Datasheet

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APPLICATIONINFORMATION
OperatingFeaturesandSystemConsiderationsforthePTMA40XXDC/DCConverters
Primary-SecondaryIsolation
UndervoltageLockout
OutputOvervoltageClamp
OvercurrentProtection
Soft-StartPowerUp
PTMA403033
PTMA402050
PTMA401120
SLTS259B–FEBRUARY2006–REVISEDMAY2007
Theseconvertersincorporateelectricalisolationbetweentheinputterminals(primary)andtheoutputterminals
(secondary).Allconvertersaretestedtoawithstandvoltageof1500Vdc.ThiscomplieswithUL/cUL60950and
EN60950andtherequirementsforfunctionalisolation.Itallowstheconvertertobeconfiguredforeithera
positiveornegativeinputvoltagesource.ThedatasheetTerminalFunctionstableprovidesguidanceastothe
correctreferencethatmustbeusedfortheexternalcontrolsignals.
Theundervoltagelockout(UVLO)isdesignedtopreventtheoperationoftheconverteruntiltheinputvoltageis
closetotheminimumoperatingvoltage.TheconverterisheldoffwhentheinputvoltageisbelowtheUVLO
threshold,andturnsonwhentheinputvoltagerisesabovethethreshold.Thispreventshighstart-upcurrent
duringnormalpowerupoftheconverter,andminimizesthecurrentdrainfromtheinputsourceduringlowinput
voltageconditions.Theconvertermeetsfullspecificationswhentheminimumspecifiedinputvoltageisreached.
TheUVLOcircuitryalsooverridestheoperationoftheenableorsynchronizationcontrols.Onlywhentheinput
voltageisabovetheUVLOthresholddoesthisinputbecomefunctional.
Themoduleisprotectedfromanovervoltageontheoutputusinganinternalclamp.Thisprotectsagainsta
breakinthefeedbackpathaswellasagroundfaultontheexternalTrimresistor,whichwouldcausetheoutput
voltagetoincrease.
Toprotectagainstloadfaults,theseconvertersincorporateoutputovercurrentprotection.Applyingaloadtothe
outputthatexceedstheconverterovercurrentthreshold(seeapplicablespecification)causestheoutputvoltage
tomomentarilyfoldback,andthenshutdown.Followingshutdown,themoduleperiodicallyattemptsto
automaticallyrecoverbyinitiatingasoft-startpowerup.Thisisoftendescribedasahiccupmodeofoperation,
wherebythemodulecontinuesinthecycleofsuccessiveshutdownandpowerupuntiltheloadfaultisremoved.
Oncethefaultisremoved,theconverterautomaticallyrecoversandreturnstonormaloperation.
Whentheconverterisfirstpowered,theinternalsoft-startcircuitlimitshowfasttheoutputvoltagecanrise.The
soft-startcircuitfunctionsafteravalidinputsourceisappliedandtheoutputisenabled,aftertheconverter
outputisenabledusingtheEnableinput,oronarecoveryfromaloadfault.Thepurposeofthesoft-startfeature
istolimitthesurgeofcurrentdrawnfromtheinputsourcewhentheconverterbeginstooperate.Bylimitingthe
rateatwhichtheoutputvoltagerises,themagnitudeofcurrentrequiredtochargeuptheloadcircuit
capacitanceissignificantlyreduced.
Figure13showsthepower-upcharacteristicofaPTMA403033converter.Theoutputvoltageis3.3V.The
soft-startcircuitcausesaslow,softriseoftheoutputvoltage.Theoutputvoltagewillbegintoriseafteratime
delay(typically85ms-100ms)onceavalidinputvoltageisapplied.Theoutputthenprogressivelyrisestothe
voltageset-point.Thewaveformswererecordedwitharesistiveloadof3A.
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