LM4670FilterlessHighEfficiency3WSwitchingAudioAmplifierJuly2006
LM4670
FilterlessHighEfficiency3WSwitchingAudioAmplifier
GeneralDescription
TheLM4670isafullyintegratedsingle-supplyhighefficiencyswitchingaudioamplifier.ItfeaturesaninnovativemodulatorthateliminatestheLCoutputfilterusedwithtypicalswitchingamplifiers.Eliminatingtheoutputfilterreducesexternalcom-ponentcount,simplifiescircuitdesign,andreducesboardarea.TheLM4670processesanaloginputswithadelta-sigmamodulationtechniquethatlowersoutputnoiseandTHDwhencomparedtoconventionalpulsewidthmodula-tors.
TheLM4670isdesignedtomeetthedemandsofmobilephonesandotherportablecommunicationdevices.Operat-ingonasingle5Vsupply,itiscapableofdrivinga4Ωspeakerloadatacontinuousaverageoutputof2.3Wwithlessthan1%THD+N.Itsflexiblepowersupplyrequirementsallowoperationfrom2.4Vto5.5V.
TheLM4670hashighefficiencywithspeakerloadscom-paredtoatypicalClassABamplifier.Witha3.6Vsupplydrivingan8Ωspeaker,theIC’sefficiencyfora100mWpowerlevelis77%,reaching88%at600mWoutputpower.TheLM4670featuresalow-powerconsumptionshutdownmode.ShutdownmaybeenabledbydrivingtheShutdownpintoalogiclow(GND).
ThegainoftheLM4670isexternallyconfigurablewhichallowsindependentgaincontrolfrommultiplesourcesbysummingthesignals.
KeySpecifications
jEfficiencyat3.6V,100mWinto8Ωspeaker77%(typ)jEfficiencyat3.6V,600mWinto8Ωspeaker88%(typ)jEfficiencyat5V,1Winto8ΩspeakerjQuiescentcurrent,3.6VsupplyjTotalshutdownpowersupplycurrentjSinglesupplyrange
87%(typ)4.8mA(typ)0.01µA(typ)2.4Vto5.5V
Features
nnnnnnnn
NooutputfilterrequiredforinductiveloadsExternallyconfigurablegain
Veryfastturnontime:1.35ms(typ)Minimumexternalcomponents
\"Clickandpop\"suppressioncircuitryMicro-powershutdownmodeShortcircuitprotection
Availableinspace-savingmicroSMDandLLPpackages
Applications
nMobilephonesnPDAs
nPortableelectronicdevices
TypicalApplication
20089901
FIGURE1.TypicalAudioAmplifierApplicationCircuit
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LM4670ConnectionDiagrams
9BumpmicroSMDPackage
microSMDMarking
200899C6
TopViewX—DateCodeT—DieTraceabilityG—BoomerFamilyE6–LM4670ITL
20089936
TopView
OrderNumberLM4670ITL,LM4670ITLXSeeNSPackageNumberTLA09ZZALeadlessLeadframePackage(LLP)
LLPMarking
20089949
TopView
OrderNumberLM4670SD
SeeNSPackageNumberSDA08AContactNSCSalesOfficeforAvailability
20089951
TopViewZ—PlantCodeXY—DateCodeTT—DieTraceabilityL4670—LM4670
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LM4670AbsoluteMaximumRatings(Notes1,2)
IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.SupplyVoltage(Note1)StorageTemperatureVoltageatAnyInputPinPowerDissipation(Note3)ESDSusceptibility(Note4)ESDSusceptibility(Note5)JunctionTemperature(TJMAX)ThermalResistance
6.0V
−65˚Cto+150˚C
VDD+0.3V≥V≥GND-0.3V
InternallyLimited
2.0kV200V150˚C
θJA(microSMD)θJA(LLP)
SolderingInformation
220˚C/W73˚C/W
SeeAN-1112\"microSMDWafersLevelChipScalePackage.\"
OperatingRatings(Note1)(Note2)
TemperatureRangeTMIN≤TA≤TMAXSupplyVoltage(Note12)
−40˚C≤TA≤85˚C2.4V≤VDD≤5.5V
(Notes1,2)
ThefollowingspecificationsapplyforAV=2V/V(RI=150kΩ),RL=15µH+8Ω+15µHunlessotherwisespecified.Limitsap-plyforTA=25˚C.
