DD 2.X
Preliminary
PowerPC 750FX RISC Microprocessor
5.8.3 Minimum Heat Sink Requirements
The worst-case power dissipation (PD) for the 750FX is shown in Table 3-5. A conservative thermal manage-
ment design will provide sufficient cooling to maintain the junction temperature (T ) of the 750FX below 105C
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at maximum PD and worst-case ambient temperature and airflow conditions.
Many factors affect the 750FX power dissipation, including V , T , core frequency, process factors, and the
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code that is running on the processor. In general, PD increases with increases in T , V , Fcore, process
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variables, and the number of instructions executed per second.
For various reasons, a designer may determine that the power dissipation of the 750FX in their application
will be less than the maximum value shown in the Datasheet. Assuming a lower PD will result in a thermal
management system with less cooling capacity than would be required for the maximum Datasheet PD. In
this case, the designer may decide to determine the actual maximum 750FX PD in the particular application.
Contact your IBM PowerPC FAE for more information.
In addition to the system factors that must be considered in a cooling system analysis, three things should be
noted. First, 750FX PD rises as T increases, so it is most useful to measure PD while the 750FX junction
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temperature is at maximum. While not specified or guaranteed, this rise in PD with T is typically less than 1W
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per 10C. So regardless of other factors, the minimum cooling solution must have a maximum temperature
rise of no more than 10C/W.
This minimum cooling solution is not generally achievable without a heat sink. A heat sink or heat spreader of
some sort must always be used in 750FX applications.
Second, due to process variations, there can be a significant variation in the PD of individual 750FX devices.
In addition, IBM will occasionally supply "downbinned" parts. These are faster parts that are shipped in lieu of
the speed that was ordered. For example, some parts that are marked as 600MHz may actually run as fast as
700MHz. These 700MHz parts will dissipate more power at 600MHz than the 600MHz parts. So power dissi-
pation analysis should be conducted using the fastest parts available.
Finally, regardless of methodology, IBM only supports system designs that successfully maintain the
maximum junction temperature within Datasheet limits. IBM also supports designs that rely on the maximum
PD values given in this Datasheet, and supply a cooling solution sufficient to dissipate that amount of power
while keeping the maximum junction temperature below the maximum T .
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Body_750FX_DS_DD2.X.fm.2.0
June 9, 2003
5. System Design Information
Page 53 of 63
IBM [ IBM ]
