MPC855T datasheet(12/76 Pages) MOTOROLA

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MPC860 Family Hardware Speciﬁcations

MOTOROLA

Estimation Using Simulation

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 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.

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)

where:

JT = thermal characterization parameter

TT = thermocouple temperature on top of package

PD = power dissipation in package

The thermal characterization parameter is measured per JEDEC JESD51-2 speciﬁcation

using a 40 gauge type T thermocouple epoxied to the top center of the package case. The

thermocouple should be positioned so that the thermocouple junction rests on the package.

A small amount of epoxy is placed over the thermocouple junction and over about 1 mm of

wire extending from the junction. The thermocouple wire is placed ﬂat against the package

case to avoid measurement errors caused by cooling effects of the thermocouple wire.

MPC855T 데이터시트(PDF) 12 Page - Motorola, Inc