APPLICATION NOTES

HEAT SINKING

To determine if a heat sink is necessary for your application and

if so, what type, refer to the thermal model and governing equation

below.

Governing Equation:

In our example the amplifier application requires the output to

drive a 20 volt peak sine wave across a 20 ohm load for 1 amp of

output current. For a worst case analysis we will treat the 1 amp

peak output current as a D.C. output current. The power supplies

are ±40 VDC.

1.) Find Power Dissipation

=(25mA) x (80V)+(20V) x (1A)

=2W+20W

=22W

=((125°C -50°C)/22W) - (0.55°C/W) - (0.15°C/W)

=2.71°C/W

The heat sink in this example must have a thermal resistance of

no more than 2.71°C/W to maintain a junction temperature of no

more than +125°C.

Rev. A 6/02

3

Where

Thermal Model:

CURRENT LIMIT

The MSK 115 has an on-board current limit scheme designed to

shut off the output drivers anytime output current exceeds a prede-

termined limit. The following formula may be used to determine the

value of current limit resistance necessary to establish the desired

current limit.

The 0.01 ohm term takes into account any wire bond and lead

resistance. Since the 0.65 volt term is obtained from the base

emitter voltage drop of a bipolar transistor: the equation only holds

true for operation at +25°C case temperature. The curve below

illustrates the effect of case temperature on current limit.

POWER SUPPLY BYPASSING

Both the negative and the positive power supplies must be

effectively decoupled with a high and low frequency bypass circuit

to avoid power supply induced oscillation. An effective decoupling

scheme consists of a 0.1 microfarad ceramic capacitor in parallel

with a 4.7 microfarad tantalum capacitor from each power supply

pin to ground. It is also a good practice with very high power

op-amps, such as the MSK 115, to place a 30-50 microfarad

non-electrolytic capacitor with a low effective series resistance in

parallel with the other two power supply decoupling capacitors.

This capacitor will eliminate any peak output voltage clipping which

may occur due to poor power supply load regulation. All power

supply decoupling capacitors should be placed as close to the

package power supply pins as possible (pins 7 and 12).