LM4670
Symbol
Parameter
Conditions
VI=0V,AV=2V/V,VDD=2.4Vto5.0VVDD=2.4Vto5.0V,InputReferred
VDD=2.4Vto5.0VVIC=VDD/2to0.5V,
VIC=VDD/2toVDD–0.8V,InputReferredVDD=5.0V,VI=5.8VVDD=5.0V,VI=–0.3VVIN=0V,NoLoad,VDD=5.0V
IDDQuiescentPowerSupplyCurrent
VIN=0V,NoLoad,VDD=3.6VVIN=0V,NoLoad,VDD=2.4V
ISDVSDIHVSDILROSDAVRSDShutdownCurrent(Note9)ShutdownvoltageinputhighShutdownvoltageinputlowOutputImpedanceGain
ResistancefromShutdownPintoGND
RL=15µH+4Ω+15µH,THD=10%(max)f=1kHz,22kHzBWVDD=5VVDD=3.6VVDD=2.5V
RL=15µH+4Ω+15µH,THD+N=1%(max)f=1kHz,22kHzBWVDD=5V,VDD=3.6V,VDD=2.5V,VSHUTDOWN=0.4VVSHUTDOWN=0VVDD=2.4Vto5.0V
64Typical(Note6)
|VOS|
DifferentialOutputOffsetVoltage
Limit(Notes7,8)
25
Units(Limits)mV(max)
dB
ElectricalCharacteristics
PSRRGSMGSMPowerSupplyRejectionRatio
CMRRGSMGSMCommonModeRejectionRatio
LogicHighInputCurrentLogicLowInputCurrent
80dB
|IIH||IIL|
2017.04.83.80.011.00.8
100510511.40.4270kΩ/RI330kΩ/RIµA(max)µA(max)mA(max)mAmA(max)µA(max)V(min)V(max)kΩV/V(min)V/V(max)
kΩ
>100
300kΩ/RI300
POOutputPower(Notes11,12)
3.01.5675WWmW
2.31.2550WWmW
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LM4670ElectricalCharacteristics(Notes1,2)
ThefollowingspecificationsapplyforAV=2V/V(RI=150kΩ),RL=15µH+8Ω+15µHunlessotherwisespecified.LimitsapplyforTA=25˚C.(Continued)
LM4670
Symbol
Parameter
Conditions
RL=15µH+8Ω+15µH,THD=10%(max)f=1kHz,22kHzBWVDD=5VVDD=3.6VVDD=2.5V
RL=15µH+8Ω+15µH,THD+N=1%(max)f=1kHz,22kHzBWVDD=5V,VDD=3.6V,VDD=2.5V,
VDD=5V,PO=1WRMS,f=1kHz
THD+N
TotalHarmonicDistortion+Noise
VDD=3.6V,PO=0.5WRMS,f=1kHz
VDD=3.6V,PO=0.5WRMS,f=5kHz
VDD=3.6V,PO=0.5WRMS,f=10kHz
VDD=3.6V,
VRipple=200mVPPSine,fRipple=217Hz
InputstoACGND,CI=0.1µ,InputReferred
VDD=3.6V,
VRipple=200mVPPSine,fRipple=1kHz
InputstoACGND,CI=0.1µFInputReferred
VDD=3.6V,
VRipple=200mVPPSine,fRipple=217Hz
fIN=1kHz,PO=10mWRMSInputReferred
SNR
SignaltoNoiseRatio
VDD=5V,PO=1WRMSVDD=3.6V,f=20Hz–20kHzInputstoACGND,CI=0.1µFNoWeighting,InputReferredVDD=3.6V,InputstoACGNDCI=0.1µF,AWeightedInputReferred
CMRRTWUTSDCommonModeRejectionRatioWake-upTimeShutdownTime
VDD=3.6V,VRipple=1VPPSinefRipple=217Hz,InputReferredVDD=3.6VVDD=3.6V
Typical(Note6)
Limit(Notes7,8)
Units(Limits)
POOutputPower(Note11)
1.65850400WmWmW
1.356803250.350.300.300.30
600
WmW(min)mW
%%%%
68dB
65dB
PSRRPowerSupplyRejectionRatio
62dB
9385
dBµVRMSeOUTOutputNoise
65
µVRMS801.350.01
dBmsms
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LM4670Note1:Allvoltagesaremeasuredwithrespecttothegroundpin,unlessotherwisespecified.
