AN-1645 LM4702 Driving a MOSFET Output Stage (Rev. A)

1 ApplicationReportSNAA045A May2007 RevisedMay2013AN-1645LM4702 Drivinga , + May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013, (Reading)POUT= 1W/Channel,RL= 8 2SK1058 (Reading)POUT= 40W/Channel,RL= 8 2SK1058 (Reading)POUT= 100W/Channel,RL= 8 2SK1058 +NvsFrequencyPOUT/Channel,RL= 8 , 80kHzBW2SK1058 +NvsOutputPower/ChannelRL= 8 , 80kHzBW2SK1058 +NvsOutputPower/ChannelRL= 8 , 1kHz2SK1058 100W(0dB),RL= 8 , 2SK1058 60W(0dB),RL= 8 , 2SK1058 (Reading)POUT= 1W/Channel,RL= 8 , BUZ901 (Reading)POUT= 40W/Channel,RL= 8 , BUZ901 (Reading)POUT= 100W/Channel,RL= 8 , BUZ901 +NvsFrequencyPOUT/Channel,RL= 8 , 80kHzBWBUZ901 +NvsOutputPower/ChannelRL= 8 , 80kHzBWBUZ901 +NvsOutputPower/ChannelRL= 8 , 1kHzBUZ901 100W(0dB),RL= 8 , BUZ901 60W(0dB),RL= 8 BUZ901 (Reading)POUT= 1W/Channel,RL= 8 2SK1530 (Reading)POUT= 40W/Channel,RL= 8 2SK1530 (Reading)POUT= 100W/Channel,RL= 8 2SK1530 +NvsFrequencyPOUT/Channel,RL= 8 , 80kHzBW2SK1530 +NvsOutputPower/ChannelRL= 8 , 80kHzBW2SK1530 +NvsOutputPower/ChannelRL= 8 , 1kHz2SK1530 100W(0dB),RL= 8 2SK1530 60W/Channel(0dB),RL= 8 , 2SK1530 (Reading)POUT= 1W/Channel,RL= 8 IRFP240 (Reading)POUT= 40W/Channel,RL= 8 IRFP240 (Reading)POUT= 100W/Channel,RL= 8 IRFP240 +NvsFrequencyPOUT/Channel,RL= 8 , 80kHzBWIRFP240 +NvsOutputPower/ChannelRL= 8 , 80kHzBWIRFP240 +NvsOutputPower/ChannelRL= 8 , 1kHzIRFP240 100W(0dB),RL= 8 IRFP240 MOSFETO utputStageSNAA045A May2007 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2007 2013, + + + May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013, highlyreliable,fullyintegrated, s wide-bandwidth, 100 Voperatingvoltagerangeallowsforpowerleve lsupto450 Winto8 and900 Winto4.

2 ThisuniqueICsolutionprovidesa quickandeasywaytomanufactureanultrahigh- fidelityamplifiersolution,withthepiece-o f-mindobtainedfroma fullyoptimized, availablein versionhasanoperatingvoltageupto 75V,whiletheB versionoperatesupto availablein a theAN-1490LM4702 PowerAmplifierApplicationReport(SNAA031) .2 OverviewThisapplicationreportwillcoverth edesignofa 2 125W,8 with 1%THD+ N amplifiersolutionusingtheLM4702directlyd rivinganoutputstagethatusesa suitableMOSFET device,settingbiaslevels,performanceopti mizations, MOSFET devicehassomepreferredfeaturescomparedto a (SOA) concernwithlesssensitivitytotemperaturet hana (minoritycarriers) MOSFET outputstagecomparedtootherdevices,a highlysubjectivecriteriabutonethatis importantin theevaluationofanamplifier' MOSFETT hechoiceoftheMOSFET deviceis alsothevoltagefromGate/BasetoSource/Emit ter(VGSorVBE) ofbothdevicesin minimumdraincurrentof100mAwitha 125W/8 theminimumoutputdrivecurrentof3mA( ).Althoughanadditionaldriverstagecanbeem ployedtoremovetheLM4702'sdrivecurrentlim itation,thedesignin parallelthatcanbedrivenwitha parallelwillnotbecoveredin MOSFET outputstagecomparedtoa BJToutputstagewiththeLM4702is drivenfromhighersupplyrails,themaximumvo ltageswingwitha MOSFET outputstagemaybelowerthana a resultofthedifferencein VGScomparedtoVBE.

