OPERATING

CONSIDERATIONS

PA44

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; heat sink selection; Apex’s

complete Application Notes library; Technical Seminar Work-

book; 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 33 ohms, however for optimum reliability the

resistor value should be set as high as possible. The value can

be estimated as follows with the maximum practical value of

500 ohms.

3

R

I

LIM

Use the typical performance graphs as a guide for expected

variations in current limit value with a given R

over temperature. The selected value of R

the specified typical value of output resistance to calculate the

total output resistance. Since the load current passes through

R

according to:

V

R

CL

where V

When the amplifier is current limiting, there may be small

signal spurious oscillation present during the current limited

portion of the negative half cycle. The frequency of the oscil-

lation is not predictable and depends on the compensation,

gain of the amplifier, and load. The oscillation will cease as the

amplifier comes out of current limit.

INPUT PROTECTION

The PA44 inputs are protected against common mode

voltages up the supply rails and differential voltages up to

±16

volts as well as static discharge. Differential voltages exceed-

ing 16 volts will be clipped by the protection circuitry. However,

if more than a few milliamps of current is available from the

overload source, the protection circuitry could be destroyed.

The protection circuitry includes 300 ohm current limiting

resistors at each input, but this may be insufficient for severe

overloads. It may be necessary to add external resistors to the

application circuit where severe overload conditions are ex-

pected. Limiting input current to 1mA will prevent damage.

STABILITY

The PA44 has sufficient phase margin when compensated

for unity gain to be stable with capacitive loads of at least 10 nF.

However, the low pass circuit created by the sumpoint (–in)

capacitance and the feedback network may add phase shift

and cause instabilities. As a general rule, the sumpoint load

resistance (input and feedback resistors in parallel) should be

1K ohm or less at low gain settings (up to 10). Alternatively, use

*

a bypass capacitor across the feedback resistor. The time

constant of the feedback resistor and bypass capacitor com-

bination should match the time constant of the sumpoint

resistance and sumpoint capacitance.

The PA44 is externally compensated and performance can

be tailored to the application. Use the graphs of small signal

gain and phase response as well as the graphs for slew rate

and power response as a guide. The compensation capacitor

C

capacitor and associated resistor R

to the amplifier pins to avoid spurious oscillation. An NPO

capacitor is recommended for compensation.

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 die metallization.

2. The temperature of the output MOSFETs.

NOTE: The output stage is protected against transient

flyback. However, for protection against sustained, high en-

ergy flyback, external fast-recovery diodes should be used.

SUPPLY TO OUTPUT DIFFERENTIAL, V

10

20

30

50

100

200

300

500

2

3

4

5

10

20

30

40

50

100

DC,

T

C

=

125

°C

DC,

T

C

=

85

°C

DC

300mS

PULSE CURVES @ 10% DUTY CYCLE MAX

PA44 SOA

200mS

120

200

HEATSINKING

The PA44 package has a large exposed integrated copper

heatslug to which the monolithic amplifier is directly attached.

The solder connection of the heatslug to a minimum 1 square

inch foil area of the printed circuit board will result in thermal

performance of 25

°C/W junction to air rating of the PA44.

Solder connection to an area of 1 to 2 square inches of foil is

recommended. This may be adequate heatsinking but the

large number of variables involved suggest temperature mea-

surements be made on the top of the package. Do not allow the

temperature to exceed 85

°C. The heatslug is tied internally to –Vs.

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.

PA44U REV. C APRIL 2000

© 2000 Apex Microtechnology Corp.