr.itis
possibletocalculatetheminimumcapacitanceneededforthedefinedripple,supposingthattheesriszero,用
usingequation6:
参数
f
istheswitchingfrequencyand
∆
visthemaximumallowedripple.
withachosenripplevoltageof10mv,aminimumcapacitanceof100µfisneeded.thetotalrippleislarger
duetotheesroftheoutputcapacitor.thisadditionalcomponentoftheripplecanbecalculatedusing
equation7:
Anadditionalrippleof80mVistheresultofusingatantalumcapacitorwithalowESRof80m
Ω
.thetotalripple
isthesumoftheripplecausedbythecapacitanceandtheripplecausedbytheesrofthecapacitor.inthis
例子,thetotalrippleis90mv.additionalrippleiscausedbyloadtransients.thismeansthattheoutput
capacitanceneedstobelargerthancalculatedabovetomeetthetotalripplerequirements.
theoutputcapacitormustcompletelysupplytheloadduringthechargingphaseoftheinductor.areasonable
valueoftheoutputcapacitancedependsonthespeedoftheloadtransientsandtheloadcurrentduringtheload
改变.withthecalculatedminimumvalueof100µfandloadtransientconsiderations,arecommendedoutput
capacitancevalueisinaround220µf.foreconomicalreasonsthisusuallyisatantalumcapacitor.becauseof
thisthecontrolloophasbeenoptimizedforusingoutputcapacitorswithanESRofabove30m
Ω
.theminimum
valuefortheoutputcapacitoris22µf.
SmallSignalStability
whenusingoutputcapacitorswithloweresr,likeceramics,itisrecommendedtousetheadjustablevoltage
版本.themissingesrcanbeeasilycompensatedthereinthefeedbackdivider.typicallyacapacitorinthe
rangeof10pfinparalleltor3helpstoobtainsmallsignalstabilitywithlowestesroutputcapacitors.formore
detailedanalysisthesmallsignaltransferfunctionoftheerroramplifierandregulator,whichisgiveninequation
8,canbeused.
asforallswitchingpowersupplies,thelayoutisanimportantstepinthedesign,especiallyathighpeakcurrents
andhighswitchingfrequencies.ifthelayoutisnotcarefullydone,theregulatorcouldshowstabilityproblemsas
wellasemiproblems.因此,usewideandshorttracesforthemaincurrentpathandforthepowerground
轨道.theinputcapacitor,outputcapacitor,andtheinductorshouldbeplacedascloseaspossibletotheic.
Useacommongroundnodeforpowergroundandadifferentoneforcontrolgroundtominimizetheeffectsof
groundnoise.connectthesegroundnodesatanyplaceclosetooneofthegroundpinsoftheic.
thefeedbackdividershouldbeplacedascloseaspossibletothecontrolgroundpinoftheic.tolayoutthe
controlground,itisrecommendedtouseshorttracesaswell,separatedfromthepowergroundtraces.这个
avoidsgroundshiftproblems,whichcanoccurduetosuperimpositionofpowergroundcurrentandcontrol
groundcurrent.
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