3 Thedifferencein MOSFETO utputStageSNAA045A May2007 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2007 2013, Table1 aredevicesfromthreesuppliersthatwillmeet thedesigncriteriabasedontheLM4702limitat ionsfordirectdriveandthetargetoutputpowe rspecificationintoa is notanexhaustivelistingbutrepresentsthose deviceswhicharecommonlyusedinMOSEFET amplifierdesigns,wereavailableatthetimeo fwriting, 140 Vminimumbreakdownvoltagerating(VDSS) orhigheranddraincurrent(ID) ( ) is usedtodetermineif 'sSafeOperatingArea(SOA) 2SJ161 Renesas2SK1058/ 2SJ162 RenesasBUZ900/ BUZ905 MagnatecBUZ901/ BUZ906 Magnatec2SK1529/ 2SJ200 Toshiba2SK1530/ 2SJ201 ToshibaIRFP240/ IRFP9240 InternationalRectifierOnlyonepairfromeac hsupplier,thehighervoltageorhighercurren tversions, ,anexampleofhighVTdevices,thepopularInte rnationalRectifierIRFP240/IRFP9240pair,i s thedominantdistortion, ,theLM4702,thebiasstage,andtheoutputstag e(therearenoprotectioncircuits).Eachstag ewillbecoveredin basicunregulatedsupplyconsistingofa transformer,a bridgerectifier,withnoiseandreservoircap acitorsis a highvoltagedriverthatincludestheinputsta geandvoltageamplifierstage(VAS) ,withfeedbackfromtheoutputstage,setstheg ainandis ,slewrate, ,it setstheDCbiasvoltageandresultingbiascurr entin theoutputstageforClassA,ABorB ,it allowsthermalcompensationthatmaintainsst eadybiascurrentastheoutputstagedevicesva ryin ,certaindevicesdonotneedtemperaturecompe nsationandthebiasstagebecomesassimpleasa a basicSource-Followerstageusinga May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013, Figure2 is a 1+RF/Ri(V/V).

4 TheCicapacitorsetsDCgaintounityandthelow frequencyresponsebycreatinga high-passfilterwitha -3dBpointatf-3dB= 1/(2 CiRi) (Hz).Forsonicquality,thedesigndoesnotuse anACcoupling, ,CC, : Setthegainin therangeof10V/Vto50 Usea ,theheatsinktemperatureshouldnotriseabov e55 C. RF, RIN= Ri. theassociatedpairsensuresthattheinputbia scurrentswillhavenegligibleeffectonthein putoffsetvoltage. Usea silvermicatypecapacitorforthecompensatio ncapacitors,CCcapacitorsontheCOMP pins,placedclosetotheLM4702. Theslewrateshouldbesettothehighestpossib lewhilemaintainingstabilitythroughthepow erandfrequencyrangesofoperation. Thefeedbackcapacitor,Ci, 'svalueshouldbesetusingthesamerecommenda tionsgiventoselectingtheinputcapacitor's value. Theinputnoisecapacitor,CN, is typically15pF- MOSFETO utputStageSNAA045A May2007 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2007 2013, SupplybypassingandPCBdesigneachhavea ,filmcapacitorbypassingis ;a basic, ,ClassABorClassB,thelevelofbiasvoltageis non-compensateddesigninvolvesa simpleresistor(orpotentiometerforeasybia sadjustment) ,RB2, is usedtosetaminimumbiasvoltagewhilethepote ntiometeris ,suchastheparallelcombinationofa highvalue( F)electrolyticwitha smallvalue(pF) ,theveryusefulVBEmultiplieris VBEmultiplierworksbyusingthevoltageacros stheBase-Emitterjunctionofa BJTtransistor,Q1, parallelwiththeVBEvoltageofQ1.

