MPC855T datasheet(13/80 Pages) FREESCALE

13 / 80 page

MPC860 PowerQUICC™ Family Hardware Specifications, Rev. 8

Freescale Semiconductor

Thermal Calculation and Measurement

Figure 2. 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 × 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 by 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.

7.5

Experimental Determination

To determine the junction temperature of the device in the application after prototypes are available, the

thermal characterization parameter (

ΨJT) can be used to determine the junction temperature with a

measurement of the temperature at the top center of the package case using the following equation:

TJ = TT + (ΨJT × PD)

Board Temperature Rise Above Ambient Divided by Package Power

MPC855T 数据表(PDF) 13 Page - Freescale Semiconductor, Inc