MIC39100-3.3WS datasheet(11/12 Pages) MICREL

11 / 12 page

August 2005

M9999-082505-B

MIC39100/39101/39102

Micrel

ΔT = TJ(max) – TA(max)

TJ(max) = 125°C

TA(max) = maximum ambient operating temperature

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:

PD = (VIN – VOUT) IOUT + VIN·IGND

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:

PD = (3.3V – 2.5V) × 1A + 3.3V × 11mA

PD = 800mW + 36mW

PD = 836mW

From Figure 4, the minimum amount of copper required to

operate this application at a ΔT of 75°C is 160mm2.

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 copper is 160mm2.

The θJA of this package is ideally 63°C/W, but it will vary

depending upon the availability of copper ground plane to

which it is attached.

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