Note2:AbsoluteMaximumRatingsindicatelimitsbeyondwhichdamagetothedevicemayoccur.OperatingRatingsindicateconditionsforwhichthedeviceisfunctional,butdonotguaranteespecificperformancelimits.ElectricalCharacteristicsstateDCandACelectricalspecificationsunderparticulartestconditionswhichguaranteespecificperformancelimits.ThisassumesthatthedeviceiswithintheOperatingRatings.Specificationsarenotguaranteedforparameterswherenolimitisgiven,however,thetypicalvalueisagoodindicationofdeviceperformance.
Note3:ThemaximumpowerdissipationmustbederatedatelevatedtemperaturesandisdictatedbyTJMAX,θJA,andtheambienttemperatureTA.ThemaximumallowablepowerdissipationisPDMAX=(TJMAX–TA)/θJAorthenumbergiveninAbsoluteMaximumRatings,whicheverislower.FortheLM4670,TJMAX=150˚C.ThetypicalθJAis220˚C/WforthemicroSMDpackageand64˚C/WfortheLLPpackage.Note4:Humanbodymodel,100pFdischargedthrougha1.5kΩresistor.Note5:MachineModel,220pF–240pFdischargedthroughallpins.
Note6:Typicalspecificationsarespecifiedat25˚Candrepresenttheparametricnorm.Note7:TestedlimitsareguaranteedtoNational’sAOQL(AverageOutgoingQualityLevel).
Note8:Datasheetmin/maxspecificationlimitsareguaranteedbydesign,test,orstatisticalanalysis.
Note9:Shutdowncurrentismeasuredinanormalroomenvironment.ExposuretodirectsunlightwillincreaseISDbyamaximumof2µA.TheShutdownpinshouldbedrivenascloseaspossibletoGNDforminimalshutdowncurrent.SeetheApplicationInformationsectionunderSHUTDOWNFUNCTIONformoreinformation.Note10:TheperformancegraphsweretakenusingtheAudioPrecisionAUX-0025SwitchingAmplifierMeasurementFilterinserieswiththeLCfilterontheboard.Note11:TypicaloutputpowernumbersarefortheLM4670intheITL(µSMD)package.IntheLLP(SDA)package,theoutputpowerwillbelowerduetohigherresistanceseenfromtheICoutputpadtoPCBtrace.Thedifferenceincreaseswithlowerimpedanceloads.
Note12:ThemaximumoperatingvoltagefortheLM4670intheSDA(LLP)packagewhendriving4Ωloadstogreaterthan10%THD+Nis5.0V.
ExternalComponentsDescription
(Figure1)Components1.2.
CSRIFunctionalDescription
Supplybypasscapacitorwhichprovidespowersupplyfiltering.RefertothePowerSupplyBypassingsectionforinformationconcerningproperplacementandselectionofthesupplybypasscapacitor.Gainsettingresistor.DifferentialgainissetbytheequationAV=2*150kΩ/Ri(V/V).
