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1、附录AAmatteroflight：PWMdimmingBySamehSarhanandChrisRichardson,NationalSemiconductorWhetheryoudriveLEDswithabuck,boost,buck-boostorlinearregulator,thecommonthreadisdrivecircuitrytocontrolthelightoutput.AfewapplicationsareassimpleasONandOFF,butthegreaternumbero
2、fapplicationscallfordimmingtheoutputbetweenzeroand100percent,oftenwithfineresolution.Thedesignerhastwomainchoices:adjusttheLEDcurrentlinearly(analogdimming),oruseswitchingcircuitrythatworksatafrequencyhighenoughfortheeyetoaveragethelightoutput(digitaldimmin
3、g).Usingpulse-widthmodulation(PWM)tosettheperiodanddutycycle(Fig.1)isperhapstheeasiestwaytoaccomplishdigitaldimming,andabuckregulatortopologywilloftenprovidethebestpeRFormance.Figure1:LEDdriverusingPWMdimming,withwaveforms.PWMdimmingpreferredAnalogdimmingis
4、oftensimplertoimplement.WevarytheoutputoftheLEDdriverinproportiontoacontrolvoltage.AnalogdimmingintroducesnonewfrequenciesaspotentialsourcesofEMC/EMI.However,PWMdimmingisusedinmostdesigns,owingtoafundamentalpropertyofLEDs:thecharacterofthelightemittedshifts
5、inproportiontotheaveragedrivecurrent.FormonochromaticLEDs,thedominantwavELengthchanges.ForwhiteLEDs,thecorrelatedcolortemperature(CCT)changes.It'sdifficultforthehumaneyetodetectachangeofafewnanometersinared,green,orblueLED,especiallywhenthelightintensityisa
6、lsochanging.Achangeincolortemperatureofwhitelight,however,iseasilydetected.MostwhiteLEDsconsistofadiethatemitsphotonsinthebluespectrum,whichstrikeaphosphorcoatingthatinturnemitsphotonsoverabroadrangeofvisiblelight.Atlowcurrentsthephosphordominatesandtheligh
7、ttendstobemoreyellow.AthighcurrentstheblueemissionoftheLEDdominates,givingthelightabluecast,leadingtoahigherCCT.InapplicationswithmorethanonewhiteLED,adifferenceinCCTbetweentwoadjacentLEDscanbebothobviousandunpleasant.Thatconceptextendstolightsourcesthatble
8、ndlightfrommultiplemonochromaticLEDs.Whenwehavemorethanonelightsource,anydifferencebetweenthemjarsthesenses.LEDmanufacturersspecifyacertaindrivecurrentintheelectricalcharacteristicstablesoftheirproduct
