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PowerDissipationandJunctionTemperature
p
d
(
最大值
)
t
j
最大值
t
一个
右
ja
(1)
p
d
v
入点
v
出点
我
出点
(2)
ProgrammingtheTPS73001AdjustableLDORegulator
v
出点
v
参考
1
右
1
右
2
(3)
右
1
v
出点
v
参考
1
右
2
(4)
c
1
(3 x 10
7
) x (右
1
右
2
)
(右
1
x 右
2
)
(5)
tps73001,tps73018
TPS73025
tps73030,tps73033
SBVS054A–NOVEMBER2004–REVISEDNOVEMBER2004
applicationinformation(续)
Specifiedregulatoroperationisassuredtoajunctiontemperatureof125
°
c;themaximumjunctiontemperature
shouldberestrictedto125
°
cundernormaloperatingconditions.thisrestrictionlimitsthepowerdissipationthe
regulatorcanhandleinanygivenapplication.toensurethejunctiontemperatureiswithinacceptablelimits,
calculatethemaximumallowabledissipation,p
d(最大值)
,andtheactualdissipation,p
d
,whichmustbelessthanor
equaltoP
d(最大值)
.
themaximumpowerdissipationlimitisdeterminedusingequation1:
在哪里:
•
t
j
maxisthemaximumallowablejunctiontemperature.
•
右
θ
ja
isthethermalresistancejunction-至-ambientforthepackage(seethedissipationratingstable).
•
t
一个
istheambienttemperature.
theregulatordissipationiscalculatedusingequation2:
powerdissipationresultingfromquiescentcurrentisnegligible.excessivepowerdissipationtriggersthethermal
protectioncircuit.
TheoutputvoltageoftheTPS73001adjustableregulatorisprogrammedusinganexternalresistordivideras
showninfigure17.theoutputvoltageiscalculatedusingequation3:
在哪里:
•
v
参考
=1.225vtyp(theinternalreferencevoltage)
resistorsr1andr2shouldbechosenforapproximately50-µadividercurrent.lowervalueresistorscanbe
usedforimprovednoiseperformance,butthesolutionconsumesmorepower.higherresistorvaluesshouldbe
avoidedasleakagecurrentinto/outoffbacrossr1/r2createsanoffsetvoltagethatartificiallyin-
折痕/decreasesthefeedbackvoltageandthuserroneouslydecreases/increasesv
出点
.therecommended
designprocedureistochooser2=30.1k
Ω
tosetthedividercurrentat50µa,c1=15pfforstability,andthen
calculater1usingequation4:
inordertoimprovethestabilityoftheadjustableversion,itissuggestedthatasmallcompensationcapacitorbe
placedbetweenoutandfb.forvoltages&指示灯;1.8v,thevalueofthiscapacitorshouldbe100pf.forvoltages>
1.8v,theapproximatevalueofthiscapacitorcanbecalculatedasshowninequation5:
thesuggestedvalueofthiscapacitorforseveralresistorratiosisshowninthetablebelow.ifthiscapacitoris
notused(suchasinaunity-gainconfiguration)orifanoutputvoltage&指示灯;1.8vischosen,thentheminimum
recommendedoutputcapacitoris4.7µfinsteadof2.2µf.
8
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