User's Guide SNVA342E–July2008–RevisedApril2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board 1 Introduction TheLM3402/02HVandLM3404/04HVarebuckregulatorderivedcontrolledcurrentsourcesdesignedto driveaseriesstringofhighpower,highbrightnessLEDs(HBLEDs)atforwardcurrentsofupto0.5A (LM3402/02HV)or1.0A(LM3404/04HV).Thisevaluationboarddemonstratestheenhancedthermal performance,fastdimming,andtrueconstantLEDcurrentcapabilitiesoftheLM3402andLM3404 devices. 2 Circuit Performance with LM3404 Thisevaluationboard(seeFigure1)usestheLM3404toprovideaconstantforwardcurrentof700mA ±10%toastringofuptofiveseries-connectedHBLEDswithaforwardvoltageofapproximately3.4V eachfromaninputof18Vto36V. 3 Thermal Performance ThePSOP-8packageispin-for-pincompatiblewiththeSO-8packagewiththeexceptionofthethermal pad,orexposeddieattachpad(DAP).TheDAPiselectricallyconnectedtosystemground.Whenthe DAPisproperlysolderedtoanareaofcopperonthetoplayer,bottomlayer,internalplanes,or combinationsofvariouslayers,theθ oftheLM3404/04HVcanbesignificantlylowerthanthatoftheSO- JA 8package.ThePSOP-8evaluationboardistwolayersof1ozcoppereach,andmeasures1.25"x1.95". TheDAPissolderedtoapproximately1/2squareinchoftopandtwosquareinchesofbottomlayer copper.ThreethermalviasconnecttheDAPtothebottomlayerofthePCB.ArecommendedDAP/via layoutisshowninFigure2. Alltrademarksarethepropertyoftheirrespectiveowners. SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 1 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ThermalPerformance www.ti.com VDIM 1N4148 VDIM VIN Dual JMP-1 D2 C6 External Voltage Source Optional R2 R3 4V to 6V Q1 LM3404 5 CS GND 4 6 RON DIM 3 C3 VOUT R6 7 VCC BOOT 2 Q3 L1 2 8 VIN SW 1 R4 U1 C1 C2 C5 D1 C4 Optional Q4 R5 CONN-1 Q3 1 LEDs on separate PCB Single package (SC70-6) R1B R1A Complementary N+P Channel Figure1.LM3402/04Schematic 90 mil 10 mil 10 mil 90 mil 35 mil 35 mil Figure2.LM3402/04PSOPThermalPADandViaLayout 2 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com ConnectingtoLEDArray 4 Connecting to LED Array TheLM3402/04evaluationboardincludestwostandard94milturretconnectorsforthecathodeand anodeconnectionstoaLEDarray. 5 Low Power Shutdown TheLM3402/04canbeplacedintoalowpowershutdownstate(I typically90µA)bygroundingtheDIM Q terminal.Duringnormaloperationthisterminalshouldbeleftopen-circuit. 6 Constant On Time Overview TheLM3402andLM3404arebuckregulatorswithawideinputvoltagerangeandalowvoltage reference.Thecontrolledon-time(COT)architectureisacombinationofhystereticmodecontrolanda one-shoton-timerthatvariesinverselywithinputvoltage.WiththeadditionofaPNPtransistor,theon- timercanbemadetobeinverselyproportionaltotheinputvoltageminustheoutputvoltage.Thisisoneof theapplicationimprovementsmadetothisdemonstrationboardthatwillbediscussedlater(improved averageLEDcurrentcircuit). TheLM3402/04weredesignedwithafocusofcontrollingthecurrentthroughtheload,notthevoltage acrossit.Aconstantcurrentregulatorisfreeofloadcurrenttransients,andhasnoneedforoutput capacitancetosupplytheloadandmaintainoutputvoltage.Therefore,inthisdemonstrationboardin ordertodemonstratethefasttransientcapabilities,Ihavechosentoomittheoutputcapacitor.Withany Buckregulator,dutycycle(D)canbecalculatedwiththefollowingequations. t t D = tON O+N tOFF = TONS = tON x fSW (1) TheaverageinductorcurrentequalstheaverageLEDcurrentwhetheranoutputcapacitorisusedornot. ’i