MPC8245 datasheet(54/68 Pages) FREESCALE

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MPC8245 Integrated Processor Hardware Specifications, Rev. 10

Freescale Semiconductor

System Design

Dow-Corning Corporation

800-248-2481

Dow-Corning Electronic Materials

2200 W. Salzburg Rd.

Midland, MI 48686-0997

Internet: www.dow.com

Shin-Etsu MicroSi, Inc.

888-642-7674

10028 S. 51st St.

Phoenix, AZ 85044

Internet: www.microsi.com

The Bergquist Company

800-347-4572

18930 West 78th St.

Chanhassen, MN 55317

Internet: www.bergquistcompany.com

Thermagon Inc.

888-246-9050

4707 Detroit Ave.

Cleveland, OH 44102

Internet: www.thermagon.com

7.8.3

Heat Sink Usage

An estimation of the chip junction temperature, T

J, can be obtained from the equation:

TJ = TA + (RθJA × PD)

where

TA = ambient temperature for the package (°C)

RθJA = junction-to-ambient thermal resistance (°C/W)

PD = power dissipation in the package (W)

The junction-to-ambient thermal resistance is an industry-standard value that provides a quick and easy

estimation of thermal performance. Unfortunately, two values are in common usage: the value determined

on a single-layer board and the value obtained on a board with two planes. Which value is closer to the

application depends on the power dissipated by other components on the board. The value obtained on a

single-layer board is appropriate for the tightly packed printed-circuit board. The value obtained on the

board with the internal planes is usually appropriate if the board has low power dissipation and the

components are well separated.

When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-case thermal

resistance and a case-to-ambient thermal resistance:

RθJA = RθJC + RθCA

where

RθJA = junction-to-ambient thermal resistance (°C/W)

RθJC = junction-to-case thermal resistance (°C/W)

RθCA = case-to-ambient thermal resistance (°C/W)

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