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Revision 1.1

Package Specifications (Continued)

8.1.1

Heatsink Considerations

Table 8-2 shows the maximum allowed thermal resistance of a heatsink for particular operating environments. The calcu-

lated values, defined as

θCA, represent the required ability of a particular heatsink to transfer heat generated by the SC1100

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 8-2, it can be seen that as ambient temperature (TA) increases, θCA decreases, and that as power con-

sumption of 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 effi-

cient as the amount of heat they are trying to dissipate increases. For this reason, heatsinks are typically specified by

graphs that plot heat dissipation (in watts) vs. mounting surface (case) temperature rise above ambient (in

°C). This method

is necessary because ambient and case temperatures fluctuate constantly during normal operation of the system. The sys-

tem 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 7-2 on page 281) 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 temperature rise above ambi-

ent that the heatsink can maintain.

Table 8-2. Case-to-Ambient Thermal Resistance Examples @ 85

°C

Core Voltage

(VCORE)

(Nominal)

Core

Frequency

Maximum

Power (W)

θCA for Different Ambient Temperatures (°C/W)

°C25°C30°C35°C40°C

TBD

300 MHz

TBD

2.0V

266 MHz

2.98

21.84

20.16

18.48

16.8

15.12

1.8V

233 MHz

2.77

23.52

21.71

19.9

18.09

16.28

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