SC3200 datasheet(440/447 Pages) AMD

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440

AMD Geode™ SC3200 Processor Data Book

Package Specifications

Revision 5.1

10.1.1

Heatsink Considerations

Table 10-2 on page 439 shows the maximum allowed ther-

mal resistance of a heatsink for particular operating envi-

ronments.

The

calculated

values,

defined

θCA,

represent the required ability of a particular heatsink to

transfer heat generated by the SC3200 processor from its

case into the air, thereby maintaining the case temperature

at or below 85

°C. Because θCA is a measure of thermal

resistivity, it is inversely proportional to the heatsinks ability

to dissipate heat or its thermal conductivity.

Note:

A “perfect” heatsink would be able to maintain a

case temperature equal to that of the ambient air

inside the system chassis.

Looking at Table 10-2, it can be seen that as ambient tem-

perature (TA) increases, θCA decreases, and that as power

consumption

the

processor

(P)

increases,

θCA

decreases. Thus, the ability of the heatsink to dissipate

thermal energy must increase as the processor power

increases and as the temperature inside the enclosure

increases.

While

θCA is a useful parameter to calculate, heatsinks are

not typically specified in terms of a single

θCA.This is

because the thermal resistivity of a heatsink is not constant

across power or temperature. In fact, heatsinks become

slightly less efficient as the amount of heat they are trying

to dissipate increases. For this reason, heatsinks are typi-

cally specified by graphs that plot heat dissipation (in

watts) vs. mounting surface (case) temperature rise above

ambient (in

°C). This method is necessary because ambi-

ent and case temperatures fluctuate constantly during nor-

mal operation of the system. The system designer must be

careful to choose the proper heatsink by matching the

required

θCA with the thermal dissipation curve of the

device under the entire range of operating conditions in

order to make sure that the maximum case temperature

(from Table 9-3 on page 370) is never exceeded. To

choose the proper heatsink, the system designer must

make sure that the calculated

θCA falls above the curve

(shaded area). The curve itself defines the minimum tem-

perature rise above ambient that the heatsink can main-

tain.

Figure 10-1 is an example of a particular heatsink under

consideration

Figure 10-1. Heatsink Example

Example 1

Assume P (max) = 5W and TA (max) = 40°C.

Therefore:

CA = 9

The heatsink must provide a thermal resistance below 9

°C/

W. In this case, the heatsink under consideration is more

than adequate since at 5W worst case, it can limit the case

temperature rise above ambient to 40

°C (θ

CA =8).

Example 2

Assume P (max) = 9W and TA (max) = 40°C.

Therefore:

CA = 5

In this case, the heatsink under consideration is NOT ade-

quate to limit the case temperature rise above ambient to

°C 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.

Note:

The power dissipations P used in these examples

are not representative of the power dissipation of

the SC3200 processor, which is always less than 4

Watts.

θCA = 45/9 = 5

Heat Dissipated - Watts

θCA = 45/5 = 9

CA =

TC − TA

CA =

− 40

CA =

TC − TA

CA =

− 40

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