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 4

to pin 18 and pin 9 to pin 13.

EXTERNAL COMPENSATION

External compensation is only necessary at gains of 30v/v or

less. For larger gains, the compensation resistor and capacitor

may be omitted. An effective 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.

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.

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INPUT PROTECTION

Input protection circuitry within the MSK 103 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 drive pin of the amplifier and the

load. A small tradeoff with bandwidth must be made in this

configuration. The graph below illustrates the effect of capaci-

tive load on open loop gain.

The MSK 103 has sufficient phase margin when compen-

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

least 10nF. However, it is recommended 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 re-

sistor and input capacitance. The user can tailor the perfor-

mance of the MSK 103 to their application using the external

compensation pins. The graphs of small signal gain and phase

as well as the graphs of slew rate and power response demon-

strate the effect of various forms of compensation. The com-

pensation capacitor must be rated at 350 volts working voltage

if maximum power supply voltages are used. The compensa-

tion resistor and capacitor lead lengths must be kept as short

as possible to minimize spurious oscillations. A high quality

NPO capacitor is recommended for the compensation capaci-

tor.

STABILITY

Rev. B 1/02

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