Load Dump and Cranking Protection for …

1 ApplicationReportSNVA681A March2015 LoadDumpandCrankingProtectionforAutomoti veBacklightLEDP owerSupplyAtulSinghABSTRACTA utomotiveelectronicsystemscan be subjectedto overvoltageprotectionis particularlycommoninautomotive12 V and 24 V systemswherepeakloaddumptransientscan be as highas 60 V. Crankingconditionsagsthe batteryvoltageto a low valuewhenthe batteryis supplyingcurrentto an powersupplydesignusingLM5088-Q1(wideinpu trangenonsynchronousbuckcontroller)with protectionfor loaddump,reversepolarityand cold crankingconditionsconduciveto the of Figures1 TypicalBatteryVoltageProfileUndera = 10 F at the Output(DelayedResponse)..69 ResponseTimeMeasurementWithC2 = 1 F at the Output(FastResponse)..610 ApplicationSchematic:AutomotivePowerSupp lyfor of Tables1 TypicalValuesof the UnsuppressedISO7637-2:2004[1]Pulse5 (loaddumppulse).

2 4 All trademarksare the propertyof March2015 LoadDumpand CrankingProtectionfor AutomotiveBacklightLEDP owerSupplySubmitDocumentationFeedbackCop yright 2015,TexasInstrumentsIncorporatedBattery the batteryvoltageis commonin ,the outputof a 12 Vvehiclebatteryvariesfrom9 V to 16 V. Therefore,all circuitspoweredby the batteryshouldbe designedto accountfor the ,if a cold-crankingconditionis to be considered,thebatteryvoltagecan dropdownto 3 V V, that is, V in the worstcase,for a shortdurationofaround15 ms. Then,the batteryvoltagegoesup to 6 V, whereit remainsfor a few seconds,beforereturningto the nominalvoltagerangewithina rise time of a few cold-crankingcondition,the batteryvoltageprofileis similarfor with respecttothe voltageleveland timing,therewill be variationsamongthe illustratestypicalbatteryvoltageprofileu ndera ColdCrankingConditionat low durationof the conditionis not long,importantautomotiveequipmentlike LCDpanelsand safetyelectronicsarestill requiredto maintainnormaloperationduringthis TypicalBatteryVoltageProfileUndera Cold-CrankingConditionIt is becomingincreasinglyimportantfor automotivesubsystemsto increasefuel economy.

3 Somenew automobilesare beingequippedwith automatedstart-stop systemsthat shut downthe combustionenginewhenthe automobileis stoppedat a trafficlight or ina trafficjam. As the brakepedalor the clutchis released,the electricstartermotorcold cranksthe batteryto get the safetyand convenience,the powermanagementsystemmustsustainthe inputoperatingvoltagelevelsin suchstart-stopsystemsto insurecontinuousoperationofelectroniccom ponentsfoundin navigationsystems,dashboardelectronics,L EDbrakelights, requirespowermanagementsolutionscapableo f operationat very low qualifiedcontrollercan be configuredto operateoveran ultra-wideinputvoltagerangeof V to 75 V. For systemswherethe voltagelevelis V or above,the LM5088-Q1providesa systemoperation,whenthe inputgoesbelowthe outputofthe controller(LM5088-Q1),can be takencareby havinga bridgerectifierfollowedby a SuperCap orhigh valuealuminumelectrolyticcapacitorsat the input,thus,providingcontinuousoperationi n cold-crankingconditionfor the durationspecifiedin LM5088is availablein two versions.

4 The LM5088-1providesa 5% frequencyditheringfunctiontoreducethe conductedand radiatedEMI,whilethe LM5088-2providesa versatilerestarttimerfor low dropoutbias regulator,tri-levelenableinputto controlshutdownand standbymodes,soft-startand deviceis availablein a thermallyenhancedTSSOP-16 EPpin showsthe simplifiedapplicationschematicof the CrankingProtectionfor AutomotiveBacklightLEDP owerSNVA681A March2015 SupplySubmitDocumentationFeedbackCopyrig ht 2015,TexasInstrumentsIncorporatedLoadXAV IN ( )CINCDITHER/RESTARTCVCCRRAMPCRAMPCSSRRTR UV2 RUV1 CBOOTQDRSLCOUT1 COUT2 RFB2 RFB1 VOUTRCOMPCCOMPCHFVINENDITH/ SimplifiedApplicationSchematicLM50882 LoadDumpA load dumpoccurswhenthe load to whicha generatoris deliveringcurrentis ,this appliesto disconnectinga batterywhileit is beingchargedby the be "as high as 120 V and may take up to 400 ms to decay".

5 Figure3 showstypicalload dumpcondition,whenthe alternatorgets disconnectedfromthe TypicalLoadDumpConditionISO7637-2:2004[1 ]pulse5 (loaddumppulse)is designedto simulatethe voltagesurgeproducedbyspinningalternator swhenthe batteryor someothersignificantload is protectingan automotiveelectronicsubsystemis to shuntthe supplyvoltageat the ESAinputwith a manycases,however,the devicehas to absorbmoreenergythanit can safelyhandle,leadingto the failureof the unsuppressedISO7637-2:2004[1]pulse5 (loaddumppulse)is shownin March2015 LoadDumpand CrankingProtectionfor AutomotiveBacklightLEDP owerSupplySubmitDocumentationFeedbackCop yright 2015,TexasInstrumentsIncorporated+VBVGS( )010 5ms-UUs0,9Us0, LoadDumpPulseThe typicalvaluesof the unsuppressedISO7637-2:2004[1]pulse5 (loaddumppulse)for a 12 V and 24 Vbatterysystemis shownin Table1:Table1.

