www.德州仪器.com
地
在
EN1
EN2
OC1
OC2
OUT1
OUT2
TPS2062
R
pullup
V+
POWERDISSIPATIONANDJUNCTIONTEMPERATURE
THERMALPROTECTION
undervoltagelockout(uvlo)
tps2061,tps2062,tps2063
tps2065,tps2066,tps2067
SLVS490B–DECEMBER2003–REVISEDDECEMBER2004
applicationinformation(持续)
figure26.typicalcircuitfortheocpin
thelowon-resistanceonthen-channelmosfetallowsthesmallsurface-mountpackagestopasslarge
电流.thethermalresistancesofthesepackagesarehighcomparedtothoseofpowerpackages;itisgood
designpracticetocheckpowerdissipationandjunctiontemperature.beginbydeterminingther
ds(在)
ofthe
n-channelmosfetrelativetotheinputvoltageandoperatingtemperature.asaninitialestimate,usethe
highestoperatingambienttemperatureofinterestandreadr
ds(在)
fromfigure20.usingthisvalue,thepower
dissipationperswitchcanbecalculatedby:
•
P
D
=r
ds(在)
×
I
2
multiplythisnumberbythenumberofswitchesbeingused.thissteprendersthetotalpowerdissipationfrom
然后-channelmosfets.
最终,calculatethejunctiontemperature:
•
T
J
=P
D
xR
Θ
JA
+T
一个
在哪里:
•
T
一个
=Ambienttemperature
°
C
•
R
Θ
JA
=Thermalresistance
•
P
D
=totalpowerdissipationbasedonnumberofswitchesbeingused.
comparethecalculatedjunctiontemperaturewiththeinitialestimate.iftheydonotagreewithinafewdegrees,
repeatthecalculation,usingthecalculatedvalueasthenewestimate.twoorthreeiterationsaregenerally
sufficienttogetareasonableanswer.
thermalprotectionpreventsdamagetotheicwhenheavy-overloadorshort-circuitfaultsarepresentfor
extendedperiodsoftime.thetps206ximplementsathermalsensingtomonitortheoperatingjunction
temperatureofthepowerdistributionswitch.inanovercurrentorshort-circuitcondition,thejunctiontemperature
risesduetoexcessivepowerdissipation.oncethedietemperaturerisestoapproximately140
°
Cdueto
overcurrentconditions,theinternalthermalsensecircuitryturnsthepowerswitchoff,thuspreventingthepower
switchfromdamage.hysteresisisbuiltintothethermalsensecircuit,andafterthedevicehascooled
approximately10
°
c,theswitchturnsbackon.theswitchcontinuestocycleinthismanneruntiltheloadfaultor
inputpowerisremoved.theocxopen-drainoutputisasserted(activelow)whenanovertemperatureshutdown
orovercurrentoccurs.
anundervoltagelockoutensuresthatthepowerswitchisintheoffstateatpowerup.whenevertheinput
voltagefallsbelowapproximately2v,thepowerswitchisquicklyturnedoff.thisfacilitatesthedesignof
hot-insertionsystemswhereitisnotpossibletoturnoffthepowerswitchbeforeinputpowerisremoved.这
uvloalsokeepstheswitchfrombeingturnedonuntilthepowersupplyhasreachedatleast2v,evenifthe
switchisenabled.onreinsertion,thepowerswitchisturnedon,withacontrolledrisetimetoreduceemiand
voltageovershoots.
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