Create Colorful and Bright LED Light with an LED...
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P371, EN Create Colorful and Bright LED Light with an LED Matrix Dimmer By Keith Szolusha, Applications Engineering Section Leader, Power Products, Linear Technology RGB LEDs are used in projector, architectural, display, stage and automotive lighting systems that require efficient, bright output. To produce predictable colors from an RGB LED, each of its component LEDs (red, green and blue) requires individual, accurate dimming control. High end systems can use an optical feedback loop to allow a microcontroller to adjust the LEDs for color accuracy. Adding a white LED to an RGB LED to produce an RGBW LED extends the hue, saturation and brightness values available in the color system. Each RGBW LED requires accurate dimming of four component LEDs. Two RGBW LEDs require eight “channels.” There are various ways to drive RGBW LEDs for color and brightness control. One way to drive and dim RGBW LEDs is to use four separate LED drivers, one for each color (R, G, B and W) as shown in Figure 1a. In such a system, the LED current, or PWM dimming, of each individual LED or string is driven by separate drivers and control signals. In this solution, though, the number of LED drivers increases quickly with the number of RGBW LEDs. Any lighting system with a significant number of RGBW LEDs requires a substantial number of drivers and synchronization of the control signals to those drivers. A much simpler (and more elegant) approach is to drive all of the LEDs with a single driver/converter at a fixed current, while using a matrix of shunting power MOSFETs to PWM dim the individual LEDs for brightness control. The matrix dimmer and single LED driver shown in Figure 1b reduces the circuit size of the Figure 1a solution. Furthermore, a single communications bus to control the matrix LED dimmer makes RGBW color-mixing LED systems relatively simple and compact, while driving high current RGBW LEDs with accurate color and brightness control.
Transcript of Create Colorful and Bright LED Light with an LED...
P371,EN
CreateColorfulandBrightLEDLightwithanLEDMatrixDimmerByKeithSzolusha,ApplicationsEngineeringSectionLeader,PowerProducts,LinearTechnology
RGBLEDsareusedinprojector,architectural,display,stageandautomotivelightingsystemsthatrequireefficient,brightoutput.ToproducepredictablecolorsfromanRGBLED,eachofitscomponentLEDs(red,greenandblue)requiresindividual,accuratedimmingcontrol.HighendsystemscanuseanopticalfeedbacklooptoallowamicrocontrollertoadjusttheLEDsforcoloraccuracy.AddingawhiteLEDtoanRGBLEDtoproduceanRGBWLEDextendsthehue,saturationandbrightnessvaluesavailableinthecolorsystem.EachRGBWLEDrequiresaccuratedimmingoffourcomponentLEDs.TwoRGBWLEDsrequireeight“channels.”
TherearevariouswaystodriveRGBWLEDsforcolorandbrightnesscontrol.OnewaytodriveanddimRGBWLEDsistousefourseparateLEDdrivers,oneforeachcolor(R,G,BandW)asshowninFigure1a.Insuchasystem,theLEDcurrent,orPWMdimming,ofeachindividualLEDorstringisdrivenbyseparatedriversandcontrolsignals.Inthissolution,though,thenumberofLEDdriversincreasesquicklywiththenumberofRGBWLEDs.AnylightingsystemwithasignificantnumberofRGBWLEDsrequiresasubstantialnumberofdriversandsynchronizationofthecontrolsignalstothosedrivers.
Amuchsimpler(andmoreelegant)approachistodrivealloftheLEDswithasingledriver/converteratafixedcurrent,whileusingamatrixofshuntingpowerMOSFETstoPWMdimtheindividualLEDsforbrightnesscontrol.ThematrixdimmerandsingleLEDdrivershowninFigure1breducesthecircuitsizeoftheFigure1asolution.Furthermore,asinglecommunicationsbustocontrolthematrixLEDdimmermakesRGBWcolor-mixingLEDsystemsrelativelysimpleandcompact,whiledrivinghighcurrentRGBWLEDswithaccuratecolorandbrightnesscontrol.
Figures1aand1b(1a)EightseparateLEDdriversandPWMsignalscanbeusedtodrivetwoRGBWLEDsinahighpowercolor-mixingapplicationor(1b)asingleboost-buckLEDdriverandmatrixLEDdimmerwithserialcommunicationscanbeusedforamuchsmallerandcompactsolution.
