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T
J
T
一个
P
D
最大值 x
R
θ
JC
R
θ
CS
R
θ
SA
(2)
T
J
T
一个
P
D
最大值 x R
θ
JA
(3)
R
θ
JA
T
J
–T
一个
P
D
最大值
(4)
至-220powerdissipation
P
D
最大值
(
3.3 – 2.5
)
V x 3 一个
2.4 W
(5)
R
θ
JA
最大值
(125 – 55)
°
C
2.4 W
29
°
C
W
(6)
tps75801,tps75815
tps75818,tps75825
TPS75833
SLVS330D–JUNE2001–REVISEDMARCH2004
thermalinformation(持续)
equation2summarizesthecomputation:
TheR
Θ
JC
isspecifictoeachregulatorasdeterminedbyitspackage,引线框架,anddiesizeprovidedinthe
调整器'sdatasheet.ther
Θ
SA
isafunctionofthetypeandsizeofheatsink.forexample,
blackbodyradiator
typeheatsinks,liketheoneattachedtotheto-220packageinfigure21(一个),canhaver
Θ
CS
valuesrangingfrom
5
°
c/wforverylargeheatsinksto50
°
c/wforverysmallheatsinks.ther
Θ
CS
isafunctionofhowthepackageis
attachedtotheheatsink.forexample,ifathermalcompoundisusedtoattachaheatsinktoato-220package,
R
Θ
CS
of1
°
c/wisreasonable.
Evenifnoexternal
blackbodyradiator
typeheatsinkisattachedtothepackage,theboardonwhichthe
regulatorismountedwillprovidesomeheatsinkingthroughthepinsolderconnections.somepackages,likethe
至-263andti'stssoppowerpad™packages,useacopperplaneunderneaththepackageorthecircuit
板'sgroundplaneforadditionalheatsinkingtoimprovetheirthermalperformance.computeraidedthermal
modelingcanbeusedtocomputeveryaccurateapproximationsofanintegratedcircuit'sthermalperformancein
differentoperatingenvironments(e.g.,differenttypesofcircuitboards,differenttypesandsizesofheatsinks,
differentairflows,等.).usingthesemodels,thethreethermalresistancescanbecombinedintoonethermal
resistancebetweenjunctionandambient(r
Θ
JA
).thisr
Θ
JA
isvalidonlyforthespecificoperatingenvironment
usedinthecomputermodel.
equation2simplifiesintoequation3:
rearrangingequation3givesequation4:
usingequation3andthecomputermodelgeneratedcurvesshowninfigure22andfigure25,adesignercan
quicklycomputetherequiredheatsinkthermalresistance/boardareaforagivenambienttemperature,电源
消耗,andoperatingenvironment.
theto-220packageprovidesaneffectivemeansofmanagingpowerdissipationinthrough-holeapplications.
theto-220packagedimensionsareprovidedinthe
MechanicalData
sectionattheendofthedatasheet.一个
heatsinkcanbeusedwiththeto-220packagetoeffectivelylowerthejunction-至-ambientthermalresistance.
toillustrate,thetps75825inato-220packagewaschosen.forthisexample,theaverageinputvoltageis3.3
v,theaverageoutputvoltageis2.5v,theaverageoutputcurrentis3a,theambienttemperature55
°
c,theair
flowis150lfm,andtheoperatingenvironmentisthesameasdocumentedbelow.neglectingthequiescent
电流,themaximumaveragepoweris:
SubstitutingT
J
maxforT
J
intoequation4givesequation6:
fromfigure22,r
Θ
JA
vsheatsinkthermalresistance,aheatsinkwithr
Θ
SA
=22
°
c/wisrequiredtodissipate
2.4w.themodeloperatingenvironmentusedinthecomputermodeltoconstructfigure22consistedofa
standardjedechigh-kboard(2s2p)witha1oz.internalcopperplaneandgroundplane.sincethepackage
pinsweresolderedtotheboard,450mm
2
oftheboardwasmodeledasaheatsink.figure23showstheside
viewoftheoperatingenvironmentusedinthecomputermodel.
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