6 TypicalValuesof the UnsuppressedISO7637-2:2004[1]Pulse5 (loaddumppulse)Parameter12 V System24 V SystemUs65 V to 87 V123 V to 174 to 4 1 to 8 td40 ms to 400 ms100 ms to 350 mstrAn alternativeapproachto addressthe load dumptransientsis to disconnectthe inputsupplyto thecontrolleror electronicsubsystemfor the specifieddurationof the pulseand thenreconnectwith a fixeddelaywhenconditionsreturnto the inputoperatingrangeup to 75 V that isacceptablefor typicalload dumptransients,thus,allowingthe controlleroperationbut for transientsabovethe controllerupperoperatingrange,inputsuppl yneedsto be be electronicsubsystemcan be disconnectedin casethe systemoperationis not so can be doneusinga N-channelorP-channelMosfetin serieswith the presentsthe typicalapplicationcircuitfor the typicallypreferredbecauseof the lowerRdsONwhencomparedtoa ReversePolarityProtectionWithN-ChannelMo sfetBut for the automotivesystemswherethe reversepolarityconnectionerrorsare unavoidable,a bridgerectifierat the inputsupplyis the cheapestbut less efficientsolutionas the continuouspowerdissipationinthe diodesreducesthe CrankingProtectionfor AutomotiveBacklightLEDP owerSNVA681A March2015 SupplySubmitDocumentationFeedbackCopyrig ht 2015.

7 TexasInstrumentsIncorporated1 + = VrefCATHODEREFANODE+ Q1012C310uf circuitin Figure6 is the seriesdisconnectcircuitusedto preventLM5088-Q1againstload circuitis designedusingTL431 Aprecisionprogrammablereferenceusedfor LoadDumpProtectionApplicationCircuitThe circuitaboveusesTL431 Afor precisionseriesdisconnect;for a 12 V batterysystemthe trip pointcanbe selectedto be 20 V, whereas,for 24 V batterysystem30 V can be set as the trip TL431 Aisa three-terminaladjustableshuntregulator,w ith specifiedthermalstabilityoverapplicablea utomotive,commercial,and outputvoltagecan be set to any valuebetweenVref( V) and 36 V, with two typicaloutputimpedanceof . Activeoutputcircuitryprovidesa very sharpturn-oncharacteristic,makingthesede vicesexcellentreplacementsfor Zenerdiodesin manyapplicationssuchas onboardregulation,adjustablepowersupplie sand showsthe functionalblockdiagramofTL431A.

8 The internalreferencevoltageof the comparatorVref (invertinginput)is V. The non-invertingend of the comparatorcan be set accordinglyto drivethe outputtransistorthat finallydecidesthe FunctionalBlockDiagramTL431 AThe ON statevoltageof TL431 Ais 2 V, whichturnsOFFthe N-channelMosfet(Q2)havingVgs(min)= (Q1)also turnsOFF,thereafter,disconnectingthe inputload trip pointcan be V as belowit, the FET s Q1 and Q2 enterthe OFFstatebecauseof insufficientinputvoltageto satisfygateto sourcethresholdsof the FET s. ZenerdiodesD1 and D2 are necessaryto clampthe gateto sourcevoltageof the FET s, whichshouldnotexceedbeyondVgs max (typically20 V) of the FET s Q1 and Q2. The Vtrippointcan be calculatedas:5 SNVA681A March2015 LoadDumpand CrankingProtectionfor AutomotiveBacklightLEDP owerSupplySubmitDocumentationFeedbackCop yright 2015, responseof load dumpprotectioncircuitrywith C2 = 10 F (Figure6) is shownin Figure8.

9 The trippointvoltagedecaysslowlybecauseof C2 (10 F) at the output,the natureof the expectedload dumptransientwith the availablecapacitancewill determinethe showstheresponsetime with C2 = 1 F at the ResponseTimeMeasurementWithC2 = 10 F at the Output(DelayedResponse)Figure9. ResponseTimeMeasurementWithC2 = 1 F at the Output(FastResponse)The load dumpprotectioncircuithas no protectionfor reversepolarityconnectionas the bodydiodeof theP-channelFET conductsin caseof reversepolarityprotectionfor the applicationcircuitis providedby meansof the bridgerectifierso that the circuitremainsfunctionalevenin showsthe completeapplicationschematicfor the automotivebacklightLEDpowersupplyhavingl oad dump ,reversebatteryand cold CrankingProtectionfor AutomotiveBacklightLEDP owerSNVA681A March2015 SupplySubmitDocumentationFeedbackCopyrig ht 2015, F!

10 F!R915m"CSD18531Q5AM1L133 H!19m" F!00Q1C310 F!R815k0 REVERSE POLARITY PROTECTION12V BATTERYCRANKING PROTECTIONLOAD dump F!C7C6560 F!C5560 F!D2 MUR460D7 MUR460D1 MUR460D6 MUR460J112 BAT12R1210kU2TL431AD81N4734AC1110 F ApplicationSchematic:AutomotivePowerSupp lyfor BacklightLEDD riverUsingLM5088-Q17 SNVA681A March2015 LoadDumpand CrankingProtectionfor AutomotiveBacklightLEDP owerSupplySubmitDocumentationFeedbackCop yright 2015,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedand its subsidiaries(TI) reservethe rightto makecorrections,enhancements,improvement sand otherchangesto its semiconductorproductsand servicesper JESD46,latestissue,and to discontinueany productor serviceper JESD48, latestrelevantinformationbeforeplacingor dersand shouldverifythat suchinformationis semiconductorproducts(alsoreferredto hereinas components )