TheLT3965matrixLEDdimmerenablessuchadesign,asshowninFigure5.EachLT39658-switchmatrixdimmercanpairwithexactlytwoRGBWLEDs,allowingcontroloftheindividualbrightnessofeachLED(red,green,blueandwhite)inPWMstepsof1/256betweenzeroand100%brightness.Two-wireI2Cserialcommandsprovidebothcolorandbrightnesscontroltoalleightchannels.I2CserialcodetothematrixLEDdimmerICdeterminesthebrightnessstateofalleightLEDsandcancheckforopenandshortLEDsincaseofafault.
SinceeachRGBWLEDisdesignedasasinglepointsource,thered,green,blue,andwhitelightcombinetoproducecolorvariety,withsaturation,hue,andbrightnesscontrol.EachLEDcanbesetin1/256stepsbetweenzero(0/256)and100%(256/256)withthehigh-speedLT3965matrixdimmer.
ACCURATE0–256RGBWCOLORANDBRIGHTNESSCONTROL
RGBWLEDscanproduceaccuratecolorandbrightnesswithPWMdimmingoftheindividualcomponentred,green,blueandwhiteLEDs.IndividualPWMbrightnesscontrolcansupport256-to-1orhigherdimmingratios.AnalternativetoPWMdimmingistosimplyreducethedrivecurrentforeachLED,butaccuracysuffersinthismethod,allowingonly10-to-1dimmingratios,andincurringcolordriftintheLEDsthemselves.AmatrixdimmingapproachusingPWMdimmingoutperformsdrive-currentschemesinaccuracyofcolorandbrightness.
ThebandwidthandtransientresponseoftheLEDdriver(thesourceofthe500mALEDcurrent)affectsthecoloraccuracy.Withover10kHzcrossoverfrequencyandlittleornooutputcapacitor,thecompactboost-buckconverterinFigure5reactsquicklytochangesinthenumberofdrivenLEDsasthematrixdimmerturnsitsswitchesonandoff.
Toillustratehowimportantthisistoaccuracy,red,greenandblueLEDsarerunseparatelyatdifferentPWMdutycyclesandmeasuredforlightoutputwithanRGBopticalsensor.TheresultsinFigure3showuniformslopesofeachcolorfrom4/256to256/256,withaslightchangeinslopebelowthat.Ofcourse,red,greenandblueLEDsarenotperfectintheircolor,sosomecolorfromotherbandssneaksoutevenwhenonlyoneisdriven.Overall,thisisahighlyaccuratesystem.
Accuracycanbeimproveddownto1/256PWMdimmingusingaveryhighbandwidth(>40kHz)buckconverterLEDdriver,butthatinvolveseithertheexpenseofaddinganotherstep-upconvertertocreatearegulated,greaterthan30Voutputvoltage,orhavinganinputvoltagesourceabove30V.Unlessahighlevelofaccuracyatextremelylowlightisnecessary,thereislittlereasontoforgotheFigure5boost-buck’sversatility,simplicityandcompactsizebyaddinganextraconverter.
ThematrixdimmedRGBWLEDcolormixersystemdescribedhereachievesabroadcolorgamut,asshowninFigure4.Addingadditionalcolors,suchasamber,canexpandthegamut.RGBWALEDs(withanamberLEDcomponent)canproducedeepyellowsandorangesthatRGBWLEDscannot.TheseLEDscanalsobedrivenwiththematrixdimmer,buttheeightchannelsofthematrixdimmermatchwelltotwoRGBWLEDs.
The256-leveldimmingschemeoftheLT3965easilytranslatestotypicalRGBpaintprogramsandcommoncolor-mixingalgorithms.Forinstance,ifyouopenastandardPCpaintprogram,youwillseethatcolorsaremixedusinga256-valueRGBsystem,asshowninFigure6.Forexample,theLEDcurrentwaveformsinFigure2producepurplelightfromanRGBWmatrixLEDsystemcontrolledbyabasicPC-
basedpaintprogram.BecausethedesigndescribedinthisarticleproducesaccuratecurrentdriveandPWMcontrol,RGBWLEDscanbepredictablycolor-calibratedbyadjustingthedutycyclesofthecomponentLEDs,easilyaccountingforinherentvariationsinLEDbrightness.
