HEAT SINKING

To select the correct heat sink for your application, refer to the

thermal model and governing equation below.

Thermal Model:

APPLICATION NOTES

CURRENT LIMIT

The MSK 0021 has an on-board current limit scheme de-

signed to limit the output drivers anytime output current

exceeds a predetermined limit. The following formula may

be used to determine the value of the current limit resistance

necessary to establish the desired current limit.

Current Limit Connection

3

Rev. A 6/03

0.7

___

See "Application Circuits" in this data sheet for additional

information on current limit connections.

POWER SUPPLY BYPASSING

Both the negative and the positive power supplies must

be effectively decoupled with a high and low frequency by-

pass 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 high power op-amps, such as the MSK

0021, to place a 30-50 microfarad 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 pack-

age power supply pins as possible.

SAFE OPERATING AREA

The safe operating area curve is a graphical representation

of the power handling capability of the amplifier under vari-

ous conditions. The wire bond current carrying capability,

transistor junction temperature and secondary breakdown

limitations are all incorporated into the safe operating area

curves. All applications should be checked against the S.O.A.

curves to ensure high M.T.B.F.

Governing Equation:

Where

= Junction Temperature

= Total Power Dissipation

R

θJC

= Junction to Case Thermal Resistance

R

θCS

= Case to Heat Sink Thermal Resistance

R

θSA

= Heat Sink to Ambient Thermal Resistance

= Case Temperature

= Ambient Temperature

= Sink Temperature

In our example the amplifier application requires the output to drive

a 10 volt peak sine wave across a 10 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

±15 VDC.

1.) Find Power Dissipation

=(3.5 mA) X (30V) + (5V) X (1A)

=0.1W + 6W

=6.1W

4.) R

5.) Rearrange governing equation to solve for R

θSA:

R

= (150°C - 25°C) / 6.1W - (2.0°C/W) - (0.15°C/W)

= 18.5°C/W

The heat sink in this example must have a thermal resistance of no

more than 18.5°C/W to maintain a junction temperature of less than

+150°C.