6-42 System Design
AMD-K5 Processor Technical Reference Manual 18524C/0—Nov1996
reduces noise without additional PCB planes. The split plane
should be made from a portion of copper that is cut out and iso-
lated from the PCB 5-V power plane. This cutout region sup-
plies a separate power source for the processor and allows
installation of bypass decoupling capacitors. The capacitors
should be placed across the split power plane to provide signal-
return termination. The processor power plane should overlap
the output pin of the voltage regulator circuit to provide a low-
impedance current path.
The ground plane should never be split because it provides a
low-impedance current sink and reference. Use generous
decoupling to ensure that clean power is supplied to the pro-
cessor.
6.8 Thermal Design
In virtually all system designs, the processor’s case tempera-
ture must be kept cool with some type of heatsink device. Typ-
ically, the heatsink is combined with an airflow device, such as
a fan. In general, the trade-off is heat-sink size and cost versus
airflow quantity and temperature. A small, low-cost heat sink
requires more airflow than a larger, more efficient heat sink.
Such cooling products are widely available. For detailed speci-
fications and assistance is selecting a product, contact your
AMD field application engineer or browse the AMD home page
on the World Wide Web (see Section 6.9 for details).
When gluing a heat sink to the processor case, follow these
guidelines:
■ Use thermal paste. This optimizes heat transfer.
■ Apply the thermal paste in a thin, smooth, even layer across
the entire processor package. Do not allow air gaps between
the processor package and the heatsink. If air gap exits, the
heatsink will be ineffective.
In addition to the above guidelines for gluing heatsinks to the
processor, observe the following general design guidelines to
minimize the adverse effects of system-generated heat on the
processor and other heat-sensitive system components:
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