MATRIXLEDCOLORMIXERWITHBOOST-BUCK
ThematrixdimmerrequiresasuitableLEDdrivertopowerthestringofeightLEDsfromavarietyofinputs:standard12V±10%,9V–16V(auto)or6V–8.4V(Li-ion).OnesuchsolutionistheLT3952boost-buckLEDdriver,whichbothstepsupandstepsdowninput-to-LEDvoltage,whileprovidinglowrippleinputandoutputcurrent.Withlittleornooutputcapacitorinitsfloatingoutputtopology,itcanreactquicklytochangesinLEDvoltageastheindividualLEDsarePWM-dimmedonandofftocontrolcolorandbrightness(Figure2).
Figure2TheRGBW500mALEDcurrentsarePWMdimmedandphasedbytheLT3965matrixdimmertocreatecolorsandpatterns.TheLT3952boost-buckconverter/LEDdrivereasilykeepsupwiththerapidchangesinLEDvoltageasindividualLEDsarePWMdimmed.
Figure3Red,green,blue,andwhitebrightnesscontrolversus0–256(outof256)PWMdimmingdutycyclecontrolledbythematrixLEDdimmerwhenpairedwiththeLT3952boost-buckLEDdriverinFigure5.
Figure4RGBLEDsfeatureawidecolorgamut.Addingwhiteisonewaytosimplifythealgorithmicmixingofspecificcolors.Insomemixingschemes,whiteisusedtochangethesaturation,whilered,greenandbluesetthehue.
TheLT3952500mAboost-buckLEDdrivershowninFigure5pairswiththeLT39658-switchmatrixLEDdimmerandtwoRGBW500mALEDs.Thisnewboost-bucktopologygracefullyoperatesovertheentirerangeofzero-to-eightLEDsinseries,withavoltageof0Vto25V.TheinstantaneousseriesLEDvoltagechanges,determinedbywhich,andhowmanyLEDsareenabledanddisabledbythematrixdimmeratanygivenmoment.The60VOUTvoltageofthisconverter/topology(asumofVINandVLED),andtheconverterdutycycle,areratedforthefullinputrangeof6Vto20Vandoutputrange(LEDseriesvoltage)of0Vto25Vat500mA.
ThematrixdimmercontrolsLEDbrightnessbyshuntingtheLEDswithparallelpowerMOSFETs.TheLEDsdonotneedtobeconnectedtogroundwitheitherthefloatingoutputboost-buckLEDdriverorwiththematrixLEDdimmer.AslongastheVINpinoftheLT3965isconnectedtoSKYHOOK,whichisatleast7.1VaboveLED+,alloftheshuntMOSFETsworkproperly.SKYHOOKcanbecreatedwithachargepumpfromtheswitchingconverteroritcanbesuppliedwitharegulatedsourcethatisatleast7.1VgreaterthanthehighestexpectedLED+voltage(inthiscase,20VVINmaxplus25VLEDmax).ThetinyLT8330boostconverterina3mm×2mmDFNisagoodchoicetogenerateSKYHOOK.
Anoptionalexternalclockingdeviceisusedtosynchronizethesystemat350kHz,whichissuitableforautomotiveenvironments,relativelyefficientandallowstheuseofcompactcomponents.Althoughthissystemcouldjustaswellrunat2MHz(abovetheAMband),350kHz(belowtheAMband)enablesthisboost-buckconvertertoregulatewithoutpulse-skippingwhenallLEDsareshortedbythematrixdimmerandtheLEDstringvoltagedropsto330mΩ•500mA•8=1.3V.ThisfrequencyalsosupportshighdimmingratioswithoutvisibleLEDflicker.
START-UPSEQUENCEWITHLEDsONOROFF
ThematrixLEDdimmersystemcanbesettostartwithalloftheLEDsonoroff.StartingupwithalloftheLEDsoffallowsthemtofadeonsoftlyortostartataprogrammedcolorandbrightness,suchasgreen-blueat10%brightness.IfalloftheLEDsstartwithfull500mAcurrentbeforetheserial
communicationsbegintellingthedimmerwhattodo,thenfullbright“white”lightmaybeobservedbeforeserialcommunicationsstart.
