PR8275
High Voltage Start-Up PWM Controller
is,ꢀ
thusꢀ canꢀ beꢀ eliminatedꢀ onꢀ theꢀ externalꢀ
circuit.ꢀ
1
ꢀ
V+ = (Vcomp − 2Vf )
3
Aꢀpullꢁhighꢀresistorꢀisꢀembeddedꢀinternallyꢀ
Oscillator and Switching Frequency
ConnectingꢀaꢀresistorꢀfromꢀRTꢀpinꢀtoꢀGNDꢀ
Theꢀsuggestedꢀoperatingꢀfrequencyꢀrangeꢀ
ofꢀPR8275ꢀisꢀwithinꢀ50KHzꢀtoꢀ130KHz.ꢀ
accordingꢀtoꢀtheꢀequationꢀcanꢀprogramꢀtheꢀ
normalꢀswitchingꢀfrequency:ꢀ
65
fsw
=
100(KHz)
RT (KΩ)
Internal Slope Compensation
ꢀ
Aꢀ fundamentalꢀ issueꢀ ofꢀ currentꢀ modeꢀ
controlꢀ isꢀ theꢀ stabilityꢀ problemꢀ whenꢀ itsꢀ
dutyꢁcycleꢀisꢀoperatedꢀmoreꢀthanꢀ50%.ꢀToꢀ
stabilizeꢀ theꢀ controlꢀ loop,ꢀ theꢀ slopeꢀ
compensationꢀisꢀneededꢀinꢀtheꢀtraditionalꢀ
UC384Xꢀ designꢀ byꢀ injectingꢀ theꢀ rampꢀ
ꢀ
signalꢀ fromꢀ theꢀ RT/CTꢀ pinꢀ throughꢀ aꢀ
couplingꢀcapacitor.ꢀInꢀPR8275,ꢀtheꢀinternalꢀ
slopeꢀ compensationꢀ circuitꢀ hasꢀ beenꢀ
implementedꢀtoꢀsimplifyꢀtheꢀexternalꢀcircuitꢀ
design.ꢀ
On/Off Control
TheꢀPR8275ꢀcanꢀbeꢀcontrolledꢀtoꢀturnꢀoffꢀ
disabledꢀimmediatelyꢀunderꢀsuchꢀcondition.ꢀ
Theꢀ offꢀ modeꢀ canꢀ beꢀ releasedꢀ whenꢀ theꢀ
pullꢁlowꢀsignalꢀisꢀremoved.ꢀ
byꢀ pullingꢀ COMPꢀ pinꢀ toꢀ lowerꢀ thanꢀ 1.2V.ꢀ
Theꢀ gateꢀ outputꢀ pinꢀ ofꢀ PR8275ꢀ willꢀ beꢀ
ꢀ
Dual-Oscillator Green-Mode Operation
Thereꢀareꢀmanyꢀdifferenceꢀtopologiesꢀhasꢀ
ofꢀallꢀtheseꢀapproachesꢀintendedꢀtoꢀreduceꢀ
theꢀ switchingꢀ cyclesꢀ underꢀ lightꢁloadꢀ orꢀ
noꢁloadꢀconditionꢀeitherꢀbyꢀskippingꢀsomeꢀ
switchingꢀ pulsesꢀ orꢀ reduceꢀ theꢀ switchingꢀ
frequency.ꢀ
beenꢀimplementedꢀinꢀdifferentꢀchipsꢀforꢀtheꢀ
greenꢁmodeꢀorꢀpowerꢀsavingꢀrequirementsꢀ
suchꢀ
asꢀ
“burstꢁmodeꢀ
control”,ꢀ
“skippingꢁcycleꢀ Mode”,ꢀ “variableꢀ offꢁtimeꢀ
controlꢀ“…etc.ꢀTheꢀbasicꢀoperationꢀtheoryꢀ
ꢀ
Over Load Protection (OLP)
ꢀ
Toꢀ protectꢀ theꢀ circuitꢀ fromꢀ theꢀ damageꢀ
duringꢀ overꢀ loadꢀ conditionꢀ orꢀ shortꢀ
condition,ꢀ aꢀ smartꢀ OLPꢀ functionꢀ isꢀ
implementedꢀ inꢀ theꢀ PR8275.ꢀ Figureꢀ 7ꢀ
showsꢀ theꢀ waveformsꢀ ofꢀ theꢀ OLPꢀ
operation.ꢀUnderꢀsuchꢀfaultꢀcondition,ꢀtheꢀ
feedbackꢀsystemꢀwillꢀforceꢀtheꢀvoltageꢀloopꢀ
towardꢀ theꢀ saturationꢀ andꢀ thusꢀ pullꢀ theꢀ
voltageꢀ onꢀ COMPꢀ pinꢀ (VCOMP)ꢀ toꢀ high.ꢀ
Wheneverꢀ theꢀ VCOMP tripsꢀ theꢀ OLPꢀ
thresholdꢀ 5.0Vꢀ andꢀ keepsꢀ longerꢀ thanꢀ
30mSꢀ (whenꢀ switchingꢀ frequencyꢀ isꢀ
65KHz),ꢀ theꢀ protectionꢀ isꢀ activatedꢀ andꢀ
thenꢀturnsꢀoffꢀtheꢀgateꢀoutputꢀtoꢀstopꢀtheꢀ
switchingꢀofꢀpowerꢀcircuit.ꢀTheꢀ30mSꢀdelayꢀ
timeꢀisꢀtoꢀpreventꢀtheꢀfalseꢀtriggerꢀfromꢀtheꢀ
powerꢁonꢀandꢀturnꢁoffꢀtransient.ꢀAꢀdivideꢁ2ꢀ
counterꢀ isꢀ implementedꢀ toꢀ reduceꢀ theꢀ
averageꢀ powerꢀ underꢀ OLPꢀ behavior.ꢀ
WheneverꢀOLPꢀisꢀactivated,ꢀtheꢀoutputꢀisꢀ
latchedꢀoffꢀandꢀtheꢀdivideꢁ2ꢀcounterꢀstartsꢀ
toꢀcountꢀtheꢀnumberꢀofꢀUVLO(off).ꢀTheꢀ ꢀ
V1.0
7/10
ETC [ ETC ]
