PC755 datasheet(21/50 Pages) ATMEL

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PC755/745

2138D–HIREL–06/03

jc is the junction-to-case thermal resistance

int is the adhesive or interface material thermal resistance

sa is the heat sink base-to-ambient thermal resistance

d is the power dissipated by the device

During operation the die-junction temperatures (T

j) should be maintained less than the

value specified in Table 5. The temperature of the air cooling the component greatly

depends upon the ambient inlet air temperature and the air temperature rise within the

electronic cabinet. An electronic cabinet inlet-air temperature (T

a) may range from 30 to

°C. The air temperature rise within a cabinet (T

r) may be in the range of 5 to 10°C.

The thermal resistance of the thermal interface material (

int) is typically about 1°C/W.

Assuming a T

a of 30°C, a Tr of 5

oC, a CBGA package

jc = 0.03, and a power consump-

tion (P

d) of 5.0 watts, the following expression for Tj is obtained:

Die-junction temperature: T

j = 30°C + 5°C + (0.03°C/W + 1.0°C/W + θsa) * 5.0 W

For a Thermalloy heat sink #2328B, the heat sink-to-ambient thermal resistance (

sa)

versus airflow velocity is shown in Figure 9.

Figure 9. Thermalloy #2328B Heat Sink-to-Ambient Thermal Resistance Versus Air-

flow Velocity

Assuming an air velocity of 0.5 m/s, we have an effective R

sa of 7°C/W, thus

j = 30°C+ 5°C+ (0.03°C/W +1.0°C/W + 7°C/W) * 5.0 W,

resulting in a die-junction temperature of approximately 81

°C which is well within the

maximum operating temperature of the component.

Other heat sinks offered by Chip Coolers, IERC, Thermalloy, Wakefield Engineering,

and Aavid Engineering offer different heat sink-to-ambient thermal resistances, and may

or may not need air flow.

0.5

1.5

2.5

3.5

Thermalloy #2328B Pin±fin Heat Sink

Approach Air Velocity (m/s)

(25 x28 x 15 mm)

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