Power Designer 131 - Precision Output Current Limiting Using Average

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1、POWERdesignerExperttips,tricks,andtechniquesforpowerfuldesignsNo.131PrecisionOutputCurrentLimitingUsingAverageCurrentMonitorFeature—ByEricLee,ApplicationsEngineerandRobertBell,DesignCenterDirectorIntroductionTraditionally,currentlimitingofabuckregulatorOncetheMOSFETisturnedoff,theinductorcur

2、rentisaccomplishedbymonitoringtheswitchcurrent.willdecayduringthebalanceoftheclockperiodtoaThismethodprotectstheswitcheffectively,butthevalleylevel.TheMOSFETwillbeturnedonagainatmaximumoutputcurrentlimitwillvarywidely.Inthestartofthenextclockperiodandiftheoverloadsomeemergingapplicationssuch

3、asbattery/super-conditionstillexists,thecurrentwillriseagaintothecapacitorchargers,USBpowersupplies,orLEDdrivers,peaklimitinglevel.verytightcontrolofthemaximumoutputcurrentisnowmandatory.Achievinghigheraccuracyofcurrentlimit/regulationisacompetitiveadvantage.VINVOUTInthisarticle,auniqueavera

4、geoutputcurrent+DrivermonitorfeatureofNationalSemiconductor’sLM5117synchronousbuckcontrollerwillbeintroducedandamethodtousethisfeatureinordertoimplementprecisecurrentlimitingwillbeshown.AnexampleofCLKCurrentLimitThreshold(VCS)Slope-aconstantcurrentregulatorprovidingoutputover-CompensationQS+

5、voltageprotectionwillalsobepresented.QR+VOUT-OutputCurrentLimitVariationWhenUsingPeak-+CurrentLimitREFErrAmpShowninFigure1isablockdiagramofacurrentmode-controlledbuckregulatorimplementingtraditionalcycle-by-cyclepeakcurrentlimiting.ThispopularFigure1.TraditionalCycle-by-CyclePeakCurrentLimit

6、ingcurrentlimitingschemesensesthecurrentthroughtheMOSFETduringtheon-time.ThecurrentcanbeInapplicationswheretheinputvoltageisrelativelysensedusingtheMOSFET's‘on’resistance,inductorlargeincomparisontotheoutputvoltage,controllingresistance,adedicatedsenseresistor,oraSenseFET.narrowpulsewidthsan

7、dshortdutycyclesarenecessaryThecurrentlimitcomparatorturnsofftheMOSFETforregulation.However,traditionalpeakcurrent-ifthesensedcurrent(andtheadditionofslopemodecontrolneedsablankingtimewhichblankscompensation)exceedsthecurrentlimitthreshold.largelea