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

PA94

PA94U



GENERAL

PleasereadApplicationNote1"GeneralOperatingConsiderations"whichcoversstability,supplies,heatsinking,

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

thathelpautomatetaskssuchascalculationsforstability,internalpowerdissipation,currentlimit;heatsinkselec-

tion;Apex’scompleteApplicationNoteslibrary;TechnicalSeminarWorkbook;andEvaluationKits.

CURRENT LIMIT

For proper operation, the current limit resistor (R

nection diagram. The minimum value is 3.5 ohm, however for optimum reliability the resistor value

should be set as high as possible. The value is calculated as follows; with the maximum practical value

of30ohms.

.7

R

I

LIM

CoMMoN ModE INPUT RANGE

Operationalamplifiersareusuallydesignedtohaveacommonmodeinputvoltagerangethatapproximatesthe

powersupplyvoltagerange.However,tokeepthecostaslowaspossibleandstillmeettherequirementsofmost

applicationsthecommonmodeinputvoltagerangeofthePA94isrestricted.Theinputpinsmustalwaysbealeast

30Vfromeithersupplyvoltagebutnevermorethan450V.ThismeansthatthePA94cannotbeusedinapplications

wherethesupplyvoltagesareextremelyunbalanced.Forexample,supplyvoltagesof+800Vand–100Vwouldnot

beallowedinanapplicationwherethenon-invertingpinisgroundedbecauseinnormaloperationbothinputpins

wouldbeat0Vandthedifferencevoltagebetweenthepositivesupplyandtheinputpinswouldbe800V.Inthis

kindofapplication,however,supplyvoltages+450Vand-100Vdoesmeettheinputcommonmodevoltagerange

requirementssincethemaximumdifferencevoltagebetweentheinputspinsandthesupplyvoltageis450V(the

maximumallowed).Theoutputhasnosuchrestrictionsonitsvoltageswing.Theoutputcanswingwithin24Vof

eithersupplyvoltageregardlessofvaluesolongasthetotalsupplyvoltagedoesnotexceed900V.

INTERNAL PoWER dISSIPATIoN ANd HEATSINK SELECTIoN

WiththeuniquecombinationofhighvoltageandspeedofthePA94,traditionalformulasforheatsinkselectionwill

falselylowertheapparentpowerhandlingcapabilityofthisamplifier.Tomoreaccuratelypredictoperatingtempera-

Findinternaldissipation(PD)resultingfromdrivingtheload.UsePowerDesignorrefertoApexApplicationsNote

1,GeneralOperatingConsidertaions,paragraph7.Findtotalquiescentpower(PD

(totalsupplyvoltage).Findoutputstagequiescentpower(PD

Calculateaheatsinkratingwhichwillmaintainthecaseat85°Corlower.

Where:T

T

Calculateaheatsinkratingwhichwillmaintainoutputtransistorjunctionsat150°Corlower.

Where:T

R

Usethelargerheatsinkofthesetwocalculations.

PowerDesignisanExcelspreadsheetavailablefreefromwww.cirrus.com