AN668 datasheet(11/19 Pages) STMICROELECTRONICS

11 / 19 page

11/19

AN668 APPLICATION NOTE

on the efficiency of the heat transfer leads (design

and thermal conductivity) and on the die size.

Rth(j-a) is due to the size of the dissipating elements

on the board, according to Figure 13, which shows

improved thermal resistance from 76 to 50°C/W

when the dissipating area is increased from 0 to 6 cm

sq.

The PowerSO-20/36 can be used in place of medium

power SO, with the slug soldered on the board.

Figure 13. Rth(j-a) vs. board dissipating area

The first experimental evidence shows a very good

heat exchange with the substrate, due to the large

contact area associated with the basic footprint of

Figure 14. Rth(j-a) is about 50°C/W in this configura-

tion.

Figure 14. Pad layout

When the two dissipating elements are added and

the footprint of Figure 15 is used, additional improve-

ment is observed. For example, Rth(j-a) is 35°C/W

with dissipation elements of 6 sq cm.

Figure 15. Footprint with dissipating element

Margin

over

the

traditional

medium

power

SO(12+4+4) is about 40%, with dissipation increased

from 1 W to 1,4 W always with Delta Tj = 50°C. Im-

provement can be also understood taking into ac-

count

that

the

junction

substrate

thermal

resistance is equivalent to Rth(j-p) = 12-14°C/W for

medium power SO(12+4+4) and equivalent to Rth(j-

c) = 0.5-2.0°C/W for PowerSO-20/36.

3.3

APPLICATIONS WITH 2-5 WATT DISSIPA-

TION

3.3.1 Dissipating elements and ground layer

It must be noticed that previous data are observed

with a simple PCB, with single or double face. In ap-

plications using multilayer boards, a much better effi-

ciency is observed due to the heat spreading effect of

the ground layer.

Looking at the board structure of Figure 16, heat gen-

erated at the junction is transferred to the dissipating

areas on the board surface and then to the ground

layer, which works as a large heat exchange element

with the ambient.

Rth(j-a) offered by this solution is 20-25°C/W for 2.0

- 2.5W dissipation.

Figure 16. Mounting on PCB with ground layer

0123456789

on board heat sink area (sq.cm)

D94AN007

die pad = 140 x 220sq.mils

die size = 120 x 130sq.mils

Pd = 1W

Rth(j-a) (˚C/W)

1.7

9.4 15.4

0.7

1.27

14.8

17.7

D94AN008

D94AN009

Ground

layer

D94AN010

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