IF L ILED(t) VIN - VOUT VOUT L L t DTS TS Figure3.BuckConverterInductorCurrentWaveform Avoltagesignal,V ,iscreatedastheLEDcurrentflowsthroughthecurrentsettingresistor,R ,to SNS SNS ground.V isfedbacktotheCSpin,whereitiscomparedagainsta200mVreference(V ).A SNS REF comparatorturnsonthepowerMOSFETwhenV fallsbelowV .ThepowerMOSFETconductsfora SNS REF controlledon-time,t ,setbyanexternalresistor,R . ON ON SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 3 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ConstantOnTimeOverview www.ti.com ILED CS + RSNS VSNS - Figure4.V Circuit SNS 6.1 Setting the Average LED Current KnowingtheaverageLEDcurrentdesiredandtheinputandoutputvoltages,theslopesofthecurrents withintheinductorcanbecalculated.Thefirststepistocalculatetheminimuminductorcurrent(LED current)point.ThisminimumlevelneedstobedeterminedsothattheaverageLEDcurrentcanbe determined. iPEAK ’i L IF iTARGET ’iD iLED-MIN ILED(t) t tON tOFF tD Figure5.I CurrentWaveform SENSE UsingFigure3andFigure5andtheequationsofaline,calculateI . LED-MIN ’i I = I - L LED-MIN F 2 (2) 4 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com StandardOn-TimeSetCalculation Where I =I (3) F LED-Average Thedeltaoftheinductorcurrentisgivenby: ’2i = VIN -2 VLOUT x tON (4) Thereisa220nsdelay(t )fromthetimethatthecurrentsensecomparatortripstothetimeatwhichthe D controlMOSFETactuallyturnson.Wecansolvefori knowingthereisadelay. TARGET ’i ITARGET = IF - 2L+(cid:3)’iD (5) Δi isthemagnitudeofcurrentbeyondthetargetcurrentandequalto: D V ’iD = OLUT tD (6) Therefore: V - V V iTARGET = IF - IN 2LOUT x tON + OLUT x tD (7) ThepointatwhichyouwantthecurrentsensecomparatortogivethesignaltoturnontheFETequals: i xR =0.20V (8) TARGET SNS Therefore: V - V V 0.2V = RSNS IF - IN 2LOUT x tON + OLUT x tD (9) FinallyR canbecalculated. SNS 0.20V R = SNS V - V V x t (IF) - IN2LOUT x tON + OUTL D (10) 7 Standard On-Time Set Calculation ThecontrolMOSFETon-timeisvariable,andissetwithanexternalresistorR (R2fromFigure1).On- ON timeisgovernedbythefollowingequation: R ON t = k x ON V IN (11) Where k=1.34x10-10 (12) Attheconclusionoft thecontrolMOSFETturnsoffforaminimumOFFtime(t )of300ns,and ON OFF-MIN oncet iscompletetheCScomparatorcomparesV andV again,waitingtobeginthenextcycle. OFF-MIN SNS REF TheLM3402/04haveminimumONandOFFtimelimitations.Theminimumontime(t )is300ns,and ON theminimumallowedofftime(t )is300ns. OFF Designingforthehighestswitchingfrequencypossiblemeansthatyouwillneedtoknowwhenminimum ONandOFFtimesareobserved. MinimumOFFtimewillbeseenwhentheinputvoltageisatitslowestallowedvoltage,andtheoutput voltageisatitsmaximumvoltage(greatestnumberofseriesLEDs). TheoppositeconditionneedstobeconsideredwhendesigningforminimumONtime.MinimumONtime isthepointatwhichtheinputvoltageisatitsmaximumallowedvoltage,andtheoutputvoltageisatits lowestvalue. SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 5 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ApplicationCircuitCalculations www.ti.com 8 Application Circuit Calculations TobetterexplaintheimprovementsmadetotheCOTLM3402/04demonstrationboard,acomparisonis shownbetweentheunmodifiedaverageoutputLEDcurrentcircuittotheimprovedcircuit.Design Examples1and2usetwooriginalLM3402/04circuits.Theswitchingfrequencieswillbemaximizedto provideasmallsolutionsize. DesignExample3isanimprovedaveragecurrentapplication.Example3willbecomparedagainst