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T
J
T
一个
P
D
最大值
R
θ
JA
(6)
R
θ
JA
T
J
T
一个
P
D
最大值
(7)
SOT223PowerDissipation
0
100
120
140
160
180
pcb 铜 范围 - 在
2
- 热的 阻抗 -
θ
JA
R c/w
°
非 空气 流动
80
60
40
20
0.1 1 10
P
D
最大值
(
3.3
2.5
)
V
1一个
800mW
(8)
R
θ
JA
最大值
(125
55)
°
C
800mW
87.5
°
C
W
0
1
2
3
6
0 25 50 75 100 150125
T
一个
= 25
°
C
T
一个
(
°
c)
4
5
4 在
2
pcb 范围
0.5 在
2
pcb 范围
P
D
(w)
tps79501,tps79516
tps79518,tps79525
tps79530,tps79533
SLVS350B–OCTOBER2002–REVISEDOCTOBER2004
equation5simplifiesintoequation6:
rearrangingequation6givesequation7:
UsingEquation6andthecomputermodelgenerated
curvesshowninfigure26,adesignercanquickly
computetherequiredheatsinkthermalresist-
ance/boardareaforagivenambienttemperature,
powerdissipation,andoperatingenvironment.
TheSOT223packageprovidesaneffectivemeansof
managingpowerdissipationinsurfacemountappli-
cations.thesot223packagedimensionsarepro-
videdinthe
MechanicalData
sectionattheendof
thedatasheet.theadditionofacopperplane
directlyunderneaththeSOT223packageenhances
thethermalperformanceofthepackage.
figure26.sot223thermalresistancevspcb
CopperArea
toillustrate,thetps79525inasot223package
waschosen.forthisexample,theaverageinput
FromthedatainFigure26andrearrangingequation
voltageis3.3v,theoutputvoltageis2.5v,这
6,themaximumpowerdissipationforadifferent
averageoutputcurrentis1a,theambienttempera-
groundplaneareaandaspecificambienttempera-
ture55
°
c,noairflowispresent,andtheoperating
turecanbecomputed(seefigure27).
environmentisthesameasdocumentedbelow.
neglectingthequiescentcurrent,themaximumaver-
agepowerisequation8:
SubstitutingT
J
maxforT
J
intoEquation4gives
equation9:
(9)
fromfigure26,r
Θ
JA
vspcbcopperarea,这
groundplaneneedstobe0.55in
2
forthepartto
dissipate800mw.theoperatingenvironmentused
toconstructfigure26consistedofaboardwith1oz.
copperplanes.thepackageissolderedtoa1oz.
copperpadonthetopoftheboard.thepadistied
throughthermalviastothe1oz.groundplane.
figure27.sot223maximumpowerdissipation
vsAmbientTemperature
11
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