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4

OPERATING

CONSIDERATIONS

PA08 • PA08A

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, in-

ternal power dissipation, current limit and heat sink selection.

The "Application Notes" and "Technical Seminar" sections

contain a wealth of information on specific types of applications.

Package outlines, heat sinks, mounting hardware and other

accessories are located in the "Packages and Accessories"

section. Evaluation Kits are available for most Apex product

models, consult the "Evaluation Kit" section for details. For

the most current version of all Apex product data sheets, visit

www.apexmicrotech.com.

SAFE OPERATING AREA (SOA)

The output stage of most power amplifiers has two distinct

limitations:

1. The current handling capability of the transistor geometry

and the wire bonds.

2. The second breakdown effect which occurs whenever the

simultaneous collector current and collector-emitter voltage

exceeds specified limits.

The SOA curves combine the effect of these limits. For a

given application, the direction and magnitude of the output

current should be calculated or measured and checked against

the SOA curves. This is simple for resistive loads but more

complex for reactive and EMF generating loads. However, the

following guidelines may save extensive analytical efforts.

1. Under transient conditions, the following capacitive and

inductive loads are safe with the current limits set to the

maximum:

±V

S

C(MAX)

L(MAX)

150V

.4µF

280mH

125V

.9µF

380mH

100V

2µF

500mH

75V

10µF

1200mH

50V

100µF

13H

2. The amplifier can handle any EMF generating or reactive

load and short circuits to the supply rails or simple shorts

to common if the current limits are set as follows:

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SHORT TO ±V

SC,

SHORT TO

±V

S

C, L, OR EMF LOAD

COMMON

150V

20mA

67mA

125V

27mA

90mA

100V

42mA

130mA

75V

67mA

200mA

50V

130mA

200mA

These simplified limits may be exceeded with further analysis

using the operating conditions for a specific application.

3. The output stage is protected against transient flyback.

However, for protection against sustained, high energy

flyback, external fast-recovery diodes should be used.

INDUCTIVE LOADS

Two external diodes as shown in Figure 1, are required

to protect these amplifiers from flyback (kickback) pulses

exceeding the supply voltages of the amplifier when driving

inductive loads. For component selection, these external diodes

must be very quick, such as ultra fast recovery diodes with no

more than 200 nanoseconds of reverse recovery time. The

diode will turn on to divert the flyback energy into the supply

rails thus protecting the output transistors from destruction due

to reverse bias.

A note of caution about the supply. The energy of the flyback

pulse must be absorbed by the power supply. As a result, a tran-

sient will be superimposed on the supply voltage, the magnitude

of the transient being a function of its transient impedance and

current sinking capability. If the supply voltage plus transient

exceeds the maximum supply rating or if the AC impedance

of the supply is unknown, it is best to clamp the output and the

supply with a zener diode to absorb the transient.

INPUT PROTECTION

The input is protected against common mode voltages up to

the supply rails and differential voltages up to ±50V. Increased

protection against differential input voltages can be obtained by

adding 2 resistors, 2 capacitors and 4 diode connected FETs

as shown in Figure 2.

CURRENT LIMITING

Proper operation requires the use of two current limit resis-

tors, connected as shown in the external connection diagram.

The minimum value for R

reliability it should be set as high as possible. Refer to the

“General Operating Considerations” section of the handbook

for current limit adjust details.

PROTECTION, INDUCTIVE LOAD

PROTECTION, OVERVOLTAGE

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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.

PA08U REV K NOVEMBER 2004 © 2004 Apex Microtechnology Corp.