标签: 数字式恒流

ThisdocumentdescribesapowerLEDdriversolutionusingthePIC12HV615microcontroller(MCU).ThePIC12HV615isan8-pinMCUwithmanyintegratedanalogfeatures.TheLEDdrivercircuitisabuck(step-down)solutionandthecircuitpresentedherecanoperatefrommostanyinputvoltagesourceaslongasitexceedstheforwardvoltageoftheLEDstobedriven.Aproportional-integral(PI)controlleralgorithmisusedtoregulatetheLEDcurrenttoaconstantvalue.ThePIcontrollerisexecutedatarateof976Hz,leavingplentyofCPUtimeavailableforothertasks.Althoughthissampleratewouldprovideinadequatecontrolresponseformostpowersupplyapplications,itworkswellforLEDapplicationsbecausetheLEDpresentsaconstantloadtothepowerstage.Therefore,thecontrollerdoesnotneedtomakefrequentadjustments.TheLEDcurrentissampledusingaresistorinserieswiththesourceoftheMOSFETinthebuckcircuitandamplifiedusingasingleopamp.TheLEDcurrentissampledusingoneoftheavailableADCinputsonthePIC12HV615.TheEnhancedCaptureComparePWM(ECCP)moduleoftheMCUisusedinPWMmodetodrivethebuckcircuit.SincetheMCUhasaninternalvoltageregulatorand8MHzoscillator,veryfewexternalcomponentsarerequiredtocompletethecircuit.AN1138ADigitalConstantCurrentPowerLEDDriverdrivecurrentforthistypeofLEDwouldbe350mAandAuthor:StephenBowlingatypicalforwardvoltagewouldbe3.5V.Secondly,weMicrochipTechnologyInc.willassumethatthecircuitinputvoltageis12V.Theinductorvaluewillbechosentoallowamaximumcurrentrippleof+/-20%.TheeyewillnotbeabletoINTRODUCTIONperceivethiscur……