STPC12 datasheet(100/111 Pages) STMICROELECTRONICS

100 / 111 page

DESIGN GUIDELINES

100/111

Issue 1.0 - July 24, 2002

6.4.5. THERMAL DISSIPATION

6.4.5.1. Power saving

Thermal dissipation of the STPC depends mainly

on supply voltage. When the system does not

need to work at the upper voltage limit, it may

therefore be beneficial to reduce the voltage to the

lower voltage limit, where possible. This could

save a few 100’s of mW.

The second area to look at is unused interfaces

and functions. Depending on the application,

some input signals can be grounded, and some

blocks not powered or shutdown. Clock speed

dynamic adjustment is also a solution that can be

used

along

with

the

integrated

power

management unit.

6.4.5.2. Thermal balls

The standard way to route thermal balls to ground

layer implements only one via pad for each ball

pad, connected using a 8-mil wire.

With such configuration the Plastic BGA package

does 90% of the thermal dissipation through the

ground balls, and especially the central thermal

balls which are directly connected to the die. The

remaining 10% is dissipated through the case.

Adding a heat sink reduces this value to 85%.

As a result, some basic rules must be followed

when routing the STPC in order to avoid thermal

problems.

As the whole ground layer acts as a heat sink, the

ground balls must be directly connected to it, as

illustrated in Figure 6-31. If one ground layer is not

enough, a second ground plane may be added.

When possible, it is important to avoid other

devices

on-board

using

the

PCB

for

heat

dissipation, like linear regulators, as this would

heat the STPC itself and reduce the temperature

range of the whole system, In case these devices

can not use a separate heat sink, they must not be

located just near the STPC

Figure 6-31. Ground Routing

Thru hole to ground layer

Top

Lay

er: S

igna

Pow

er la

yer

Inte

rnal

Lay

er:

Sig

nals

Bott

Lay

er: G

rou

laye

Pad for ground ball

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