MIC37101 datasheet(13/15 Pages) MICREL

13 / 15 page

Micrel, Inc.

MIC37100/37101/37102

September 2007

M9999-090607

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 maximum allowable temperature rise

must be calculated to determine operation along which

curve.

∆T = TJ(max) – TA(max)

TJ(max) = 125°C

TA(max) = maximum ambient operating temp-

erature.

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 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 ∆T of 75°C is 160mm

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 maximum 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 160mm

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.

100

200

300

400

500

600

700

800

900

0.25 0.50 0.75 1.00 1.25 1.50

POWER DISSIPATION (W)

A =85°C

50°C 25°C

TJ = 125°C

Figure 5. Copper Area vs. Power-SOIC

Power Dissipation

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