PA98 datasheet(6/7 Pages) CIRRUS

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PA98 • PA98A

PA98U

P r o d u c t I n n o v a t i o n F r o m

INPUT PROTECTION

AlthoughthePA98canwithstanddifferentialvoltagesupto±25V,additionalexternalprotectionisrecommended.

SincethePA98isahighspeedamplifier,lowleakage,lowcapacitanceJFETsconnectedasdiodesarerecom-

mended(e.g.2N4416,Q1-Q4inFigure1).Thedifferentialinputvoltagewillbeclampedto±1.4V.Thisissufficient

overdrivetoproducemaximumpowerbandwidth.

POWER SUPPLY PROTECTION

Unidirectionalzenerdiodetransientsuppressorsarerecommendedas

protectiononthesupplypins.Thezenersclamptransientstovoltages

withinthepowersupplyratingandalsoclamppowersupplyreversalsto

ground.Whetherthezenersareusedornot,thesystempowersupply

shouldbeevaluatedfortransientperformanceincludingpower-onover-

shootandpower-offpolarityreversalsaswellaslineregulation.

Conditionswhichcancauseopencircuitsorpolarityreversalsoneither

powersupplyrailshouldbeavoidedorprotectedagainst.Reversalsor

opensonthenegativesupplyrailisknowntoinduceinputstagefailure.

Unidirectional transzorbs prevent this, and it is desirable that they be

bothelectricallyandphysicallyasclosetotheamplifieraspossible.

STABILITY

ThePA98isexternallycompensatedandperformancecanbetailoredtotheapplication.Usethegraphsofsmall

signalresponseandpowerresponseasaguide.ThecompensationcapacitorC

Cmustberatedat500Vworking

voltage.AnNPOcapacitorisrecommended.ThecompensationnetworkC

CRCmustbemountedcloselytotheam-

plifierpins7and8toavoidspuriousoscillation.

INTERNAL POWER DISSIPATION AND HEATSINK SELECTION

WiththeuniquecombinationofhighvoltageandspeedofthePA98,traditionalformulasforheatsinkselectionwill

falselylowertheapparentpowerhandlingcapabilityofthisamplifier.Tomoreaccuratelypredictoperatingtempera-

turesusePowerDesign1 revision10orhigher,orusethefollowingprocedure:

Findinternaldissipation(PD)resultingfromdrivingtheload.UsePowerDesignorrefertoApexPrecisionPower

ApplicationsNote1,GeneralOperatingConsidertaions,paragraph7.Findtotalquiescentpower(PD

Q)bymultiply-

ing0.025AbyV

SS(totalsupplyvoltage).Findoutputstagequiescentpower(PDQOUT)bymultiplying0.001byVSS.

Calculateaheatsinkratingwhichwillmaintainthecaseat85°Corlower.

C - TA

PD + PD

ØSA =

-0.1°C/W

Where: T

C=maximumcasetemperatureallowed

A=maximumambienttemperatureencountered

Calculateaheatsinkratingwhichwillmaintainoutputtransistorjunctionsat150°Corlower.

J - TA - (PD + PDQOUT) * RØJC

PD + PD

ØSA =

-0.1°C/W

Where: T

J=maximumjunctiontemperatureallowed.

ØJC=ACorDCthermalresistancefromthespecificationtable.

Usethelargerheatsinkofthesetwocalculations.

PowerDesignisanExcelspreadsheetavailablefreefromwww.cirrus.com

+VS

–IN

+IN

11, 12

7, 8

PA98

–VS

FIGURE 1.

OVERVOLTAGE

PROTECTION

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