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4

OPERATING

CONSIDERATIONS

PA88 • PA88A

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. 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

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.

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INPUT PROTECTION

Although the PA88 can withstand differential input voltages

up to ±25V, additional external protection is recommended,

and required at total supply voltages above 300 volts. In most

applications 1N4148 or 1N914 signal diodes are sufficient (D1,

D2 in Figure 2a). In more demanding applications where low

leakage or low capacitance are of concern 2N4416 or 2N5457-

2N5459 JFETs connected as diodes will be required (Q1, Q2

in Figure 2b). In either case the input differential voltage will

be clamped to ±.7V. 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. 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 reversal 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 PA88 has sufficient phase margin to be stable with

most capacitive loads at a gain of 4 or more, using the rec-

ommended phase

compensation.

The PA88 is exter-

nally 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

be rated at 500V

working voltage. An

NPO capacitor is

recommended. The

compensation net-

work C

mounted closely to

the amplifier pins 7

and 8 to avoid spuri-

ous oscillation.

FIGURE 2. OVERVOLTAGE PROTECTION

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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 notic

PA88U REV L FEBRUARY 2005 © 2005 Apex Microtechnology Corp.