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LM4670TypicalPerformanceCharacteristics
THD+NvsFrequency
VDD=2.5V,RL=15µH+4Ω+15µH
POUT=375mW,22kHzBW
(Note10)
THD+NvsFrequency
VDD=3.6V,RL=15µH+4Ω+15µH
POUT=750mW,22kHzBW
2008994120089943
VDDTHD+NvsFrequency
=5V,RL=15µH+4Ω+15µHPOUT=1.5W,22kHzBW
VDDTHD+NvsFrequency
=2.5V,RL=15µH+8Ω+15µHPOUT=200mW,22kHzBW
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VDDTHD+NvsFrequency
=3.6V,RL=15µH+8Ω+15µHPOUT=500mW,22kHzBW
VDDTHD+NvsFrequency
=5V,RL=15µH+8Ω+15µHPOUT=1W,22kHzBW
2008994420089946
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LM4670TypicalPerformanceCharacteristics
THD+NvsOutputPowerRL=15µH+4Ω+15µHf=1kHz,22kHzBW
(Note10)(Continued)
THD+NvsOutputPowerRL=15µH+8Ω+15µHf=1kHz,22kHzBW
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CMRRvsFrequency
VDD=3.6V,RL=15µH+8Ω+15µHVCM=1VP-PSineWave,22kHzBWPSRRvsFrequency
VDD=3.6V,RL=15µH+8Ω+15µHVCM=200mVP-PSineWave,22kHzBW
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EfficiencyandPowerDissipation
vsOutputPower
RL=15µH+4Ω+15µH,f=1kHz,THD<2%EfficiencyandPowerDissipation
vsOutputPower
RL=15µH+8Ω+15µH,f=1kHz,THD<1%
2008991120089912
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LM4670TypicalPerformanceCharacteristics
OutputPowervsSupplyVoltage
RL=15µH+4Ω+15µH,f=1kHz,22kHzBW
(Note10)(Continued)
OutputPowervsSupplyVoltage
RL=15µH+8Ω+15µH,f=1kHz,22kHzBW
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SupplyCurrent(RMS)vsOutputPower
RL=15µH+4Ω+15µH,f=1kHzSupplyCurrent(RMS)vsOutputPower
RL=15µH+8Ω+15µH,f=1kHz
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ShutdwonThresholdRL=15µH+8Ω+15µHShutdwonThresholdvsSupplyVoltage
RL=15µH+8Ω+15µH
20089918200899H5
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LM4670TypicalPerformanceCharacteristics
SupplyCurrentvsShutdownVoltage
RL=15µH+8Ω+15µH
(Note10)(Continued)
SupplyCurrentvsSupplyVoltage
RL=15µH+8Ω+15µH
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SupplyCurrentvsSupplyVoltage
RL=DifferentµHloadsDifferentialGainvsSupplyVoltage
RL=15µH+8Ω+15µH,Ri=150kΩ,f=1kHz
2008993920089913
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LM4670ApplicationInformation
GENERALAMPLIFIERFUNCTION
TheoutputsignalsgeneratedbytheLM4670consistoftwo,BTLconnected,outputsignalsthatpulsemomentarilyfromneargroundpotentialtoVDD.ThetwooutputscanpulseindependentlywiththeexceptionthattheybothmayneverpulsesimultaneouslyasthiswouldresultinzerovoltsacrosstheBTLload.Theminimumwidthofeachpulseisapproxi-mately350ns.However,pulsesonthesameoutputcanoccursequentially,inwhichcasetheyareconcatenatedandappearasasinglewiderpulsetoachieveaneffective100%dutycycle.Thisresultsinmaximumaudiooutputpowerforagivensupplyvoltageandloadimpedance.TheLM4670canachievemuchhigherefficienciesthanclassABamplifierswhilemaintainingacceptableTHDperformance.
Theshort(350ns)drivepulsesemittedattheLM4670out-putsmeansthatgoodefficiencycanbeobtainedwithmini-malloadinductance.Thetypicaltransducerloadonanaudioamplifierisquitereactive(inductive).Forthisreason,theloadcanactasit’sownfilter,sotospeak.This\"filter-less\"switchingamplifier/transducerloadcombinationismuchmoreattractiveeconomicallyduetosavingsinboardspaceandexternalcomponentcostbyeliminatingtheneedforafilter.
singleinputamplifiers.Thecommon-moderejectioncharac-teristicofthedifferentialamplifierreducessensitivitytogroundoffsetrelatednoiseinjection,especiallyimportantinhighnoiseapplications.