5 UsingOhm'sLaw,thecurrentthroughRB2andRPO T willbe:I = VBE/ (RB2+ RPOT) (A)(1)Ignoringthesmallamountofcurrenttha tflowsintothebaseofQ1, 'sLawagain,thevoltageacrossRB1is equalto:VB1= RB1* I (V)(2)Thetotalbiasvoltageis equaltotheCollectortoEmittervoltage,VCE, ofQ1, whichis alsothesameasthevoltageacrossRB1, RPOT, andRB2. CombiningthevoltageacrossRB2andRPOT, whichis thesameastheVBEvoltageofQ1, andthevoltagefoundaboveacrossRB1resultsi n:VBIAS= VCE= RB1* I + VBE(V)(3)Substitutingforthecurrent,I, resultsin:VBIAS= RB1* [VBE/(RB2+ RPOT)] + VBE(V)(4)7 SNAA045A May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013, thewellknownVBEmultiplierequation:VBIAS= VBE* [1+ RB1/(RB2+ RPOT)] (V)(5)Ascanbeseen,thebiasvoltagewilltrac ktheVBEvoltageofQ1ata givenbiasvoltage,theoutputstage' ,sinceQ1is mountedalongsidetheoutputdevices,its(Fig ure4) 's base,whichresultsin a biasvoltagethatchangesin ordertomaintaina stablebiascurrentin ClassABamplifierdesign,biascurrentis chosensuchthatcrossoverdistortionis Figure3, differentbiascurrentlevelsareshownin theFFTversusFrequencygraphsandoscillosco peviews(Figure5 toFigure10).

6 Foreachgraph,theoutputpowerlevelis 40 Wintoan8 setequaltothevoltagefor40 Winto8 .Thefirstgraph,Figure5, hasa biascurrentof50mAandshowsa THDthatis theFFTofFigure7 andtheoscilloscopeviewin andFigure10showtheharmoniccontentwhenthe biasis MOSFETO utputStageSNAA045A May2007 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2007 2013, , Figure12, andFigure13showthedifferencein harmonicswithbiaslevelsof100mA,200mA,and 300mA,respectively,usingtheMagnatecBUZ90 1 May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013, , Table3, andTable4 listtheresulting1kHzTHD+ N measurementat40 Wintoan8 THD+ N ata +N1kHzTHD+Nat40W/8 BiasCurrentSingleChannel, +N1kHzTHD+Nat40W/8 BiasCurrentSingleChannel, MOSFETO utputStageSNAA045A May2007 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2007 2013, +N1kHzTHD+Nat40W/8 BiasCurrentSingleChannel, Table2, Table3, andTable4 indicatethatthea rangeof100mAto200mAofbiascurrentin (A20k resistormaybeusedwiththebiasvoltagereduc edto7V).

7 Thebiascurrentis ,theDrainCurrentversesGate- Sourcevoltage( ) graphin wherestablebiascurrentcanbeachievedata , (< ).TheToshiba2SK1530/2SJ201pairandInternationalRectifierIRFP240/IRFP9240pairgraphshaveanintersectionofthethreeplotsata veryhighdraincurrent(>5A).Infact, is determinedthattheRenesasandMagnatecpairs canbebiasedin biaslevelof150mAin theoutputstagewillbeusedforthe2SK1530 properlybiasedsincetheintersectionpointo ntheID- VGSis May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013, maybedesirabletohavehigherbiascurrenttor educedistortionharmonics,improveTHDperfo rmance,affectsonicqualities, 'sID- ,theamountofvariationmaybeacceptableata ,theRenesas2SK1058 thebiasis biasstabilityis resistorbias(fixedbiasvoltage) notinvestigatedin turnedoff(Time= 0) , , theRenesas2SK1058/2SJ162pairoperatingata 'sID-VGSgrapheachbiascurrentsettingis biascurrentof115mAwillbeusedfortheRenesa s2SK1058 Figure17andFigure18.

