APPLICATION NOTES

CURRENT LIMIT

Current limit resistor value can be calculated as follows:

It is recommended that the user set up the value of current limit

as close as possible to the maximum expected output current

to protect the amplifier. The minumum value of current limit

resistance is 33 ohms. The maximum practical value is 500

ohms. Current limit will vary with case temperature. Refer to

the typical performance graphs as a guide. Since load current

passes through the current limit resistor, a loss in output volt-

age swing will occur. The following formula approximates out-

put voltage swing reduction:

When the device is in current limit, there will be spurious oscil-

lations present on the negative half cycle. The frequency of

the oscillation is application dependant and can not be pre-

dicted. Oscillation will cease when the device comes out of

current limit. If current limit is not required simply short pin 3

to pin 4 and pin 10 to pin 11.

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 junction temperature of the output MOSFET's.

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.

*

INPUT PROTECTION

Input protection circuitry within the MSK 165 will clip differ-

ential input voltages greater than 16 volts. The inputs are also

protected against common mode voltages up to the supply rails

as well as static discharge. There are 300 ohm current limiting

resistors in series with each input. These resistors may be-

come damaged in the event the input overload is capable of

driving currents above 1mA. If severe overload conditions are

expected, external input current limiting resistors are recom-

mended.

OUTPUT SNUBBER NETWORK

A 100 ohm resistor and a 330pF capacitor connected in se-

ries from the output of the amplifier to ground is recommended

for applications where load capacitance is less than 330pF.

For larger values of load capacitance, the output snubber net-

work may be omitted. If loop stability becomes a problem due

to excessively high load capacitance, a 100 ohm resistor may

be added between the output of the amplifier (the junction of

width must be made in this configuration. The graph below

illustrates the effect of capacitive load on open loop gain.

The MSK 165 has sufficient phase margin when compen-

sated for unity gain to be stable with capacitive loads of at

least 10nF at gains of 2V/V or greater. However, it is recom-

mended that the parallel sum of the input and feedback resistor

be 1000 ohms or less for closed loop gains of ten or less to

minimize phase shift caused by the R-C network formed by the

input resistor, feedback resistor and input capacitance. An ef-

fective method of checking amplifier stability is to apply the

worst case capacitive 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

Rev. A 8/00

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