example2toillustratetheimprovements. Example4willusethesameconditionsandcircuitasexample3,buttheswitchingfrequencywillbe reducedtoimproveefficiency.Thereducedswitchingfrequencycanfurtherreduceanyvariationsin averageLEDcurrentwithawideoperatingrangeofseriesLEDsandinputvoltages. DesignExample1 • V =48V(±20%) IN • DrivingthreeHBLEDswithV =3.4V F • V =(3x3.4V+200mV)=10.4V OUT • I =500mA(typicalapplication) F • Estimatedefficiency=82% • f =fastaspossible SW • Designfortypicalapplicationwithint andt limitations ON OFF LED(inductor)ripplecurrentof10%to60%isacceptablewhendrivingLEDs.Withthismuchallowed ripplecurrent,youcanseethatthereisnoneedforanoutputcapacitor.Eliminatingtheoutputcapacitoris actuallydesirable.AnLEDconnectedtoaninductorwithoutacapacitorcreatesanearperfectcurrent source,andthisiswhatwearetryingtocreate. Inthisdesignwewillchoose50%ripplecurrent. Δi =500mAx0.50=250mA L I =500mA+125mA=625mA PEAK Calculatet ,t andR ON OFF ON FromthedatasheetthereareminimumcontrolMOSFETONandOFFtimesthatneedtobemet. t minimum=300ns OFF t minimum=300ns ON TheminimumONtimewilloccurwhenV isatitsmaximumvalue.ThereforecalculateR atV =60V, IN ON IN andsett =300ns. ON Aquickguidelineformaximumswitchingfrequencyallowedversusinputandoutputvoltagesarein Figure6andFigure7. 6 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com ApplicationCircuitCalculations Figure6.V vsf OUT-MAX SW Figure7.V vsf OUT-MIN SW R ON t = k x ON V IN (13) R =135kΩ (usestandardvalueof137kΩ) ON t =306ns ON Checktoseeift minimumissatisfied.ThisoccurswhenV isatitsminimumvalue. OFF IN AtV =36V,andR =137kΩ calculatet frompreviousequation. IN ON ON t =510ns ON Weknowthat: VOUT tON D = = VIN x K tON + tOFF (14) Rearrangingtheaboveequationandsolvingfort witht setto510ns OFF ON V x K tOFF = tON IN - 1 V OUT (15) t =938ns(satisfied) OFF SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 7 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ApplicationCircuitCalculations www.ti.com Table1.Example1ONandOFFTimes V (V) V (V) t t IN OUT ON OFF 36 10.4 5.10E-07 9.38E-07 48 10.4 3.82E-07 1.06E-06 60 10.4 3.06E-07 1.14E-06 CalculateSwitchingFrequency V =36V,48and60V. IN Substitutingequations: f =691kHz(V =36V,48V,and60V) SW IN CalculateInductorValue With50%rippleatV =48V IN •I =500mA F •Δi =250mA(target) L •L=57µH(68 µHstandardvalue) CalculateΔiforV =36V,48V,and60VwithL=68 µH IN Table2.Example1RippleCurrent V (V) V (V) Δi (A) IN OUT L 36 10.4 0.192 48 10.4 0.211 60 10.4 0.223 CalculateR SNS CalculateR atV typical(48V),andaverageLEDcurrent(I )setto500mA. SNS IN F iPEAK ’i IF L iLED-MIN ILED(t) t tON tOFF Figure8.InductorCurrentWaveform • I =500mA F • V =48V IN • V =10.4V OUT • L=68µH • t =220ns D • t =382ns ON UsingequationsfromtheCOTOverviewsection,calculateR . SNS 8 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com ApplicationCircuitCalculations 0.20V R = SNS V - V V x t (IF) - IN2LOUT x tON + OUTL D Or: 0.20V R = SNS V - V k x R V x t (I ) - IN OUT ON + OUT D F 2L VIN L (16) Therefore:R =467mΩ SNS CalculateAverageLEDcurrent(I ) F CalculateaveragecurrentthroughtheLEDsforV =36Vand60V. IN IF = 0R.20V + VIN 2- LVOUT (tON) - VOUTL x tD SNS (17) Table3.Example1AverageLEDCurrent V (V) V (V) I (A) IN OUT F 36 10.4 0.490 48 10.4 0.500 60 10.4 0.506 DesignExample2 