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4

OPERATING

CONSIDERATIONS

PA89 • PA89A

GENERAL

Please read Application Note 1 "General Operating Con-

siderations" which covers stability, supplies, heat sinking,

mounting, current limit, SOA interpretation, and specification

interpretation. Visit www.apexmicrotech.com for design tools

that help automate tasks such as calculations for stability,

internal power dissipation, current limit; heat sink selection;

Apex’s complete Application Notes library; Technical Seminar

Workbook; and Evaluation Kits.

STABILITY

Although the PA89 can be operated at unity gain, maximum

slew rate and bandwidth performance was designed to be

obtained at gains of 10 or more. Use the small signal response

and phase response graphs as a guide. In applications where

gains of less than 10 are required, use noise gain compensa-

tion to increase the phase margin of the application circuit as

illustrated in the typical application drawing.

SAFE OPERATING AREA (SOA)

The MOSFET output stage of this power operational ampli-

fier has two distinct limitations:

1. The current handling capability of the MOSFET geometry

and the wire bonds.

2. The junction temperature of the output MOSFETs.

NOTE: The output stage is protected against transient flyback.

However, for protection against sustained, high energy flyback,

external fast-recovery diodes should be used.

SAFE OPERATING CURVES

The safe operating area curves define the maximum ad-

ditional internal power dissipation the amplifier can tolerate

when it produces the necessary output to drive an external

load. This is not the same as the absolute maximum internal

power dissipation listed elsewhere in the specification since

the quiescent power dissipation is significant compared to

the total.

EXTERNAL COMPONENTS

The very high operating voltages of the PA89 demand

consideration of two component specifications rarely of con-

cern in building op amp circuits: voltage rating and voltage

coefficient.

The compensation capacitance C

full supply voltage range. For example, with supply voltages

of ±500V the possible voltage swing across C

addition, a voltage coefficient less than 100PPM is recom-

mended to maintain the capacitance variation to less than

5% for this example. It is strongly recommended to use the

highest quality capacitor possible rated at least twice the total

supply voltage range.

Of equal importance are the voltage rating and voltage coef-

ficient of the gain setting resistances. Typical voltage ratings of

low wattage resistors are 150 to 250V. In the above example

1000V could appear across the feedback resistor. This would

require several resistors in series to obtain the proper voltage

rating. Low voltage coefficient resistors will insure good gain

linearity. The wattage rating of the feedback resistor is also of

concern. A 1 megohm feedback resistor could easily develop

1 watt of power dissipation.

Though high voltage rated resistors can be obtained,

a 1 megohm feedback resistor comprised of five 200Kohm,

1/4 watt metal film resistors in series will produce the proper

voltage rating, voltage coefficient and wattage rating.

CURRENT LIMIT

For proper operation the current limit resistor (R

connected as shown in the external connection 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 of

150 ohms.

.7

R

I

LIM

When setting the value for R

well as the current in the feedback resistor. Also allow for the

temperature coefficient of the current limit which is approxi-

mately -0.3% /°C of case temperature rise.

CAUTIONS

The operating voltages of the PA89 are potentially lethal.

During circuit design, develop a functioning circuit at the lowest

possible voltages. Clip test leads should be used for “hands

off” measurements while troubleshooting.

POWER SUPPLY PROTECTION

Unidirectional zener diode transient absorbers are recom-

mended as protection on the supply pins. The zeners clamp

transients to voltages within the power supply rating and also

clamp power supply reversals to ground. Whether the zeners

are used or not, the system power supply should be evaluated

for transient performance including power-on overshoot and

power-off polarity reversals as well as line regulation.

Conditions which can cause open circuits or polarity reversals

on either power supply rail should be avoided or protected

against. Reversals or opens on the negative supply rail is

known to induce input stage failure. Unidirectional transzorbs

prevent this, and it is desirable that they be both electrically

and physically as close to the amplifier as possible.

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This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.

PA89U REV H MAY 2005 © 2005 Apex Microtechnology Corp.