POWERDISSIPATIONANDEFFICIENCY
Ingeneralterms,efficiencyisconsideredtobetheratioofusefulworkoutputdividedbythetotalenergyrequiredtoproduceitwiththedifferencebeingthepowerdissipated,typically,intheIC.Thekeyhereis“useful”work.Foraudiosystems,theenergydeliveredintheaudiblebandsiscon-sideredusefulincludingthedistortionproductsoftheinputsignal.Sub-sonic(DC)andsuper-soniccomponents(>22kHz)arenotuseful.ThedifferencebetweenthepowerflowingfromthepowersupplyandtheaudiobandpowerbeingtransducedisdissipatedintheLM4670andinthetransducerload.TheamountofpowerdissipationintheLM4670isverylow.ThisisbecausetheONresistanceoftheswitchesusedtoformtheoutputwaveformsistypicallylessthan0.25Ω.Thisleavesonlythetransducerloadasapo-tential\"sink\"forthesmallexcessofinputpoweroveraudiobandoutputpower.TheLM4670dissipatesonlyafractionoftheexcesspowerrequiringnoadditionalPCBareaorcop-perplanetoactasaheatsink.
PCBLAYOUTCONSIDERATIONS
Asoutputpowerincreases,interconnectresistance(PCBtracesandwires)betweentheamplifier,loadandpowersupplycreateavoltagedrop.ThevoltagelossonthetracesbetweentheLM4670andtheloadresultsisloweroutputpoweranddecreasedefficiency.HighertraceresistancebetweenthesupplyandtheLM4670hasthesameeffectasapoorlyregulatedsupply,increaserippleonthesupplylinealsoreducingthepeakoutputpower.Theeffectsofresidualtraceresistanceincreasesasoutputcurrentincreasesduetohigheroutputpower,decreasedloadimpedanceorboth.Tomaintainthehighestoutputvoltageswingandcorre-spondingpeakoutputpower,thePCBtracesthatconnecttheoutputpinstotheloadandthesupplypinstothepowersupplyshouldbeaswideaspossibletominimizetraceresistance.
TheuseofpowerandgroundplaneswillgivethebestTHD+Nperformance.Whilereducingtraceresistance,theuseofpowerplanesalsocreatesparasitecapacitorsthathelptofilterthepowersupplyline.
Therisingandfallingedgesarenecessarilyshortinrelationtotheminimumpulsewidth(350ns),havingapproximately16nsriseandfalltimes,typical,dependingonparasiticoutputcapacitance.Theinductivenatureofthetransducerloadcanalsoresultinovershootononeorbothedges,clampedbytheparasiticdiodestoGNDandVDDineachcase.FromanEMIstandpoint,thisisanaggressivewave-formthatcanradiateorconducttoothercomponentsinthesystemandcauseinterference.Itisessentialtokeepthepowerandoutputtracesshortandwellshieldedifpossible.Useofgroundplanes,beads,andmicro-striplayouttech-niquesareallusefulinpreventingunwantedinterference.AsthedistancefromtheLM4670andthespeakerincrease,theamountofEMIradiationwillincreasesincetheoutputwiresortracesactingasantennabecomemoreefficientwithlength.WhatisacceptableEMIishighlyapplicationspecific.FerritechipinductorsplacedclosetotheLM4670maybeneededtoreduceEMIradiation.Thevalueoftheferritechipisveryapplicationspecific.
