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电源-upcharacteristics
t - 时间 = 5 ms/div
I (1 一个/div)
I
V
O
(2 v/div)
V
I
(5 v/div)
UndervoltageLockout
CurrentLimitProtection
OvertemperatureProtection
PTN78000A
SLTS246A–APRIL2005–REVISEDMAY2005
whenconfiguredperthestandardapplication,theptn78000apowermoduleproducesaregulatedoutput
voltagefollowingtheapplicationofavalidinputsourcevoltage.duringpowerup,internalsoft-startcircuitryslows
theratethattheoutputvoltagerises,therebylimitingtheamountofin-rushcurrentthatcanbedrawnfromthe
inputsource.thesoft-startcircuitryintroducesashorttimedelay(typically5ms–10ms)intothepower-向上
典型的.thisisfromthepointthatavalidinputsourceisrecognized.Figure18showsthepower-向上
waveformsforaptn78000a,operatingfroma12-vinputandwiththeoutputvoltageadjustedto–5v.这
waveformsweremeasuredwitha1.5-aresistiveload.
figure18.电源-upwaveforms
theundervoltagelockout(uvlo)circuitpreventsthemodulefromattemptingtopowerupuntiltheinputvoltage
isabovetheuvlothreshold.thispreventsthemodulefromdrawingexcessivecurrentfromtheinputsourceat
powerup.belowtheuvlothreshold,themoduleisheldoff.
theptn78000modulesprotectagainstloadfaultswithacontinuouscurrentlimitcharacteristic.underaload
faultcondition,theoutputcurrentcannotexceedthecurrentlimitvalue.attemptingtodrawcurrentthatexceeds
thecurrentlimitvaluecausesthemoduletoprogressivelyreduceitsoutputvoltage.currentiscontinuously
suppliedtothefaultuntilitisremoved.onremovalofthefault,theoutputvoltagepromptlyrecovers.当
limitingoutputcurrent,theregulatorexperienceshigherpowerdissipation,whichincreasesitstemperature.ifthe
temperatureincreaseisexcessive,themodule'sovertemperatureprotectionbeginstoperiodicallyturntheoutput
voltagecompletelyoff.
athermalshutdownmechanismprotectsthemodule'sinternalcircuitryagainstexcessivelyhightemperatures.一个
riseintemperaturemaybetheresultofadropinairflow,ahighambienttemperature,orasustainedcurrent-限制
情况.ifthejunctiontemperatureoftheinternalcontrolicrisesexcessively,themoduleturnsitselfoff,
reducingtheoutputvoltagetozero.themoduleinstantlyrestartswhenthesensedtemperaturedecreasesbya
fewdegrees.
便条:
overtemperatureprotectionisalast-resortmechanismtopreventdamagetothemodule.itshouldnotbe
reliedonaspermanentprotectionagainstthermalstress.alwaysoperatethemodulewithinitstemperature
deratedlimits,fortheworst-caseoperatingconditionsofoutputcurrent,ambienttemperature,andairflow.
operatingthemoduleabovetheselimits,albeitbelowthethermalshutdowntemperature,reducesthelong-期
reliabilityofthemodule.
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