MPC8555CPXAPF datasheet(73/88 Pages) FREESCALE

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MPC8555E PowerQUICC™ III Integrated Communications Processor Hardware Specifications, Rev. 4

Freescale Semiconductor

Thermal

Chanhassen, MN 55317

Internet: www.bergquistcompany.com

Thermagon Inc.

888-246-9050

4707 Detroit Ave.

Cleveland, OH 44102

Internet: www.thermagon.com

16.2.4

Heat Sink Selection Examples

The following section provides a heat sink selection example using one of the commercially available heat

sinks.

16.2.4.1

Case 1

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

TJ = TI + TR + (θJC + θINT + θSA) × PD

where

TJ is the die-junction temperature

TI 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. See Table 4 and Table 5.

During operation the die-junction temperatures (TJ) should be maintained within the range specified in

Table 2. 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 (T

R) may be in the range of 5° to

°C. The thermal resistance of some thermal interface material (θ

INT) may be about 1°C/W. For the

purposes of this example, the

JC value given in Table 49 that includes the thermal grease interface and is

documented in note 4 is used. If a thermal pad is used,

INT must be added.

Assuming a TI of 30C, a TR of 5C, a FC-PBGA package θJC = 0.96, and a power consumption (PD) of

8.0 W, the following expression for TJ is obtained:

Die-junction temperature: TJ = 30°C + 5°C + (0.96°C/W + θSA) × 8.0 W

The heat sink-to-ambient thermal resistance (

SA) versus airflow velocity for a Thermalloy heat sink

#2328B is shown in Figure 46.

Assuming an air velocity of 2 m/s, we have an effective

SA+ of about 3.3C/W, thus

TJ = 30C + 5C + (0.96C/W + 3.3C/W) × 8.0 W,

resulting in a die-junction temperature of approximately 69C which is well within the maximum

operating temperature of the component.

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