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.