February 2005

11

MIC3975

MIC3975

Micrel

For example, the maximum ambient temperature is 50°C,

the ΔT is determined as follows:

ΔT = 125°C – 50°C

ΔT = 75°C

Using Figure 4, the minimum amount of required copper can

bedeterminedbasedontherequiredpowerdissipation.Power

dissipation in a linear regulator is calculated as follows:

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

current of 750mA, then our power dissipation is as follows:

From Figure 4, the minimum amount of copper required to

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,

625mW, the curve in Figure 5 shows that the required area

depending upon the availability of copper ground plane to

which it is attached.