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)

2 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 .ThisuniqueICsolutionprovidesa quickandeasywaytomanufactureanultrahigh- fidelityamplifiersolution,withthepiece-o f-mindobtainedfroma fullyoptimized, availablein versionhasanoperatingvoltageupto 75V,whiletheB versionoperatesupto availablein a theAN-1490LM4702 PowerAmplifierApplicationReport(SNAA031) .

3 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( ).

4 Althoughanadditionaldriverstagecanbeempl oyedtoremovetheLM4702'sdrivecurrentlimit ation,thedesignin parallelthatcanbedrivenwitha parallelwillnotbecoveredin MOSFET outputstagecomparedtoa BJToutputstagewiththeLM4702is drivenfromhighersupplyrails,themaximumvo ltageswingwitha MOSFET outputstagemaybelowerthana a resultofthedifferencein VGScomparedtoVBE. 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)

5 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)

6 ,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).TheCicapacitorsetsDCgaintou nityandthelowfrequencyresponsebycreating a high-passfilterwitha -3dBpointatf-3dB= 1/(2 CiRi) (Hz).Forsonicquality,thedesigndoesnotuse anACcoupling, ,CC, : Setthegainin therangeof10V/Vto50 Usea ,theheatsinktemperatureshouldnotriseabov e55 C.

7 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.

8 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. 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.

9 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).

10 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).