Designexample2demonstratesadesignifasingleBillofMaterials(Bom)isdesiredovermanydifferent applications(numberofseriesLEDs,V ,V etc). IN OUT • V =48V(±20%) IN • Driving3,4,or5HBLEDswithV =3.4V F • I =500mA(typicalapplication) F • Estimatedefficiency=82% • f =fastaspossible SW • Designfortypicalapplicationwithint andt limitations ON OFF Theinductor,R resistor,andtheR resistoriscalculatedforatypicaloraveragedesign. ON SNS • V =3x3.4V+200mV=10.4V OUT • V =4x3.4V+200mV=13.8V OUT • V =5x3.4V+200mV=17.2V OUT Calculatet ,t andR ON OFF ON Inthisdesignwewillmaximizetheswitchingfrequencysothatwecanreducetheoverallsizeofthe design.Inalaterdesign,aslowerswitchingfrequencyisutilizedtomaximizeefficiency.Ifthedesignisto usethehighestpossibleswitchingfrequency,youmustensurethattheminimumonandofftimesare adheredto. MinimumontimeoccurswhenV isatitsmaximumvalue,andV isatitslowestvalue. IN OUT CalculateR atV =60V,V =10.4V,andsett =300ns: ON IN OUT ON R ON t = k x ON V IN (18) R =137kΩ,t =306ns ON ON Checktoseeift minimumissatisfied: OFF t minimumoccurswhenV isatitslowestvalue,andV isatitsmaximumvalue. OFF IN OUT AtV =36V,V =17.2V,andR =137kΩ calculatet fromtheaboveequation: IN OUT ON ON SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 9 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ApplicationCircuitCalculations www.ti.com t =510ns ON V x K t IN ON = VOUT tON + tOFF (19) Rearrangetheaboveequationandsolvefort witht setto510ns OFF ON V x K tOFF = tON IN - 1 V OUT (20) t =365ns(satisfied) OFF Table4.Example2OnandOffTime ThreeSeriesLEDs V (V) V (V) R t t IN OUT ON ON OFF 36 10.4 137kΩ 5.10E-07 9.38E-07 48 10.4 137kΩ 3.82E-07 1.06E-06 60 10.4 137kΩ 3.06E-07 1.14E-06 FourSeriesLEDs 36 13.8 137kΩ 5.10E-07 5.81E-07 48 13.8 137kΩ 3.82E-07 7.08E-07 60 13.8 137kΩ 3.06E-07 7.85E-07 FiveSeriesLEDs 36 17.2 137kΩ 5.10E-07 3.65E-07 48 17.2 137kΩ 3.82E-07 4.93E-07 60 17.2 137kΩ 3.06E-07 5.69E-07 CalculateSwitchingFrequency Theswitchingfrequencywillonlychangewithoutputvoltage. V f = OUT SW VIN x K x tON (21) Substitutingequations: V OUT f = SW K x k x RON (22) Or: 1 f = SW tON + tOFF (23) •f =691kHz(V =10.4V) SW OUT •f =916kHz(V =13.8V) SW OUT •f =1.14MHz(V =17.2V) SW OUT CalculateInductorValue V - V L = IN ’iOUT x tON (24) With50%rippleatV =48V,andV =10.4V IN OUT •I =500mA AVG •Δi =250mA(target) L •L=53µH(68uHstandardvalue) CalculateΔiforV =36V,48V,and60VwithL=68 µH. IN 10 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com ApplicationCircuitCalculations Table5.Example2RippleCurrent V (V) V (V) Δi (A) IN OUT L ThreeSeriesLEDs 36 10.4 0.192 48 10.4 0.211 60 10.4 0.223 FourSeriesLEDs 36 13.8 0.166 48 13.8 0.192 60 13.8 0.208 FourSeriesLEDs 36 17.2 0.141 48 17.2 0.173 60 17.2 0.193 CalculateR SNS CalculateR atV typical(48V),withfourseriesLEDs(13.8V=V ),andaverageLEDcurrent(I )set SNS IN OUT F to500mA. • I =500mA F • V =48V IN • V =13.8V OUT • L=68µH • t =220ns D • t =382ns ON 0.20V R = SNS V - V V x t (IF) - IN OUT x tON + OUT D 2L L (25) R =446mΩ SNS CalculateAverageCurrentthroughLED AllcombinationsofV ,V withR =446mΩ IN OUT SNS IF = 0R.20V + VIN 2- LVOUT (tON) - VOUTL x tD SNS (26) Table6.Example2AverageLEDCurrent V (V) V (V) I (A) IN OUT F ThreeSeriesLEDs 36 10.4 0.511 48 10.4 0.521 60 10.4 0.526 FourSeriesLEDs 36 13.8 0.487 48 13.8 0.500 60 13.8 0.508 FiveSeriesLEDs 36 17.2 0.463 48 17.2 0.479 60 17.2 0.489 SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 11 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ModifiedCOTApplicationCircuit www.ti.com Inthisapplicationyoucanseethatthereisadifferenceof63mAbetweenthelowandhighofthe averageLEDcurrent. 