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1、附录AAmatteroflight：PWMdimmingBySamehSarhanandChrisRichardson,NationalSemiconductorWhetheryoudriveLEDswithabuck,boost,buck-boostorlinearregulator,thecommonthreadisdrivecircuitrytocontrolthelightoutput.AfewapplicationsareassimpleasONandOFF,butthegreaternumberofapplicationscallfordimmingtheoutpu
2、tbetweenzeroand100percent,oftenwithfineresolution.Thedesignerhastwomainchoices:adjusttheLEDcurrentlinearly(analogdimming),oruseswitchingcircuitrythatworksatafrequencyhighenoughfortheeyetoaveragethelightoutput(digitaldimming).Usingpulse-widthmodulation(PWM)tosettheperiodanddutycycle(Fig.1)isp
3、erhapstheeasiestwaytoaccomplishdigitaldimming,andabuckregulatortopologywilloftenprovidethebestpeRFormance.Figure1:LEDdriverusingPWMdimming,withwaveforms.PWMdimmingpreferredAnalogdimmingisoftensimplertoimplement.WevarytheoutputoftheLEDdriverinproportiontoacontrolvoltage.Analogdimmingintroduce
4、snonewfrequenciesaspotentialsourcesofEMC/EMI.However,PWMdimmingisusedinmostdesigns,owingtoafundamentalpropertyofLEDs:thecharacterofthelightemittedshiftsinproportiontotheaveragedrivecurrent.FormonochromaticLEDs,thedominantwavELengthchanges.ForwhiteLEDs,thecorrelatedcolortemperature(CCT)change
5、s.It'sdifficultforthehumaneyetodetectachangeofafewnanometersinared,green,orblueLED,especiallywhenthelightintensityisalsochanging.Achangeincolortemperatureofwhitelight,however,iseasilydetected.MostwhiteLEDsconsistofadiethatemitsphotonsinthebluespectrum,whichstrikeaphosphorcoatingthatinturnemi
6、tsphotonsoverabroadrangeofvisiblelight.Atlowcurrentsthephosphordominatesandthelighttendstobemoreyellow.AthighcurrentstheblueemissionoftheLEDdominates,givingthelightabluecast,leadingtoahigherCCT.InapplicationswithmorethanonewhiteLED,adifferenceinCCTbetweentwoadjacentLEDscanbebothobviousandunp
7、leasant.ThatconceptextendstolightsourcesthatblendlightfrommultiplemonochromaticLEDs.Whenwehavemorethanonelightsource,anydifferencebetweenthemjarsthesenses.LEDmanufacturersspecifyacertaindrivecurrentintheelectricalcharacteristicstablesoftheirproduct
