MPC866P datasheet(13/92 Pages) FREESCALE

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MOTOROLA

MPC866/859 Hardware Speciﬁcations

Thermal Calculation and Measurement

Figure 3. Effect of Board Temperature Rise on Thermal Behavior

If the board temperature is known, an estimate of the junction temperature in the environment can be made

using the following equation:

TJ = TB +(RθJB x PD)

where:

RθJB = junction-to-board thermal resistance (ºC/W)

TB = board temperature ºC

PD = power dissipation in package

If the board temperature is known and the heat loss from the package case to the air can be ignored,

acceptable predictions of junction temperature can be made. For this method to work, the board and board

mounting must be similar to the test board used to determine the junction-to-board thermal resistance,

namely a 2s2p (board with a power and a ground plane) and vias attaching the thermal balls to the ground

plane.

7.4

Estimation Using Simulation

When the board temperature is not known, a thermal simulation of the application is needed. The simple

two-resistor model can be used with the thermal simulation of the application [2], or a more accurate and

complex model of the package can be used in the thermal simulation.

100

2 04 0

6 08 0

Board Temperture Rise Above Ambient Divided by Package

Power

Freescale Semiconductor, Inc.

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