APPLICATION NOTES

SAFE OPERATING AREA (SOA)

The output stage of this power operational amplifier has three

distinct limitations:

1. The current handling capability of the die metallization.

2. The junction temperature of the output device's.

3. Secondary breakdown.

All applications should be checked against the SOA curves.

NOTE: The output stage is protected against transient flyback.

However, for protection against sustained, high energy flyback,

external fast-recovery reverse biased diodes should be connected

from the output to ground.

A value of current limit resistance can be calculated as fol-

lows:

Where:

0.057Ω is the drop in the current limit path across internal

impedances other than the actual current limit resistor.

0.809Ω volts is the voltage drop that must be developed

across the current limit connections to activate the current

limit circuit at 25°C.

It is recommended the user limit output current to a value as

close to the required output current as possible, without clip-

ping output voltage swing. Current limit will vary with case

temperature. Refer to the typical performance curves to pre-

dict current limit drift. If current limit is not required replace the

TYPICAL CONNECTION DIAGRAM

A 0.1 microfarad ceramic disc and low ESR capacitor with a

value of 10 microfarads per amp of output current should be

placed in parallel from each power supply pin to ground. These

capacitors must be rated for the full power supply voltage. Since

the MSK 155 is commonly used in circuits where the loop gain

is greater than 10 V/V, high frequency noise that enters the op-

amp through the power supply lines will be amplified and could

cause the amplifier to break into oscillation. In addition, with-

out supply bypassing, the inductance of the power supply lines

interacts with capacitive loads to form an oscillatory LC tank

circuit. The power supply decoupling capacitors will minimize

this effect and keep the circuit stable.

POWER SUPPLY DECOUPLING

Input protection circuitry within the MSK 155 will clip differ-

protected against common mode voltages up to the supply rails

as well as static discharge. There are current limiting resistors

in series with each input. These resistors may become dam-

aged in the event the input overload is capable of driving cur-

rents above 1mA. If severe overload conditions are expected,

external input current limiting resistors are recommended.

INPUT PROTECTION

It is recommended that the parallel sum of the input and feed-

back resistor be 1000 ohms or less to minimize phase shift

caused by the R-C network formed by the input resistor, feed-

back resistor and input capacitance. An effective method of

checking amplifier stability is to apply the worst case capaci-

tive load to the output of the amplifier and drive a small signal

square wave across it. If overshoot is less than 25%, the

system will typically be stable.

STABILITY

3

Rev. A 12/01