POWERSUPPLYBYPASSING
Aswithanypoweramplifier,propersupplybypassingiscriticalforlownoiseperformanceandhighpowersupplyrejectionratio(PSRR).Thecapacitor(CS)locationshouldbeascloseaspossibletotheLM4670.Typicalapplicationsemployavoltageregulatorwitha10µFanda0.1µFbypasscapacitorsthatincreasesupplystability.ThesecapacitorsdonoteliminatetheneedforbypassingonthesupplypinoftheLM4670.A1µFtantalumcapacitorisrecommended.SHUTDOWNFUNCTION
Inordertoreducepowerconsumptionwhilenotinuse,theLM4670containsshutdowncircuitrythatreducescurrentdrawtolessthan0.01µA.ThetriggerpointforshutdownisshownasatypicalvalueintheElectricalCharacteristicsTablesandintheShutdownHysteresisVoltagegraphsfoundintheTypicalPerformanceCharacteristicssection.Itisbesttoswitchbetweengroundandsupplyforminimum
10
DIFFERENTIALAMPLIFIEREXPLANATION
Aslogicsupplyvoltagescontinuetoshrink,designersareincreasinglyturningtodifferentialanalogsignalhandlingtopreservesignaltonoiseratioswithrestrictedvoltageswing.TheLM4670isafullydifferentialamplifierthatfeaturesdifferentialinputandoutputstages.Adifferentialamplifieramplifiesthedifferencebetweenthetwoinputsignals.Tra-ditionalaudiopoweramplifiershavetypicallyofferedonlysingle-endedinputsresultingina6dBreductioninsignaltonoiseratiorelativetodifferentialinputs.TheLM4670alsooffersthepossibilityofDCinputcouplingwhicheliminatesthetwoexternalACcoupling,DCblockingcapacitors.TheLM4670canbeused,however,asasingleendedinputamplifierwhilestillretainingit’sfullydifferentialbenefits.Infact,completelyunrelatedsignalsmaybeplacedontheinputpins.TheLM4670simplyamplifiesthedifferencebe-tweenthesignals.Amajorbenefitofadifferentialamplifieristheimprovedcommonmoderejectionratio(CMRR)over
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LM4670ApplicationInformation
(Continued)
currentusagewhileintheshutdownstate.WhiletheLM4670maybedisabledwithshutdownvoltagesinbetweengroundandsupply,theidlecurrentwillbegreaterthanthetypical0.01µAvalue.IncreasedTHDmayalsobeobservedwithvoltageslessthanVDDontheShutdownpinwheninPLAYmode.
TheLM4670hasaninternalresistorconnectedbetweenGNDandShutdownpins.ThepurposeofthisresistoristoeliminateanyunwantedstatechangeswhentheShutdownpinisfloating.TheLM4670willentertheshutdownstatewhentheShutdownpinisleftfloatingorifnotfloating,whentheshutdownvoltagehascrossedthethreshold.Tomini-mizethesupplycurrentwhileintheshutdownstate,theShutdownpinshouldbedriventoGNDorleftfloating.IftheShutdownpinisnotdriventoGND,theamountofadditionalresistorcurrentduetotheinternalshutdownresistorcanbefoundbyEquation(1)below.
(VSD-GND)/300kΩ
(1)
capacitorcoupledtoground(SeeFigures5–7).Settingthehigh-passfilterpointabovethepowersupplynoisefrequen-cies,217HzinaGSMphone,forexample,willfilteroutthisnoisesoitisnotamplifiedandheardontheoutput.Capaci-torswithatoleranceof10%orbetterarerecommendedforimpedancematching.
DIFFERENTIALCIRCUITCONFIGURATIONS
TheLM4670canbeusedinmanydifferentcircuitconfigu-rations.ThesimplestandbestperformingistheDCcoupled,differentialinputconfigurationshowninFigure2.Equation(2)aboveisusedtodeterminethevalueoftheRiresistorsforadesiredgain.
InputcapacitorscanbeusedinadifferentialconfigurationasshowninFigure3.Equation(3)aboveisusedtodeterminethevalueoftheCicapacitorsforadesiredfrequencyre-sponseduetothehigh-passfiltercreatedbyCiandRi.Equation(2)aboveisusedtodeterminethevalueoftheRiresistorsforadesiredgain
TheLM4670canbeusedtoamplifymorethanoneaudiosource.Figure4showsadualdifferentialinputconfiguration.Thegainforeachinputcanbeindependentlysetformaxi-mumdesignflexibilityusingtheRiresistorsforeachinputandEquation(2).InputcapacitorscanbeusedwithoneormoresourcesaswelltohavedifferentfrequencyresponsesdependingonthesourceorifaDCvoltageneedstobeblockedfromasource.
