September 2004

13

M9999-091604

MIC37100/37101/37102

Micrel

0

100

200

300

400

500

600

700

800

900

0

0.25 0.50 0.75 1.00 1.25 1.50

POWER DISSIPATION (W)

∆T

Figure 4. Copper Area vs. Power SO-8

Power Dissipation

Figure 4 shows copper area versus power dissipation with

each trace corresponding to a different temperature rise

above ambient.

From these curves, the minimum area of copper necessary

for the part to operate safely can be determined. The maxi-

mum allowable temperature rise must be calculated to deter-

mine operation along which curve.

∆T = T

T

T

For example, the maximum ambient temperature is 50

°C, the

∆T is determined as follows:

∆T = 125°C – 50°C

∆T = 75°C

Using Figure 4, the minimum amount of required copper can

be determined based on the required power dissipation.

Power dissipation in a linear regulator is calculated as fol-

lows:

P

If we use a 2.5V output device and a 3.3V input at an output

current of 1A, then our power dissipation is as follows:

P

P

P

From Figure 4, the minimum amount of copper required to

operate this application at a

Quick Method

Determine the power dissipation requirements for the design

along with the maximum ambient temperature at which the

device will be operated. Refer to Figure 5, which shows safe

operating curves for three different ambient temperatures:

25

°C, 50°C and 85°C. From these curves, the minimum

amount of copper can be determined by knowing the maxi-

mum power dissipation required. If the maximum ambient

temperature is 50

°C and the power dissipation is as above,

836mW, the curve in Figure 5 shows that the required area of

The

θ

depending upon the availability of copper ground plane to

which it is attached.

0

100

200

300

400

500

600

700

800

900

0

0.25 0.50 0.75 1.00 1.25 1.50

POWER DISSIPATION (W)

T

50

°C 25°C

T

J

= 125

°C

Figure 5. Copper Area vs. Power-SOIC

Power Dissipation