MMFT3055EL

7

Motorola TMOS Power MOSFET Transistor Device Data

INFORMATION FOR USING THE SOT–223 SURFACE MOUNT PACKAGE

MINIMUM RECOMMENDED FOOTPRINT FOR SURFACE MOUNTED APPLICATIONS

Surface mount board layout is a critical portion of the total

design. The footprint for the semiconductor packages must be

the correct size to insure proper solder connection interface

between the board and the package. With the correct pad

geometry, the packages will self align when subjected to a

solder reflow process.

SOT–223

0.079

2.0

0.15

3.8

0.248

6.3

0.079

2.0

0.059

1.5

0.059

1.5

0.059

1.5

0.091

2.3

0.091

2.3

mm

inches

SOT–223 POWER DISSIPATION

The power dissipation of the SOT–223 is a function of the

drain pad size. This can vary from the minimum pad size for

soldering to a pad size given for maximum power dissipation.

Power dissipation for a surface mount device is determined by

TJ(max), the maximum rated junction

temperature of the die, R

θJA, the thermal resistance from the

device junction to ambient, and the operating temperature, TA.

Using the values provided on the data sheet for the SOT–223

package, PD can be calculated as follows:

PD =

TJ(max) – TA

R

θJA

The values for the equation are found in the maximum

ratings table on the data sheet. Substituting these values into

the equation for an ambient temperature TA of 25°C, one can

calculate the power dissipation of the device which in this case

is 800 milliwatts.

PD =

150

°C – 25°C

156

°C/W

= 800 milliwatts

The 156

°C/W for the SOT–223 package assumes the use

of the recommended footprint on a glass epoxy printed circuit

board to achieve a power dissipation of 800 milliwatts. There

are other alternatives to achieving higher power dissipation

from the SOT–223 package. One is to increase the area of the

drain pad. By increasing the area of the drain pad, the power

dissipation can be increased. Although one can almost double

the power dissipation with this method, one will be giving up

area on the printed circuit board which can defeat the purpose

of using surface mount technology. A graph of R

θJA versus

drain pad area is shown in Figure 17.

°

A, Area (square inches)

0.0

0.2

0.4

0.6

0.8

1.0

160

140

120

100

80

Figure 17. Thermal Resistance versus Drain Pad

Area for the SOT–223 Package (Typical)

Board Material = 0.0625

″

G–10/FR–4, 2 oz Copper

TA = 25°C

0.8 Watts

1.25 Watts*

1.5 Watts

*Mounted on the DPAK footprint

Another alternative would be to use a ceramic substrate or

an aluminum core board such as Thermal Clad

™. Using a

board material such as Thermal Clad, an aluminum core

board, the power dissipation can be doubled using the same

footprint.