SINGLE-ENDEDCIRCUITCONFIGURATIONS
TheLM4670canalsobeusedwithsingle-endedsourcesbutinputcapacitorswillbeneededtoblockanyDCattheinputterminals.Figure5showsthetypicalsingle-endedapplica-tionconfiguration.TheequationsforGain,Equation(2),andfrequencyresponse,Equation(3),holdforthesingle-endedconfigurationasshowninFigure5.
Whenusingmorethanonesingle-endedsourceasshowninFigure6,theimpedanceseenfromeachinputterminalshouldbeequal.TofindthecorrectvaluesforCi3andRi3connectedtothe+INinputpintheequivalentimpedanceofallthesingle-endedsourcesarecalculated.Thesingle-endedsourcesareinparalleltoeachother.Theequivalentcapacitorandresistor,Ci3andRi3,arefoundbycalculatingtheparallelcombinationofallCivaluesandthenallRival-ues.Equations(4)and(5)belowareforanynumberofsingle-endedsources.
Ci3=Ci1+Ci2+Cin...(F)Ri3=1/(1/Ri1+1/Ri2+1/Rin...)(Ω)
(4)(5)
Withonlya0.5Vdifference,anadditional1.7µAofcurrentwillbedrawnwhileintheshutdownstate.
PROPERSELECTIONOFEXTERNALCOMPONENTSThegainoftheLM4670issetbytheexternalresistors,RiinFigure1,TheGainisgivenbyEquation(2)below.BestTHD+Nperformanceisachievedwithagainof2V/V(6dB).
AV=2*150kΩ/Ri(V/V)
(2)
Itisrecommendedthatresistorswith1%toleranceorbetterbeusedtosetthegainoftheLM4670.TheRiresistorsshouldbeplacedclosetotheinputpinsoftheLM4670.KeepingtheinputtracesclosetoeachotherandofthesamelengthinahighnoiseenvironmentwillaidinnoiserejectionduetothegoodCMRRoftheLM4670.NoisecoupledontoinputtraceswhicharephysicallyclosetoeachotherwillbecommonmodeandeasilyrejectedbytheLM4670.
Inputcapacitorsmaybeneededforsomeapplicationsorwhenthesourceissingle-ended(seeFigures3,5).InputcapacitorsareneededtoblockanyDCvoltageatthesourcesothattheDCvoltageseenbetweentheinputterminalsoftheLM4670is0V.Inputcapacitorscreateahigh-passfilterwiththeinputresistors,Ri.The–3dBpointofthehigh-passfilterisfoundusingEquation(3)below.
fC=1/(2πRiCi)(Hz)
(3)
Theinputcapacitorsmayalsobeusedtoremovelowaudiofrequencies.Smallspeakerscannotreproducelowbassfrequenciessofilteringmaybedesired.WhentheLM4670isusingasingle-endedsource,powersupplynoiseonthegroundisseenasaninputsignalbythe+INinputpinthatis
TheLM4670mayalsouseacombinationofsingle-endedanddifferentialsources.Atypicalapplicationwithonesingle-endedsourceandonedifferentialsourceisshowninFigure7.Usingtheprincipleofsuperposition,theexternalcompo-nentvaluescanbedeterminedwiththeaboveequationscorrespondingtotheconfiguration.
