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1、附录AAmatteroflight：PWMdimmingBySamehSarhanandChrisRichardson,NationalSemiconductorWhetheryoudriveLEDswithabuck,boost,buck-boostorlinearregulator,thecommonthreadisdrivecircuitrytocontrolthelightoutput.AfewapplicationsareassimpleasONandOFF,butthegreaternumberofapp
2、licationscallfordimmingtheoutputbetweenzeroand100percent,oftenwithfineresolution.Thedesignerhastwomainchoices:adjusttheLEDcurrentlinearly(analogdimming),oruseswitchingcircuitrythatworksatafrequencyhighenoughfortheeyetoaveragethelightoutput(digitaldimming).Using
3、pulse-widthmodulation(PWM)tosettheperiodanddutycycle(Fig.1)isperhapstheeasiestwaytoaccomplishdigitaldimming,andabuckregulatortopologywilloftenprovidethebestpeRFormance.Figure1:LEDdriverusingPWMdimming,withwaveforms.PWMdimmingpreferredAnalogdimmingisoftensimpler
4、toimplement.WevarytheoutputoftheLEDdriverinproportiontoacontrolvoltage.AnalogdimmingintroducesnonewfrequenciesaspotentialsourcesofEMC/EMI.However,PWMdimmingisusedinmostdesigns,owingtoafundamentalpropertyofLEDs:thecharacterofthelightemittedshiftsinproportiontoth
5、eaveragedrivecurrent.FormonochromaticLEDs,thedominantwavELengthchanges.ForwhiteLEDs,thecorrelatedcolortemperature(CCT)changes.It'sdifficultforthehumaneyetodetectachangeofafewnanometersinared,green,orblueLED,especiallywhenthelightintensityisalsochanging.Achangei
6、ncolortemperatureofwhitelight,however,iseasilydetected.MostwhiteLEDsconsistofadiethatemitsphotonsinthebluespectrum,whichstrikeaphosphorcoatingthatinturnemitsphotonsoverabroadrangeofvisiblelight.Atlowcurrentsthephosphordominatesandthelighttendstobemoreyellow.Ath
7、ighcurrentstheblueemissionoftheLEDdominates,givingthelightabluecast,leadingtoahigherCCT.InapplicationswithmorethanonewhiteLED,adifferenceinCCTbetweentwoadjacentLEDscanbebothobviousandunpleasant.Thatconceptextendstolightsourcesthatblendlightfrommultiplemonochrom
8、aticLEDs.Whenwehavemorethanonelightsource,anydifferencebetweenthemjarsthesenses.LEDmanufacturersspecifyacertaindrivecurrentintheelectricalcharacteristicstablesoftheirproduct
