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THERMALINFORMATION
T
J
T
一个
P
D
最大值 x
R
θ
JC
R
θ
CS
R
θ
SA
P
D
最大值
V
i(avg)
V
o(avg)
I
o(avg)
V
i(avg)
x I
(q)
一个
B
C
T
J
一个
R
θ
JC
T
C
B
R
θ
CS
T
一个
C
R
θ
SA
(一个)
(b)
DDPak 包装
sot223 包装
电路 板 铜 范围
B
一个
C
tps79601,tps79618,tps79625
tps79628,tps79630,tps79633
SLVS351D–SEPTEMBER2002–REVISEDOCTOBER2004
temperatureduetotheregulator'spowerdissipation.
TheamountofheatthatanLDOlinearregulator
Thetemperatureriseiscomputedbymultiplyingthe
generatesisdirectlyproportionaltotheamountof
maximumexpectedpowerdissipationbythesumof
poweritdissipatesduringoperation.allintegrated
thethermalresistancesbetweenthejunctionandthe
circuitshaveamaximumallowablejunctiontempera-
情况(r
Θ
JC
),thecasetoheatsink(r
Θ
CS
),andthe
ture(t
J
最大值)abovewhichnormaloperationisnot
heatsinktoambient(r
Θ
SA
).thermalresistancesare
使确信.asystemdesignermustdesignthe
measuresofhoweffectivelyanobjectdissipates
operatingenvironmentsothattheoperatingjunction
热温.典型地,thelargerthedevice,themore
温度(t
J
)doesnotexceedthemaximum
surfaceareaavailableforpowerdissipationandthe
junctiontemperature(t
J
最大值).thetwomainenviron-
lowertheobject'sthermalresistance.
mentalvariablesthatadesignercanusetoimprove
thermalperformanceareairflowandexternalfigure24illustratesthesethermalresistancesfor(一个)
heatsinks.thepurposeofthisinformationistoaidasot223packagemountedinajedeclow-k
thedesignerindeterminingtheproperoperatingboard,和(b)addpakpackagemountedona
environmentforalinearregulatorthatisoperatingatjedechigh-kboard.
aspecificpowerlevel.
equation5summarizesthecomputation:
ingeneral,themaximumexpectedpower(p
d(最大值)
)
consumedbyalinearregulatoriscomputedas
equation4:
(5)
TheR
Θ
JC
isspecifictoeachregulatorasdetermined
byitspackage,引线框架,anddiesizeprovidedin
(4)
theregulator'sdatasheet.ther
Θ
SA
isafunctionof
在哪里:
thetypeandsizeofheatsink.forexample,
黑色
bodyradiator
typeheatsinkscanhaveR
Θ
CS
值
•
V
i(avg)
istheaverageinputvoltage.
rangingfrom5
°
c/wforverylargeheatsinksto
•
V
o(avg)
istheaverageoutputvoltage.
50
°
c/wforverysmallheatsinks.ther
Θ
CS
isa
•
I
o(avg)
istheaverageoutputcurrent.
functionofhowthepackageisattachedtothe
•
I
(q)
isthequiescentcurrent.
散热器.forexample,ifathermalcompoundisused
toattachaheatsinktoasot223package,r
Θ
CS
的
formosttildoregulators,thequiescentcurrentis
1
°
c/wisreasonable.
insignificantcomparedtotheaverageoutputcurrent;
因此,thetermv
i(avg)
xI
(q)
canbeneglected.这
operatingjunctiontemperatureiscomputedbyadding
theambienttemperature(t
一个
)andtheincreasein
figure24.thermalresistances
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