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PA05

Power Operational Amplifier

Apex Microtechnology 

PA05 • PA05A

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.apexanalog.com for design tools that

help automate tasks such as calculations for stability, internal

power dissipation, current limit; heat sink selection; Apex Mi-

crotechnologys complete Application Notes library; Technical

Seminar Workbook; and Evaluation Kits.

CURRENT LIMIT

The two current limit sense lines are to be connected directly

across the current limit sense resistor. For the current limit to

work correctly,pin 11 must be connected to the amplifier output

side and pin 10 connected to the load side of the current limit

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resistor may not be placed anywhere in the output circuit except

where shown in Figure 1. If current limiting is not used, pins 10

and 11 must be tied to pin 7.

The value of the current limit resistor can be calculated as

follows:

Rf

Ri

INPUT

10

1

11

CL

2 PA05 CL

7 RP

RCL

RL

ILIMIT = .7/RCL

FIGURE 1. CURRENT LIMIT

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.

SOA

30

15

12

t = 200ms

9

DC

6

T

3

C = 125°C

1.5

1.2

.9

.6

.3

1

2 3 4 5 10 20 30 40 50 100

SUPPLY TO OUTPUT DIFFERENTIAL (V)

The output stage thermal protection circuit engages when

junction temperatures reach approximately 175C.If the condition

remains that caused the shutdown, the amplifier may oscillate

in and out of shutdown, creating high peak power stresses

reducing the reliability of the device.

SHUTDOWN OPERATION

To disable the output stage, pin 12 is connected to ground

via relay contacts or via an electronic switch. The switching

device must be capable of sinking 2mA to complete shutdown

and capable of standing off the supply voltage +VS. See Figure

2 for suggested circuits.

–LOGIC

K1

12 SHUTDOWN

A

–LOGIC

B

Q1

470Ω

12 SHUTDOWN

FIGURE 2. SHUTDOWN OPERATION

From an internal circuitry standpoint, shutdown is just a

special case of current limit where the allowed output current

is zero. As with current limit, however, a small current does

flow in the output during shutdown. A load impedance of 100

ohms or less is required to insure the output transistors are

turned off. Note that even though the output transistors are off

the output pin is not open circuited because of the shutdown

operating current.

BOOST OPERATION

With the VBOOST feature, the small signal stages of the ampli-

fier are operated at higher supply voltages than the amplifier’s

high current output stage. +VBOOST (pin 9), and –VBOOST (pin

5) are connected to the small signal circuitry of the amplifier.

+VS (pin 8) and –VS (pin 6) are connected to the high current

output stage. An additional 5V on the VBOOST pins is sufficient

to allow the small signal stages to drive the output transistors

into saturation and improve the output voltage swing for extra

efficient operation when required. When close swings to the

supply rails is not required the +VBOOST and +VS pins must be

strapped together as well as the –VBOOST and –VS pins.The boost

voltage pins must not be at a voltage lower than the VS pins.

COMPENSATION

The external compensation components CC and RC are con-

nected to pins 3 and 4. Unity gain stability can be achieved

at any compensation capacitance greater than 470 pF with at

least 60 degrees of phase margin.At higher gains, more phase

shift can be tolerated in most designs and the compensation

capacitance can accordingly be reduced, resulting in higher

bandwidth and slew rate. Use the typical operating curves as

a guide to select CC and RC for the application.

4

PA05U

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