9 Modified COT Application Circuit Withtheadditionofonepnptransistorandoneresistor(Q1andR3)theaveragecurrentthroughthe LEDscanbemadetobemoreconstantoverinputandoutputvoltagevariations.Refertopageone, Figure1.ResistorR (R2)andQ1turnthet equationinto: ON ON R t = k x ON ON V - V IN OUT (27) IgnorethePNPtransistor’sV voltagedrop. BE Designtothesamecriteriaasthepreviousexamplewiththeimprovedapplicationandcompareresults. 10 Modified Application Circuit Design Example 3 DesignExample1 • V =48V(±20%) IN • Driving3,4,or5HBLEDswithV =3.4V F • I =500mA(typicalapplication) F • Estimatedefficiency=82% • f =fastaspossible SW • Designfortypicalapplicationwithint andt limitations ON OFF Theinductor,R resistor,andtheR resistorarecalculatedforatypicaloraveragedesign. ON SNS •V =3x3.4V+200mV=10.4V OUT •V =4x3.4V+200mV=13.8V OUT •V =5x3.4V+200mV=17.2V OUT Calculatet ,t andR ON OFF ON MinimumONtimeoccurswhenV isatitsmaximumvalue,andV isatitslowestvalue. IN OUT CalculateR atV =60V,V =10.4V,andsett =300ns: ON IN OUT ON V - V R = t IN OUT ON ON k (28) R =111kΩ (113kΩ)t =306ns ON ON Checktoseeift minimumissatisfied. OFF AtV =36V,V =17.2V,andR =113kΩ calculatet . IN OUT ON ON: t =806ns ON V x K tOFF = tON IN - 1 V OUT (29) t =577ns(satisfied) OFF 12 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com ModifiedApplicationCircuitDesignExample3 VIN Improved Average R2 Current Circuit Q1 R3 LM3404 5 CS GND 4 6 RON DIM 3 VOUT C3 7 VCC BOOT 2 L1 8 VIN SW 1 U1 D1 C4 Optional C1 C2 C5 LEDs on separate PCB R1 Figure9.ImprovedAverageLEDCurrentApplicationCircuit Table7.Example3OnandOffTimes ThreeSeriesLEDs V (V) V (V) R t t IN OUT ON ON OFF 36 10.4 113kΩ 5.92E-07 1.09E-07 48 10.4 113kΩ 4.03E-07 1.12E-06 60 10.4 113kΩ 3.06E-07 1.14E-06 FourSeriesLEDs 36 13.8 113kΩ 6.83E-07 7.78E-07 48 13.8 113kΩ 4.43E-07 8.21E-07 60 13.8 113kΩ 3.28E-07 8.41E-07 FiveSeriesLEDs 36 17.2 113kΩ 8.06E-07 5.77E-07 48 17.2 113kΩ 4.92E-07 6.34E-07 60 17.2 113kΩ 3.54E-07 6.59E-07 SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 13 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ModifiedApplicationCircuitDesignExample3 www.ti.com CalculateSwitchingFrequency V f = OUT SW V x K x t IN ON Or: 1 f = SW tON + tOFF (30) Table8.Example3SwitchingFrequency V (V) V (V) f (kHz) IN OUT SW ThreeSeriesLEDs 36 10.4 595 48 10.4 656 60 10.4 692 FourSeriesLEDs 36 13.8 685 48 13.8 791 60 13.8 855 FiveSeriesLEDs 36 17.2 723 48 17.2 888 60 17.2 987 CalculateInductorValue V - V L = IN ’iOUT x tON R t = k x ON ON V - V IN OUT (31) Therefore: R L = ’OiN x k (32) Youcanquicklyseeonebenefitofthemodifiedcircuit.Theimprovedcircuiteliminatestheinputand outputvoltagevariationonRMScurrent. • I =500mA(typicalapplication) F • Δi =250mA(target) L • R =113kΩ ON • L=59µH(68 µHstandardvalue) • Δi =223mA(L=68 µHallcombinations) L CalculateR SNS OriginalR equation: SNS 0.20V R = SNS V - V V x t (IF) - IN OUT x tON + OUT D 2L L (33) Substituteimprovedcircuitt calculation: ON 0.20V R = SNS V - V R V x t (IF) - IN2LOUT k xV -O VN + OULT D IN OUT (34) Simplified: 14 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com ModifiedApplicationCircuitDesignExample4 0.20V R = SNS (IF) - k x2 RLON + VOUT Lx tD (35) TypicalApplication: • V =13.8V OUT • I =500mA F • R =113kΩ ON • L=68µH • t =220ns D R =462mΩ SNS ThisequationshowsthatonlyvariationsinV willaffecttheaveragecurrentovertheentireapplication OUT range.Thesevariationsshouldbeveryminorevenwithlargevariationsinoutputvoltage. CalculateAverageCurrentthroughLED Modifiedapplicationcircuitaverageforwardcurrentequation. I =0.20V + VIN - VOUT k x RON - VOUT x tD F R 2L V - V L SNS IN OUT (36) Simplified: I = 0.20V+ k x RON - VOUT x tD F R 2L L SNS (37) Table9.Example3AverageLEDCurrent V (V) V (V) I (A) IN OUT F ThreeSeriesLEDs 36 10.4 0.511 48 10.4 0.511 60 10.4 0.511 FourSeriesLEDs 36 13.8 0.500 48 13.8 0.500 60 13.8 0.500 FiveSeriesLEDs 36 17.2 0.489 48 17.2 0.489 60 17.2 0.489 Inthisapplicationyoucanseethatthereisadifferenceof22mAbetweenthelowandhighofthe averageLEDcurrent. 11 Modified Application Circuit Design Example 4 • V =48V(±20%) IN • Driving3,4,or5HBLEDswithV =3.4V F • I =500mA(typicalapplication) F • Estimatedefficiency=82% • f =500kHz(typapp) SW Theinductor,R resistor,andtheR resistorarecalculatedforatypicaloraveragedesign. ON SNS •V =3x3.4V+200mV=10.4V OUT SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 15 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ModifiedApplicationCircuitDesignExample4 www.ti.com •V =4x3.4V+200mV=13.8V OUT •V =5x3.4V+200mV=17.2V OUT Reduceswitchingfrequencyforthetypicalapplicationtoabout500kHztoincreaseefficiency. Calculatet ,t andR ON OFF ON V 1 t = OUT ON V x K f IN SW (38) • V =13.8V OUT • V =48V IN • I =500mA F • t =220ns D • η=0.85 • f =500kHz SW t ≊705ns ON t R = ON (V - V ) ON k IN OUT (39) R ≊179kΩ (usestandardvalueof182kΩ) ON CalculateInductorValue R L = ’OiN x k (40) • I =500mA F • Δi =250mA(target) L • R =182kΩ ON • L=100µH CalculateΔi withL=100 µH(V =48V,V =13.8V) L IN OUT Δi =241mA(allcombinations) L CalculateSwitchingFrequency V f = OUT SW V x K x t IN ON Or: 1 f = SW tON + tOFF (41) Table10.Example4SwitchingFrequency V (V) V (V) f (kHz) IN OUT SW ThreeSeriesLEDs 36 10.4 374 48 10.4 412 60 10.4 435 FourSeriesLEDs 36 13.8 430 48 13.8 497 60 13.8 537 FiveSeriesLEDs 36 17.2 454 48 17.2 558 60 17.2 620 16 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com Dimming CalculateR SNS 0.20V R = SNS (IF) - k x2 RLON + VOUT Lx tD (42) • V =13.8V OUT • V =48V IN • I =500mA F • t =220ns D • η=0.85 • L=100µH R =488mΩ SNS CalculateAverageCurrentthroughLED I = 0.20V+ k x RON - VOUT x tD F R 2L L SNS (43) Table11.Example4AverageLEDCurrent V (V) V (V) I (A) IN OUT F ThreeSeriesLEDs 36 10.4 0.507 48 10.4 0.507 60 10.4 0.507 FourSeriesLEDs 36 13.8 0.500 48 13.8 0.500 60 13.8 0.500 FiveSeriesLEDs 36 17.2 0.493 48 17.2 0.493 60 17.2 0.493 Inthereducedfrequencyapplicationyoucanseethatthereisadifferenceof14mAbetweenthelowand highoftheaveragecurrent. Iftheoriginalt circuitwasused(noPNPtransistor)withtheswitchingfrequencycenteredaround500 ON kHzthedifferencebetweenthehighandlowvalueswouldbeabout67mA. 12 Dimming TheDIMpinoftheLM3402/04isaTTLcompatibleinputforlowfrequencypulsewidthmodulation(PWM) dimmingoftheLEDcurrent.Dependingontheapplication,acontrastratiogreaterthanwhatthe LM3402/04internalDIMcircuitrycanprovidemightbeneeded.Thisdemonstrationboardcomeswith externalcircuitrythatallowsfordimmingcontrastratiosgreaterthan50k:1. 