Preliminary Information
AMD Athlon™ 64 and AMD Opteron™ Processors
Thermal Design Guide
26633 Rev.
3.03 September
2003
2.6.5 Fans
Improvements in fan technology have increased the range of flow capacity available to thermal
solution designers. However, increased flow may often occur at the expense of causing increased
acoustic noise. Larger fans can move more air at lower rotational speeds than smaller fans. The
recent increase in availability of 70-mm body fans allows them to be used for high-volume
thermal solutions. AMD recommends using 70-mm body fans for the AMD Athlon 64 and
AMD Opteron and processors.
Because the thermal solutions are used on a variety of motherboard designs with the fan power
connector in different locations, AMD recommends a minimum fan power cable length of 200
mm. This length should allow the thermal solution to be mounted in any orientation while
maintaining the ability to power the fan.
Because the thermal solution must meet the processor thermal requirements at elevated ambient
temperatures, a single fan speed solution results in a high fan speed to meet these requirements.
Acoustic noise is a concern for both system builders and AMD. Systems with high levels of
acoustic noise can generate customer complaints and can influence the acceptance of a product.
Therefore, AMD recommends using fans that have speed control implemented on the fan.
Usually, a thermistor is used to sense the airflow temperature at the fan to determine the fan speed.
A fan with variable speed capability can meet the thermal requirement at multiple ambient
conditions.
The significant temperatures for the AMD Athlon 64 and AMD Opteron processor systems are
32°C and 42°C. These temperatures correspond to room temperatures of 25°C and 35°C,
respectively, with an additional 7°C allowed for the temperature rise from the system external
temperature to the system internal temperature at the processor location.
At 25°C (normal room temperature), the thermal solution has an additional 10°C added to to the
system thermal budget. The required thermal resistance increases, for example, from 0.31°C/W to
0.40°C/W. While the fan speed and heatsink performance are closely coupled, the fan speed can
be significantly reduced with the additional 10°C in the thermal budget.
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AMD Athlon™ 64 and AMD Opteron™ Processors Thermal Requirements Chapter 2