LNBK20D2

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THERMAL DESIGN NOTE

During normal operation, this device dissipates some power. At maximum rated output current (400mA),

the voltage drop on the linear regulator lead to a total dissipated power that is of about 2W. The heat

generated requires a suitable heatsink to keep the junction temperature below the over temperature

protection threshold. Assuming a 40°C temperature inside the Set-Top-Box case, the total Rthj-amb has

to be less than 43°C/W.

While this can be easily achieved using a through-hole power package that can be attached to a small

heatsink or to the metallic frame of the receiver, a surface mount power package must rely on PCB

solutions whose thermal efficiency is often limited. The simplest solution is to use a large, continuous

copper area of the GND layer to dissipate the heat coming from the IC body.

The SO-20 package of this IC has 4 GND pins that are not just intended for electrical GND connection, but

also to provide a low thermal resistance path between the silicon chip and the PCB heatsink. Given an

Rthj-c equal to 15°C/W, a maximum of 28°C/W are left to the PCB heatsink. This figure is achieved if a

a multi-layer PCB, or, in a dual layer PCB, an unbroken GND area even on the opposite side where the

IC is placed. In both cases, the thermal path between the IC GND pins and the dissipating copper area

must exhibit a low thermal resistance.

In figure 4, it is shown a suggested layout for the SO-20 package with a dual layer PCB, where the IC

Ground pins and the square dissipating area are thermally connected through 32 vias holes, filled by

solder. This arrangement, when L=50mm, achieves an Rthc-a of about 28°C/W.

Different layouts are possible, too. Basic principles, however, suggest to keep the IC and its ground pins

approximately in the middle of the dissipating area; to provide as many vias as possible; to design a

dissipating area having a shape as square as possible and not interrupted by other copper traces.

SO-20 SUGGESTED PCB HEATSINK LAYOUT