Witheitherstart-upmethod,theLT3965shouldbepoweredupbeforeitreceivesI2Cserialcommunications,ortheinitialcommunicationsmaybelostwhenitperformsapower-onreset(POR).ThePORoccurswhentheEN/UVLOpincrossesabovethe1.2Vthreshold.SincethisvoltageisbasedonSKYHOOKbeingatleast7.1VaboveLED+,thiscanoccuratanytimeafterahighSKYHOOKvoltageisapplied,suchas55Vfromasmallboostregulator,oritcanhappenafteracharge-pumpedvoltagefromtheLT3952switchnodeishighenoughtocreateSKYHOOK.Inthecaseofacharge-pumpedSKYHOOK,theLEDcurrentmaybepresentbeforethecharge-pumpedSKYHOOK,sotheLEDslightupbeforetheLT3965switchescanturntheLEDsoff.ThisisasimplesolutionforadesignerwhowouldliketheLEDstoturnonfullbrightnesstostart.
TostarttheLEDsoff,SKYHOOKmustbepresentatahighvoltagebeforetheLT3952isturnedon.AsshowninFigure6,ifthePWMpinisheldlowduringstart-up,theLT3952willnotstartupuntilitiscommandedtodosobyanexternalsource,suchasthemastermicrocontroller.ThemicrocontrollercansendI2CsetupcommandstotheLT3965onceSKYHOOKispresentandsetupitsswitchestotheLEDOFFpositionbeforecurrentisflowingtothem.Then,aftersetup,theLT3952PWMcanbeassertedandthecurrentbeginstoflowthroughshortedLT3965switches,withtheLEDsoff.Afterthis,afadestartcanoccur,ortheLT3965dimmercanjumptoaparticularcolororbrightness.
Uponareset,thePWMoftheLT3952mustbepulledlowagaintoturnitoffandrestartintheLEDsoffposition.InthecaseofFigure5,asimplemicropowerboostsuchasanLT8330cansupply55Vfromthe6V–20Vinput.ThemicrocontrollerreceivesasignalthatLT3965ispoweredupandreadytoreceiveserialcommunicationsbyassertingtheALERTflag.Beforeanyoftheswitchesareshortedout,zerocurrentthroughtheLEDsshowsupaszerovoltageacrosstheswitches—interpretedandreportedasashort-circuitfault.OnlyaftertheLT3965ispoweredupbySKYHOOKistheflagasserted.
Figure5TogetherwiththeLT3952boost-buckLEDdriver,theLT3965matrixLEDdimmercontrolsindividualcolorsontwo500mARGBWLEDsforserial-controlledcolorandpatterns.
Figure6.ColorscanbechosenusingastandardPC-basedcolorpicker.The0–256valuesusedbythematrixdimmercanberelatedtothe0–255valuesusedintypicalRGBsystems.Forinstance,RGB(128,10,128)producesapurplehue.Ascanbeseeninthephotographbelow,thematrixdimmercanproducepredictablecolorswitharealRGBWLED,simplifyingtheworkofalightingdesigner.(6a)Chooseacolor.(6b)TheRGBvaluescorrespondtotheLT3965LEDmatrixdimmingratios.(6c)UseyourPCtosetthedimmingvalues,andseetheresults.
CONCLUSION
TheLT3965matrixLEDdimmersuchastheLT3965canbepairedwithaboost-buckLEDdrivertoformanaccurate-colorRGBWLEDcolormixersystem.WiththeLT3952,itcanbeusedtodrivetwoRGBWLEDsat500mAwith350kHzswitchingfrequencyfroma6Vto20Vinput.Thisversatilesystemcanbepoweredwithautomotivebatteries,12VpowerorLi-ionbatteries.Highcoloraccuracyresultsfromthefasttransientresponseofthepatent-pendingboost-buckLEDdrivertopologyandpredictabledimmingcontrolviathe256:1,I2C-controlledmatrixsystem.ItcanbesetuptostartupwithalloftheLEDsoffandcanfadetostartorjumptoaparticularcolor.Althoughnotrequired,opticalfeedback(viamicrocontroller)canbeaddedtoimprovecoloraccuracy.