13 LM3402/04 DIM Pin Operation TofullyenableanddisabletheLM3402/04,thePWMsignalshouldhaveamaximumlogiclowlevelof 0.8Vandaminimumlogichighlevelof2.2V.Dimmingfrequency,f ,anddutycycle,D ,arelimitedby DIM DIM theLEDcurrentrisetimeandfalltimeandthedelayfromactivationoftheDIMpintotheresponseofthe internalpowerMOSFET.Ingeneral,f shouldbeatleastoneorderofmagnitudelowerthanthesteady DIM stateswitchingfrequencyinordertopreventaliasing. SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 17 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ContrastRatioDefinition www.ti.com Forillustrations,seeFigure10.Theintervalt representsthedelayfromalogichighattheDIMpintothe D onsetoftheoutputcurrent.Thequantitiest andt representthetimeneededfortheLEDcurrentto SU SD slewuptosteadystateandslewdowntozero,respectively. Asanexample,assumeaDIMdutycycleD equalto100%(alwayson)andthecircuitdelivers500mA DIM ofcurrentthroughtheLEDstring.AtD equalto50%youwouldlikeexactly½ of500mAofcurrent DIM throughyourLEDstring(250mA).Thiscouldonlybepossibleiftherewerenodelays(t )betweenthe D on/offDIMsignalandtheon/offoftheLEDcurrent.Theriseandfalltimes(t andt )oftheLEDcurrent SU SD wouldalsoneedtobeeliminated.Ifwecanreducethesetimes,thelinearitybetweenthePWMsignaland theaveragecurrentwillberealized. T T T DIM D DMIN DMAX tD tSU tSD tD tSU tSD tD tSU tSD IF 1 tD + tSU T - tSD T = D = D = fPWM MIN T MAX T Figure10.ContrastRatioDefinitions 14 Contrast Ratio Definition ContrastRatio(CR)=1/D MIN D =(t +t )xf MIN D SU DIM 18 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com ExternalMOSFETDimmingandContrastRatio DIM 5V/Div 200 mA/Div I F 2 Ps/DIV Figure11.t andt (DIMPin) D SU 15 External MOSFET Dimming and Contrast Ratio MOSFETQ4anditsdrivecircuitryareprovidedonthedemonstrationPCB(seeFigure12).When MOSFETQ4isturnedon,itshortsLED+toLED-,thereforeredirectingtheinductorcurrentfromtheLED stringtotheshuntMOSFET.TheLM3402/04isneverturnedoff,andthereforebecomeaperfectcurrent sourcebyprovidingcontinuouscurrenttotheoutputthroughtheinductor(L1).Abuckconverterwithan externalshuntMOSFETistheidealcircuitfordeliveringthehighestpossiblecontrastratio.Fortypical delaysandrisetimeforexternalMOSFETdimming,seeFigure13-Figure15. SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 19 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

ExternalMOSFETDimmingandContrastRatio www.ti.com 1N4148 VDIM Dual VDIM JMP-1 From V CC LM3402/04 D2 External Voltage C5 Source Optional 4V to 6V R6 L1 Q32 R4 Optional C4 Q4 R5 CONN-1 Q3 1 LEDs on separate PCB Single package (SC70-6) R1B R1A Complementary N+P Channel 11.0 1.1 VDIM 8.0 0.8 )V )A ( M 5.0 0.5 ( D VID IEL ILED 2.0 0.2 -1.0 -0.1 8.0 0.0 8.4 16.6 24.8 33.0 TIME (Ps) Figure12.V =24V,3seriesLEDs@400mA IN 20 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com FastDimming+ImprovedAverageCurrentCircuit 11 1.1 VDIM 0.8 7 )V( M ILED 0.5 )A( DE VID IL 3 0.2 40 ns -1 -0.1 -100 -60 -20 20 60 100 TIME (ns) Figure13.t +t Graph D SU 12.0 1.00 VDIM 8.0 0.60 )V( M )A( DE VID IL 4.0 0.20 ILED 36 ns 0.0 -0.20 -100 -60 -20 20 60 100 TIME (ns) Figure14.t +t Graph D SD 16 Fast Dimming + Improved Average Current Circuit UsingboththeImprovedAverageLEDcurrentcircuitandtheexternalMOSFETfastdimmingcircuit togetherhasadditionalbenefits.IfR andtheconverter'sswitchingfrequency(f )isdeterminedandset ON SW withtheimprovedaverageLEDcurrentcircuit,theswitchingfrequencywilldecreaseonceV isshorted OUT duringfastdimming.WithMOSFETQ4on,V isequaltoV (200mV).Thet equationthenbecomes OUT FB ON almostidenticaltotheoriginalunmodifiedcircuitequation. Settingt andR : ON ON R t = k x ON ON V - V IN OUT (44) t equationbecomes: ON R t = k x ON ON V - 0.2V IN (45) whenQ4shuntMOSFETisonduringfastdimming. t equationduringnormaloperationis: OFF SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 21 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

