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PA21

Power dual operational amplifier

Apex Microtechnology 

PA21/25/26 • PA21A/25A

OPERATING

CONSIDERATIONS

GENERAL

Please read Application Note 1 "General Operating Consid-

erations" which covers stability, supplies, heat sinking, mount-

ing, current limit, SOA interpretation, and specification inter-

pretation. Visit www.apexmicrotech.com for design tools that

help automate tasks such as calculations for stability, internal

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.

4

3

2

1

EACH,EBAOCTHH, OLONAEDLEODADED

TC = 25°C

.1

1

2 3 4 56

10

20 30 40 50

SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE VS –VO (V)

CURRENT LIMIT

Current limit is internal to the amplifier, the typical value is

shown in the current limit specification.

SAFE OPERATING AREA (SOA)

The SOA curves combine the effect of all limits for this power

op amp. 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. The following guidelines may save extensive analytical

efforts.

Under transient conditions, capacitive and dynamic* induc-

tive loads up to the following maximum are safe:

±Vs

CAPACITIVE LOAD INDUCTIVE LOAD

20V

200µF

15V

500µF

10V

5mF

5V

50mF

7.5mH

25mH

35mH

150mH

* If the inductive load is driven near steady state conditions,

allowing the output voltage to drop more than 6V below the

supply rail while the amplifier is current limiting, the inductor

should be capacitively coupled or the supply voltage must be

lowered to meet SOA criteria.

NOTE: For protection against sustained, high energy flyback,

external fast-recovery diodes should be used.

MONOLITHIC AMPLIFIER

STABILITY CONSIDERATIONS

All monolithic power op amps use output stage topologies

that present special stability problems. This is primarily due to

non-complementary (both devices are NPN) output stages

with a mismatch in gain and phase response for different

polarities of output current. It is difficult for the op amp manu-

facturer to optimize compensation for all operating conditions.

The recommended R-C network of 1 ohm in series with

0.1µF from output to AC common (ground or a supply rail, with

adequate bypass capacitors) will prevent local output stage

oscillations.

This network is provided internally on the PA21 but must be

supplied externally on the PA25 and PA26. The amplifiers are

internally compensated for unity gain stability, no additional

compensation is required.

THERMAL CONSIDERATIONS

Although R θJC is the same for PA21/25/26 there are differ-

ences in the thermal interface between case and heatsink

which will limit power dissipation capability. Thermal grease or

an Apex TW03 thermal washer, R θCS = .1-.2°C/W, is the only

recommended interface for the PA21/25. The PA26 may

require a thermal washer which is electrically insulating since

the tab is tied to –VS. This can result in thermal impedances for

R θCS of up to 1°C/W or greater.

VBOOST

+VS

– IN

+IN

OUT

ISENSE

RS

– VS

FIGURE 2. PA26 EQUIVALENT SCHEMATIC (ONE CHANNEL)

APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739

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