1.) Find Driver Power Dissipation

= [(50mA) x (80V)] + [(20V) x (0.05A)]

= 4W + 1.0W

= 5Watts

= ((125°C - 50°C) / 5W) - (12°C/W) - (.15°C/W)

≅ 2.85°C/W

The heat sink in this example must have a thermal

resistance of no more than 2.85°C/W to maintain a junc-

APPLICATION NOTES

HEAT SINKING

To select the correct heat sink for your application,

refer to the thermal model and governing equation below.

Thermal Model:

Governing Equation:

CURRENT LIMIT

The output current of the MSK 1461 is internally lim-

ited to approximately ±750mA by two 0.8Ω internal cur-

rent limit resistors. Additional current limit can be achieved

through the use of two external current limit resistors.

value of the current limit resistors can be determined as

follows:

Since the 0.65V term is obtained from the base to

emitter voltage drop of a bipolar transistor, the equation

only holds true for +25°C operation. As case tempera-

ture increases, the 0.65V term will decrease making the

actual current limit set point decrease slightly.

Where

Example:

In our example the amplifier application requires the

output to drive a 20 volt peak sine wave across a 400Ω

load for 50mA of peak output current. For a worst case

analysis we will treat the 50mA peak output current as a

D.C. output current. The power supplies shall be set to

±40VDC.

The following schematic illustrates how to connect

each current limit resistor:

IN

Any designer who has worked with power operational

amplifiers is familiar with Safe Operating Area (S.O.A.)

curves. S.O.A. curves are a graphical representation of

the following three power limiting factors of any bipolar

transistor output op-amp.

1. Wire Bond Current Carrying Capability

2. Transistor Junction Temperature

3. Secondary Breakdown Limitations

Since the MSK 1461 utilizes a MOSFET output, there

are no secondary breakdown limitations and therefore no

need for S.O.A. curves. The only limitation on output

power is the junction temperature of the output drive tran-

sistors.

Whenever possible, junction temperature should be

kept below 150°C to ensure high reliability. See "Heat

Sinking" for more information involving junction tempera-

ture calculations.

SAFE OPERATING AREA

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 0.1µF ce-

ramic capacitor in parallel with a 4.7µF tantalum capaci-

tor from each power supply pin to ground.

POWER SUPPLY BYPASSING

INPUT OFFSET ADJUST CONNECTION

Rev. B 8/00

3