MPC8241EC datasheet(51/58 Pages) FREESCALE

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MPC8241 Integrated Processor Hardware Specifications, Rev. 10

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

7.7.3

Heat Sink Usage

An estimation of the chip junction temperature, TJ, 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. For packages such as the PBGA,

these values can be different by a factor of two. 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)

RθJC is device-related and cannot be influenced by the user. The user controls the thermal environment to

change the case-to-ambient thermal resistance, RθCA. For instance, the user can change the size of the heat

sink, the airflow around the device, the interface material, the mounting arrangement on the printed-circuit

board, or the thermal dissipation on the printed-circuit board surrounding the device.

To determine the junction temperature of the device in the application when heat sinks are not used, the

thermal characterization parameter (

ψJT) measures the temperature at the top center of the package case

using the following equation:

TJ = TT + (ψJT × PD)

MPC8241EC Datasheet(PDF) 51 Page - Freescale Semiconductor, Inc

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