APPLICATION NOTES CONT.

4

HEAT SINK SELECTION

To select a heat sink for the MSK 5921RH, the follow-

ing formula for convective heat flow may be used.

Governing Equation:

Where

= Junction Temperature

= Total Power Dissipation

R

θJC

= Junction to Case Thermal Resistance

R

R

= Ambient Temperature

Next, the user must select a maximum junction tem-

perature. The absolute maximum allowable junction tem-

perature is 150°C. The equation may now be rearranged

to solve for the required heat sink to ambient thermal

resistance (R

Example:

bient temperature is +25°C. The maximum desired junc-

tion temperature is +125°C.

R

greases

Power Dissipation=(3.3V-2.5V) x (3A)

=2.4Watts

Solve for R

θSA:

125°C - 25°C

2.4W

= 36.5°C/W

In this example, a heat sink with a thermal resistance

of no more than 36.5°C/W must be used to maintain a

junction temperature of no more than 125°C.

TYPICAL APPLICATIONS CIRCUIT

OUTPUT VOLTAGE SELECTION

As noted in the above typical applications circuit, the

formula for output voltage selection is

A good starting point for this output voltage selection is

to set R2=1K. By rearranging the formula it is simple to

calculate the final R1 value.

Table 1 below lists some of the most probable resistor

combinations based on industry standard usage.

R1

R2

1+

1.265

-1

R1=R2

1.5

1.8

2.0

2.5

2.8

3.3

4.0

5.0

R2

(

Ω

Ω

Ω

Ω)

1K

1K

1K

1K

1K

1K

1K

1K

R1 (nearest 1%)

(

Ω

Ω)

187

422

576

976

1.21K

1.62K

2.15K

2.94K

R

-4.0°C/W - 0.15°C/W

OUTPUT

VOLTAGE

(V)

THERMAL LIMITING

The MSK 5921RH control circuitry has a thermal shut-

down temperature of approximately 150°C. This ther-

mal shutdown can be used as a protection feature, but

for continuous operation, the junction temperature of the

pass transistor must be maintained below 150°C. Proper

heat sink selection is essential to maintain these condi-

tions.

TABLE 1

Rev. C

1/06