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1、附录AAmatteroflight：PWMdimmingBySamehSarhanandChrisRichardson,NationalSemiconductorWhetheryoudriveLEDswithabuck,boost,buck-boostorlinearregulator,thecommonthreadisdrivecircuitrytocontrolthelightoutput.AfewapplicationsareassimpleasONandOFF,butthegreaternumberofap
2、plicationscallfordimmingtheoutputbetweenzeroand100percent,oftenwithfineresolution.Thedesignerhastwomainchoices:adjusttheLEDcurrentlinearly(analogdimming),oruseswitchingcircuitrythatworksatafrequencyhighenoughfortheeyetoaveragethelightoutput(digitaldimming).Usi
3、ngpulse-widthmodulation(PWM)tosettheperiodanddutycycle(Fig.1)isperhapstheeasiestwaytoaccomplishdigitaldimming,andabuckregulatortopologywilloftenprovidethebestpeRFormance.Figure1:LEDdriverusingPWMdimming,withwaveforms.PWMdimmingpreferredAnalogdimmingisoftensimp
4、lertoimplement.WevarytheoutputoftheLEDdriverinproportiontoacontrolvoltage.AnalogdimmingintroducesnonewfrequenciesaspotentialsourcesofEMC/EMI.However,PWMdimmingisusedinmostdesigns,owingtoafundamentalpropertyofLEDs:thecharacterofthelightemittedshiftsinproportion
5、totheaveragedrivecurrent.FormonochromaticLEDs,thedominantwavELengthchanges.ForwhiteLEDs,thecorrelatedcolortemperature(CCT)changes.It'sdifficultforthehumaneyetodetectachangeofafewnanometersinared,green,orblueLED,especiallywhenthelightintensityisalsochanging.Ach
6、angeincolortemperatureofwhitelight,however,iseasilydetected.MostwhiteLEDsconsistofadiethatemitsphotonsinthebluespectrum,whichstrikeaphosphorcoatingthatinturnemitsphotonsoverabroadrangeofvisiblelight.Atlowcurrentsthephosphordominatesandthelighttendstobemoreyell
7、ow.AthighcurrentstheblueemissionoftheLEDdominates,givingthelightabluecast,leadingtoahigherCCT.InapplicationswithmorethanonewhiteLED,adifferenceinCCTbetweentwoadjacentLEDscanbebothobviousandunpleasant.Thatconceptextendstolightsourcesthatblendlightfrommultiplemo
8、nochromaticLEDs.Whenwehavemorethanonelightsource,anydifferencebetweenthemjarsthesenses.LEDmanufacturersspecifyacertaindrivecurrentintheelectricalcharacteristicstablesoftheirproduct
