MPC755 datasheet(48/56 Pages) MOTOROLA

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MPC755 RISC Microprocessor Hardware Specifications, Rev. 6.1

Freescale Semiconductor

System Design Information

Shin-Etsu MicroSi, Inc.

888-642-7674

10028 S. 51st St.

Phoenix, AZ 85044

Internet: www.microsi.com

Thermagon Inc.

888-246-9050

4707 Detroit Ave.

Cleveland, OH 44102

Internet: www.thermagon.com

8.8.3 Heat Sink Selection Example

This section provides a heat sink selection example using one of the commercially-available heat sinks. For

preliminary heat sink sizing, the die-junction temperature can be expressed as follows:

Tj = Ta + Tr + (θjc + θint + θsa) × Pd

where:

Tj is the die-junction temperature

Ta is the inlet cabinet ambient temperature

Tr is the air temperature rise within the computer cabinet

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

Pd is the power dissipated by the device

During operation the die-junction temperatures (Tj) should be maintained less than the value specified in Table 3.

The temperature of air cooling the component greatly depends on the ambient inlet air temperature and the air

temperature rise within the electronic cabinet. An electronic cabinet inlet-air temperature (Ta) may range from 30°

to 40°C. The air temperature rise within a cabinet (Tr) 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 Ta of 30°C, a Tr of 5°C, a CBGA package

Rθjc < 0.1, and a power consumption (Pd) of 5.0 W, the following expression for Tj is obtained:

Die-junction temperature:

Tj = 30°C + 5°C + (0.1°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 28.

Assuming an air velocity of 0.5 m/s, we have an effective Rsa of 7°C/W, thus

Tj = 30°C + 5°C + (0.1°C/W + 1.0°C/W + 7°C/W) × 5.0 W,

resulting in a die-junction temperature of approximately 76°C which is well within the maximum operating

temperature of the component.

Other heat sinks offered by Aavid Thermalloy, Alpha Novatech, The Bergquist Company, IERC, Chip Coolers, and

Wakefield Engineering offer different heat sink-to-ambient thermal resistances, and may or may not need airflow.

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