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4

OPERATING

CONSIDERATIONS

PA92

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.

CURRENT LIMIT

For proper operation, the current limit resistor (R

connected as shown in the external connection diagram. 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 16 ohms.

.65

R

I

LIM

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 fly-

back. 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 addi-

tional internal power dissipation the amplifier can tolerate when

it produces the necessary output to drive an external load.

INPUT PROTECTION

Although the PA92 can withstand differential voltages up

to ±20V, additional external protection is recommended. Low

leakage, low capacitance JFETs connected as diodes are

recommended (e.g. 2N4416, Q1-Q4 in Figure 2). The differ-

ential input voltage will be clamped to ±1.4V. This is sufficient

overdrive to produce maximum power bandwidth.

POWER SUPPLY PROTECTION

Unidirectional zener diode transient suppressors are recom-

mended as protection on the supply pins. See Figure 2. 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 sup-

ply 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.

STABILITY

The PA92 is externally compensated and performance can

be tailored to the application. Use the graphs of small signal

response and power response as a guide. The compensation

capacitor C

capacitor is recommended. The compensation network C

must be mounted closely to the amplifier pins 4 and 5 to avoid

spurious oscillation.

QUIESCENT CURRENT REDUCTION

When pin 3 (I

the output stage is disabled. This lowers quiescent power but

also raises distortion since the output stage is then class C

biased. The output stage bias current is nominally set at 1mA.

Pin 3 may be left open if not used.

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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.

PA92U REV H OCTOBER 2004 © 2004 Apex Microtechnology Corp.