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4

OPERATING

CONSIDERATIONS

PA141/PA142 • PA141A/PA142A

GENERAL

Please read Application Note 1 “General Operating

Considerations” 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,

internal power dissipation, current limit; heat sink selection;

Apex’s complete Application Notes library; Technical Seminar

Workbook; 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 18 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

to the specified typical value of output resistance to calculate

the total output resistance. Since the load current passes

through R

swing 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 oscillation 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 PA141/142 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

exceeding 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 expected. Limiting input current to 1mA will

prevent damage.

STABILITY

The PA141/142 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 5K 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 combination should match

the time constant of the sumpoint resistance and sumpoint

capacitance.

The PA141/142 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

C

must be rated at 350V working voltage.

The compensation capacitor and associated resistor R

C

must be mounted closely to the amplifier pins to avoid

spurious oscillation. An NPO capacitor is recommended

for compensation.

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