8 Becausetheresolutionofthedevice'sID-VGSg raphis limitedtheintersectionpointis slopesignonthetimecurvesin Figure17andFigure18, biascurrentof180mAwillbeusedfortheMagnat ecBUZ901 MOSFETO utputStageSNAA045A May2007 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2007 2013, targetbiascurrentof150mAis chosenfortheToshiba2SK1530 hasmorevariationin biasvoltagewithtemperaturethanneededfort he2SK1530 modifiedasshownin addedtoseta temperatureindependentvoltagewhilethevol tagecreatedbytherestofthecircuitwillvary withtemperature(SeeEquation5). Thecorrectamountofvariationin biasvoltagetomaintaina targetbiascurrentof150mAoveroperatingtem peratureis foundbyusingtheresistorbiascircuitshowni n Figure3. Thebiasvoltageis measuredwith150mAofbiascurrentatinitialp oweronwhenthecasetemperatureofthedevices is C andtheamountofvoltagechangeneededacrosst hetemperaturerange, 1k , RB2= 500 , RB3=390 . biasvoltageusingtheLM4702,thebiasvoltage is lowenoughthatatthehighestoperatingcasete mperaturethebiascurrentis nomorethan100mAin distortionis reducedastheamplifierheatsupduetotheincr easein biascurrentin May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013,TexasInstrumentsIncorporated BF3@$=1:2 %+55 4)#6'; :*V; a ,RG,complementaryMOSFET devices,anda snubbercircuit,RSNandCSN.

9 Theoutputstageis showninFigure21. Onlythegateresistorandbiasvoltagelevelwi llbevariedtodetermineeffectonperformance foreachofthedifferentdeviceslistedin Table1. Thedesignis a designwithmultiplepairsofcomplementaryde vicesin parallel, ,RGNandRGP, arenecessaryforstability, ,thefrequencypolelocationofthelow-passfi ltercreatedbythegateresistorandthedevice inputcapacitance,CISS, is foundusingtheformula:(6)Theexactpoleloca tionis tooverdrivetheamplifierwitha thevalueis toohighthena "bump"appearsin thevalueis a 2 VRMS, MOSFETO utputStageSNAA045A May2007 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2007 2013,TexasInstrumentsIncorporatedBF3@$=1 :2 450 %50; :*V; Figure24andFigure25wherethegateresistorv aluesaresetcorrectlywiththosefoundin Figure22andFigure23. , thevalueofRGis toolow(especiallyfortheN-channelFET) composedofRSNandCSN. F is chosenforCSNandthenastartingvalueof10 is usedforRSN. Thepolelocationcreatedbythecircuitis foundusing:(7)If small,highfrequencyoscillationis thecompleteMOSFET amplifiercircuitshownin Figure26(onlyonechannelforsimplicity).

10 Theotherchannelis identicalin ortheVBEmultiplierofFigure19willbeusedde pendingontheoutputdevicesfromTable1. May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013, MOSFETO utputStageSNAA045A May2007 RevisedMay2013 SubmitDocumentationFeedbackCopyright 2007 2013, +NPerformance9 THD+NPerformanceWiththebiaslevelsandcirc uitsnowdetermined,THD+ N ,bothchannelsoftheLM4702wereonandin benefitoftheseRenesasdevicesis - 210 ontheN-channelFETand240 - 330 ' dBrelativetothefundamental,orin otherwords,thefundamentalpeakis notchedoutbythemeasurementequipmentreduc ingit' , Figure28, andFigure29showthedistortionlevelsoftheh armonicsat1W,40W,and100 Woutputpowerlevels,respectively,witha (Reading) (Reading)POUT= 1W/Channel,RL= 8 POUT= 40W/Channel,RL= 8 2SK1058/2SJ1622SK1058 (Reading)POUT= 100W/Channel,RL= 8 2SK1058/2SJ162 Figure30andFigure31showthathighperforman ceis May2007 RevisedMay2013AN-1645LM4702 Drivinga MOSFETO utputStageSubmitDocumentationFeedbackCop yright 2007 2013,TexasInstrumentsIncorporatedTHD+ THD+ N is noise(+N)andnotharmonics(THD).