AN1703 Datasheet, PDF (8/22 Pages) STMicroelectronics – GUIDELINES FOR USING ST’S MOSFET SMD PACKAGES

AN1703 - APPLICATION NOTE
For a drain pad area of 1in² (about 600mm²) we obtain:
R_{THJ −PCB}
= 38 ^°CW
⇒ P_D
=
∆T
R_{THJ −PCB}
= 3.3W
In the next figure the maximum allowable power dissipation as function of a PCB drain pad area for
different T_J-T_A values is shown:
Figure 10: Maximum allowable power dissipation versus drain pad area
4
Tj-Ta=125°C
3
Tj-Ta=100°C
2
Tj-Ta=75°C
6. SO-8
Tj-Ta=50°C
1
Tj-Ta=25°C
0
0
200
400
600
800
1000
Area mm²
The basis of the pad design for an SO-8 MOSFET is the package footprint. In converting the footprint to
the pad set for a MOSFET, designers must make two connections: an electrical connection and a
thermal connection, to draw heat away from the package.
For the SO-8 the thermal connections are very simple. Pins 5, 6, 7 and 8 are the drain of the MOSFET
for a single MOSFET package and are connected together. In a dual package, pins 5 and 6 are one
drain, and pins 7 and 8 are the other drain.
In figure 11 the basic SO-8 footprint is shown.
Figure 11: SO-8 and its basic footprint (all dimensions are in mm)
For a small-signal device or integrated circuit, typical connections would be made with traces that are
0.020 inches wide. Since the drain pins serve the additional function of providing the thermal connection
to the package, this level of connection is inadequate.
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