OPERATING

CONSIDERATIONS

PA84 • PA84A • PA84S

GENERAL

Please read the “General Operating Considerations” sec-

tion, which covers stability, supplies, heatsinking, mounting,

current limit, SOA interpretation, and specification interpreta-

tion. Additional information can be found in the application

notes. For information on the package outline, heatsinks, and

mounting hardware, consult the “Accessory and Package

Mechanical Data” section of the handbook.

SAFE OPERATING AREA (SOA)

The bipolar output stage of this high voltage operational

amplifier has two output limitations:

1. The internal current limit which limits maximum available

output current.

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. The following capacitive and inductive loads are safe:

±V

S

C(MAX)

L(MAX)

150V

1.2

µF

.7H

125V

6.0

µF

25H

100V

12

µF

90H

75V

ALL

ALL

2. Short circuits to ground are safe with dual supplies up to

±150V or single supplies up to 150V.

3. Short circuits to the supply rails are safe with total supply

voltages up to 150V (i.e.

±75V).

OUTPUT PROTECTION

Two external diodes as shown in Figure 2, are required to

protect these amplifiers against 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.

STABILITY

Due to its large bandwidth the PA84 is more likely to oscillate

than lower bandwidth Power Operational Amplifiers such as

the PA83 or PA08. To prevent oscillations, a reasonable phase

margin must be maintained by:

1. Selection of the proper phase compensation capacitor and

resistor. Use the values given in the table under external

connections and interpolate if necessary. The phase mar-

gin can be increased by using a large capacitor and a

smaller resistor than the slew rate optimized values listed in

the table. The compensation capacitor may be connected to

common (in lieu of +V

bypassed to common. Because the voltage at pin 8 is only

a few volts below the positive supply, this ground connec-

tion requires the use of a high voltage capacitor.

2. Keeping the external sumpoint stray capacitance to ground

at a minimum and the sumpoint load resistance (input and

feedback resistors in parallel) below 500

Ω. Larger sumpoint

load resistance can be used with increased phase compen-

sation (see 1 above).

3. Connecting the amplifier case to a local AC common thus

preventing it from acting as an antenna.

FIGURE 1. PROTECTIVE,

INDUCTIVE LOAD

20

30

40

50

25

35

300

200

250

150

170

SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE (V)

t =

5ms

t =

1ms

SAFE OPERATING AREA CURVES

STEADY

STATE

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.

PA84U REV. L JANUARY 2000

© 2000 Apex Microtechnology Corp.

Be sure the diode voltage rating is greater than the total of both

supplies. 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

transient 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.