GXM Datasheet, PDF (197/244 Pages) National Semiconductor (TI) – Geode™ GXm Processor Integrated x86 Solution with MMX Support

Package Specifications (Continued)
To choose the proper heatsink, the system designer must
make sure that the calculated θ_CA falls above the curve
(shaded area). The curve itself defines the minimum tem-
perature rise above ambient that the heatsink can main-
tain.
See Figure 8-1 as an example of a particular heatsink
under consideration.
θ_{CA = 45/5 = 9}
50
40
30
θ_{CA = 45/9 = 5}
20
10
0
2
4
6
8
10
Heat Dissipated - Watts
Figure 8-1. Heatsink Example
Example 1
Assume P (max) = 5W and T_A (max) = 40°C.
Therefore:
θ_CA = -^T----^C-----_P^–-----T----^A--
θ_CA = ⁽---8---5----–5-----4---0----⁾
θ_CA = 9
In this case, the heatsink under consideration is more than
adequate since at 5W worst case, it can maintain a 40°C
case temperature rise above ambient (θ_CA = 9) when a
maximum of 45°C (θ_CA = 8) is required.
Example 2
Assume P (max) = 10W and T_A (max) = 40°C.
Therefore:
θ_CA = -^T----^C-----_P^–-----T----^A--
θ_CA
=
(_{---8---5----–-----4---0----})
9
θ_CA = 5
In this case, the heatsink under consideration is NOT ade-
quate to maintain the 45°C case temperature rise above
ambient for a 9W processor.
For more information on thermal design considerations or
heatsink properties, refer to the Product Selection Guide of
any leading vendor of Thermal Engineering solutions.
Revision 3.1
197
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