LinearitywithFastDimming www.ti.com V x K tOFF = tON IN - 1 V OUT (46) t equationthenbecomes: OFF V x K tOFF = tON IN - 1 0.2V (47) whenQ2shuntMOSFETisOFFduringfastdimming. Thisisanaddedbenefitduetothefactthatt isgreatlyincreased,andthereforetheswitching OFF frequencyisdecreased,whichleadstoimprovedefficiency(seeFigure16).InductorL1stillremains charged,andassoonasQ4turnsoffcurrentflowsthroughtheLEDstring. 34.0 0.5 fSW = 650 kHz ILED (A) 28.0 0.4 VSW (V) 22.0 0.2 )V )A ( W 16.0 fSW = 75 kHz 0.1 ( D VS IEL 10.0 -0.1 VDIM (V) 4.0 -0.3 -2.0 -0.4 -6.0 0.5 7.0 13.5 20.0 TIME (Ps) Figure15.ImprovedAvgI Circuit+FastDimming LED 17 Linearity with Fast Dimming Oncethedelaysandrise/falltimeshavebeengreatlyreduced,linearaveragecurrentvs,dutycycle(D ) DIM canbeachievedatveryhighdimmingfrequencies(f )(seeFigure17). DIM 350 300 250 )A( DE 200 fDIM = 25 kHz fDIM = 500 Hz L 150 I 100 50 fDIM = 5 kHz 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) Figure16.LinearityWithFastDimming 22 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

www.ti.com LM3404ImprovedILEDAverageandFastDimmingDemonstrationBoard 18 LM3404 Improved ILED Average and Fast Dimming Demonstration Board VDIM 1N4148 VDIM VIN Dual JMP-1 D2 C6 External Voltage Source Optional R2 R3 4V to 6V Q1 LM3404 5 CS GND 4 6 RON DIM 3 C3 VOUT R6 7 VCC BOOT 2 Q3 L1 2 8 VIN SW 1 R4 U1 C1 C2 C5 D1 C4 Optional Q4 R5 CONN-1 Q3 1 LEDs on separate PCB Single package (SC70-6) R1B R1A Complementary N+P Channel Figure17.V =9Vto18V,I =700mA,3x3.4VWhiteLEDStrings(f ≊≊500kHz) IN LED SW SNVA342E–July2008–RevisedApril2013 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent 23 SubmitDocumentationFeedback EvaluationBoard Copyright©2008–2013,TexasInstrumentsIncorporated

BillofMaterials www.ti.com 19 Bill of Materials PartID PartValue Mfg PartNumber U1 1ABuckLEDDriver NSC LM3404 SOPowerPADpkg C1,InputCap 10µF,25V,X5R TDK C3225X5R1E106M C2,C6Cap 1µF,16V,X5R TDK C1608X5R1C105M C3,V Cap 0.1µF,X5R TDK C1608X5R1H104M BOOST C4OutputCap 10µF,25V,X5R(Optional) TDK C3225X5R1E106M C5,V Cap 0.01µF,X5R TDK C1608X5R1H103M RON D1,CatchDiode 0.5V Schottky2A,30V DiodesINC B230 f R D2 DualSMTsmallsignal DiodesINC BAV199 L1 33µH CoilCraft D01813H-333 R1A,R1B 0.62Ω1%0.25W1206 ROHM MCR18EZHFLR620 R2 47.5kΩ1% Vishay CRCW08054752F R3 1.0kΩ,1% Vishay CRCW08051001F R4,R5 1Ω,1% Vishay CRCW08051R00F R6 10kΩ,1% Vishay CRCW08051002F Q1 SOT23PNP DiodesINC MMBT3906 Q4 SOT23-6N-CH2.4A,20V ZETEX ZXMN2A01E6 Q3 SC70-6,P+NChannel Vishay Si1539DL TestPoints Connector Keystone 1502-2 VIN,GND,LED+,LED- Connector Keystone 575-8 JMP-1 Jumper Molex 22-28-4023 J15 50ΩBNC Amphenol 112538 20 Layout 24 AN-1839LM3402/LM3404FastDimmingandTrueConstantLEDCurrent SNVA342E–July2008–RevisedApril2013 EvaluationBoard SubmitDocumentationFeedback Copyright©2008–2013,TexasInstrumentsIncorporated

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