PR8275
High Voltage Start-Up PWM Controller
theꢀoutputꢀfromꢀanyꢀuncertainꢀstateꢀwhichꢀ
mayꢀ causesꢀ theꢀ MOSFETꢀ workingꢀ
abnormallyꢀorꢀfalseꢀtriggeredꢁon.ꢀHowever,ꢀ
suchꢀ designꢀ won’tꢀ coverꢀ theꢀ conditionꢀ ofꢀ
disconnectionꢀofꢀgateꢀresistorꢀRgꢀthusꢀitꢀisꢀ
stillꢀ stronglyꢀ recommendedꢀ toꢀ haveꢀ aꢀ
resistorꢀ connectedꢀ onꢀ theꢀ MOSFETꢀ gateꢀ
terminalꢀ(asꢀshownꢀinꢀfigureꢀ9)ꢀtoꢀprovideꢀ
extraꢀprotectionꢀforꢀfaultꢀcondition.ꢀ
wheneverꢀtheꢀgateꢀresistorꢀisꢀdisconnectedꢀ
orꢀopenedꢀinꢀanyꢀcase.
Thisꢀexternalꢀpullꢁlowꢀresistorꢀisꢀtoꢀpreventꢀ
theꢀ MOSFETꢀ fromꢀ damageꢀ duringꢀ
powerꢁonꢀ underꢀ theꢀ gateꢀ resistorꢀ isꢀ
disconnected.ꢀ Inꢀ suchꢀ singleꢁfaultꢀ
condition,ꢀasꢀshowꢀinꢀfigureꢀ21,ꢀtheꢀresistorꢀ
R8ꢀcanꢀprovideꢀaꢀdischargeꢀpathꢀtoꢀavoidꢀ
theꢀMOSFETꢀfromꢀbeingꢀfalseꢁtriggeredꢀbyꢀ
theꢀ currentꢀ throughꢀ theꢀ gateꢁtoꢁdrainꢀ
capacitorꢀCgd.ꢀTherefore,ꢀtheꢀMOSFETꢀisꢀ
alwaysꢀ pullꢁlowꢀ andꢀ keptꢀ inꢀ theꢀ offꢁstateꢀ
Fig.9
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Protection Resistor on the Hi-V Path
Inꢀ someꢀ otherꢀ HiꢁVꢀ processꢀ andꢀ design,ꢀ
suchꢀlatchꢀupꢀbehavior.SoꢀthatꢀPR8275ꢀisꢀ
withꢀ higherꢀ capabilityꢀ toꢀ sustainꢀ negativeꢀ
voltageꢀthanꢀsimilarꢀproducts.ꢀHowever,ꢀaꢀ
thereꢀmayꢀcauseꢀaꢀparasiticꢀSCRꢀbetweenꢀ
HVꢀ pin,ꢀ Vccꢀ andꢀ GND.ꢀ aꢀ smallꢀ negativeꢀ
spikeꢀ onꢀ theꢀ HVꢀ pinꢀ mayꢀ triggerꢀ thisꢀ
parasiticꢀ SCRꢀ andꢀ causesꢀ theꢀ latchꢀ upꢀ
betweenꢀVccꢀandꢀGND.ꢀAndꢀsuchꢀlatchꢀupꢀ
isꢀeasyꢀtoꢀdamageꢀtheꢀchipꢀbecauseꢀofꢀtheꢀ
equivalentꢀshortꢁcircuitꢀwhichꢀisꢀinducedꢀbyꢀ
10K Ω resistorꢀ isꢀ recommendedꢀ toꢀ
implementꢀonꢀ theꢀHiꢁVꢀ pathꢀtoꢀ beꢀ playedꢀ
theꢀ roleꢀ asꢀ aꢀ currentꢀ limitꢀ resistorꢀ
wheneverꢀaꢀnegativeꢀvoltageꢀisꢀappliedꢀinꢀ
anyꢀcaseꢀ
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V1.0
9/10
ETC [ ETC ]
