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

PA95

PA95U

5

GENERAL

PleasereadApplicationNote1"GeneralOperatingConsiderations"whichcoversstability,supplies,heatsinking,

mounting,currentlimit,SOAinterpretation,andspecificationinterpretation.Visitwww.cirrus.comfordesigntools

thathelpautomatetaskssuchascalculationsforstability,internalpowerdissipation,currentlimit;heatsinkselec-

tion;Apex’scompleteApplicationNoteslibrary;TechnicalSeminarWorkbook;andEvaluationKits.

CURRENT LIMIT

Forproperoperation,thecurrentlimitresistor(R

gram.Theminimumvalueis3.5ohm,howeverforoptimumreliabilitytheresistorvalueshouldbesetashighas

possible.Thevalueiscalculatedasfollows;withthemaximumpracticalvalueof150ohms.

.7

R

I

LIM

COMMON MODE INPUT RANGE

Operationalamplifiersareusuallydesignedtohaveacommonmodeinputvoltagerangethatapproximatesthe

powersupplyvoltagerange.However,tokeepthecostaslowaspossibleandstillmeettherequirementsofmost

applicationsthecommonmodeinputvoltagerangeofthePA95isrestricted.Theinputpinsmustalwaysbealeast

30Vfromeithersupplyvoltagebutnevermorethan500V.ThismeansthatthePA95cannotbeusedinapplications

wherethesupplyvoltagesareextremelyunbalanced.Forexample,supplyvoltagesof+800Vand–100Vwouldnot

beallowedinanapplicationwherethenon-invertingpinisgroundedbecauseinnormaloperationbothinputpins

wouldbeat0Vandthedifferencevoltagebetweenthepositivesupplyandtheinputpinswouldbe800V.Inthis

kindofapplication,however,supplyvoltages+500Vand-100Vdoesmeettheinputcommonmodevoltagerange

requirementssincethemaximumdifferencevoltagebetweentheinputspinsandthesupplyvoltageis500V(the

maximumallowed).Theoutputhasnosuchrestrictionsonitsvoltageswing.Theoutputcanswingwithin24Vof

eithersupplyvoltageregardlessofvaluesolongasthetotalsupplyvoltagedoesnotexceed900V.

INPUT PROTECTION

AlthoughthePA95canwithstanddifferentialinputvoltagesupto

±20V,additionalexternalprotectionisrecommended.Inmostap-

plications1N4148or1N914signaldiodesaresufficient(D1,D2in

Figure1a).Inmoredemandingapplicationswherelowleakageor

lowcapacitanceareofconcern2N4416or2N5457-2N5459JFETs

connected as diodes will be required (Q1, Q2 in Figure 1b). In

eithercasetheinputdifferentialvoltagewillbeclampedto±.7V.

Thisissufficientoverdrivetoproducemaximumpowerbandwidth.

Notethatthisprotectiondoesnotautomaticallyprotecttheampli-

fierfromexcessivecommonmodeinputvoltages.

POWER SUPPLY PROTECTION

Unidirectionalzenerdiodetransientsuppressorsarerecommend-

edasprotectiononthesupplypins.Thezenersclamptransients

tovoltageswithinthepowersupplyratingandalsoclamppower

supplyreversalstoground.Whetherthezenersareusedornot,

thesystempowersupplyshouldbeevaluatedfortransientperfor-

manceincludingpower-onovershootandpower-offpolarityrever-

salaswellaslineregulation.

Conditionswhichcancauseopencircuitsorpolarityreversalson

eitherpowersupplyrailshouldbeavoidedorprotectedagainst.

Reversalsoropensonthenegativesupplyrailisknowntoinduceinputstagefailure.Unidirectionaltranszorbspre-

ventthis,anditisdesirablethattheybebothelectricallyandphysicallyasclosetotheamplifieraspossible.

FIGURE 1. OVERVOLTAGE PROTECTION