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LM4670ApplicationInformation
(Continued)
20089903
FIGURE2.Differentialinputconfiguration
20089904
FIGURE3.Differentialinputconfigurationwithinputcapacitors
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LM4670ApplicationInformation
(Continued)
20089905
FIGURE4.Dualdifferentialinputconfiguration
20089906
FIGURE5.Single-endedinputconfiguration
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LM4670ApplicationInformation
(Continued)
20089907
FIGURE6.Dualsingle-endedinputconfiguration
20089908
FIGURE7.Dualinputwithasingle-endedinputandadifferentialinput
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LM4670ApplicationInformation
(Continued)
REFERENCEDESIGNBOARDSCHEMATIC
20089909
FIGURE8.
Inadditiontotheminimalpartsrequiredfortheapplicationcircuit,ameasurementfilterisprovidedontheevaluationcircuitboardsothatconventionalaudiomeasurementscanbeconvenientlymadewithoutadditionalequipment.Thisisabalancedinput,groundeddifferentialoutputlowpassfilterwitha3dBfrequencyofapproximately35kHzandanonboardterminationresistorof300Ω(seeschematic).Notethatthecapacitiveloadelementsarereturnedtoground.Thisisnotoptimalforcommonmoderejectionpurposes,butduetotheindependentpulseformatateachoutputthereisasignificantamountofhighfrequencycommonmodecom-ponentontheoutputs.Thegroundedcapacitivefilterele-mentsattenuatethiscomponentattheboardtoreducethehighfrequencyCMRRrequirementplacedontheanalysisinstruments.
Evenwiththegroundedfiltertheaudiosignalisstilldiffer-ential,necessitatingadifferentialinputonanyanalysisin-strumentconnectedtoit.MostlabinstrumentsthatfeatureBNCconnectorsontheirinputsareNOTdifferentialre-spondingbecausetheringoftheBNCisusuallygrounded.
ThecommonlyusedAudioPrecisionanalyzerisdifferential,butitsabilitytoaccuratelyrejectfastpulsesof350nswidthisquestionablenecessitatingtheonboardmeasurementfilter.Whenindoubtorwhenthesignalneedstobesingle-ended,useanaudiosignaltransformertoconvertthedifferentialoutputtoasingleendedoutput.Dependingontheaudiotransformer’scharacteristics,theremaybesomeattenua-tionoftheaudiosignalwhichneedstobetakenintoaccountforcorrectmeasurementofperformance.
Measurementsmadeattheoutputofthemeasurementfiltersufferattenuationrelativetotheprimary,unfilteredoutputsevenataudiofrequencies.Thisisduetotheresistanceoftheinductorsinteractingwiththeterminationresistor(300Ω)andistypicallyabout-0.25dB(3%).Inotherwords,thevoltagelevels(andcorrespondingpowerlevels)indicatedthroughthemeasurementfilterareslightlylowerthanthosethatactuallyoccurattheloadplacedontheunfilteredout-puts.Thissmalllossinthefilterformeasurementgivesaloweroutputpowerreadingthanwhatisreallyoccurringontheunfilteredoutputsanditsload.
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LM4670ApplicationInformation
LM4670microSMDBOARDARTWORK
CompositeView
(Continued)
SilkScreen
2008993220089935
TopLayerInternalLayer1,GND
2008993720089933
InternalLayer2,VDDBottomLayer
2008993420089931
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LM4670ApplicationInformation
LM4670SDABOARDARTWORK
CompositeView
(Continued)
SilkScreen
2008995320089956
TopLayerInternalLayer1,GND
2008995720089954
InternalLayer2,VDDBottomLayer
2008995520089952
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LM4670RevisionHistory
Rev1.01.11.2
Date12/15/047/06/057/13/06
Description
InitialWEBoftheD/S(TLpkg).
Re-releasedD/StotheWEB(addedtheSDpackage).EditedNote9,thenre-releasedD/StotheWEB.
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LM4670PhysicalDimensions
inches(millimeters)unlessotherwisenoted
9BumpmicroSMD
OrderNumberLM4670ITL,LM4670ITLX
NSPackageNumberTLA09ZZAX1=1.463X2=1.463X3=0.600
LLP
OrderNumberLM4670SDNSPackageNumberSDA08A
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LM4670FilterlessHighEfficiency3WSwitchingAudioAmplifierNotes
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