MPC5634M datasheet(69/132 Pages) FREESCALE

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MPC5634M Microcontroller Data Sheet, Rev. 8

Freescale Semiconductor

4.3.1

General notes for specifications at maximum junction temperature

An estimation of the chip junction temperature, TJ, can be obtained from the equation:

TJ = TA + (RJA * PD)

Eqn. 1

where:

TA = ambient temperature for the package (oC)

RJA = junction-to-ambient thermal resistance (oC/W)

PD = power dissipation in the package (W)

The thermal resistance values used are based on the JEDEC JESD51 series of standards to provide consistent values for

estimations and comparisons. The difference between the values determined for the single-layer (1s) board compared to a

four-layer board that has two signal layers, a power and a ground plane (2s2p), demonstrate that the effective thermal resistance

is not a constant. The thermal resistance depends on the:

•

Construction of the application board (number of planes)

•

Effective size of the board which cools the component

•

Quality of the thermal and electrical connections to the planes

•

Power dissipated by adjacent components

Connect all the ground and power balls to the respective planes with one via per ball. Using fewer vias to connect the package

to the planes reduces the thermal performance. Thinner planes also reduce the thermal performance. When the clearance

between the vias leave the planes virtually disconnected, the thermal performance is also greatly reduced.

As a general rule, the value obtained on a single-layer board is within the normal range for the tightly packed printed circuit

board. The value obtained on a board with the internal planes is usually within the normal range if the application board has:

•

One oz. (35 micron nominal thickness) internal planes

•

Components are well separated

•

Overall power dissipation on the board is less than 0.02 W/cm2

The thermal performance of any component depends on the power dissipation of the surrounding components. In addition, the

ambient temperature varies widely within the application. For many natural convection and especially closed box applications,

the board temperature at the perimeter (edge) of the package is approximately the same as the local air temperature near the

device. Specifying the local ambient conditions explicitly as the board temperature provides a more precise description of the

local ambient conditions that determine the temperature of the device.

At a known board temperature, the junction temperature is estimated using the following equation:

TJ = TB + (RJB * PD)

Eqn. 2

where:

TB = board temperature for the package perimeter (oC)

RJB = junction-to-board thermal resistance (oC/W) per JESD51-8S

PD = power dissipation in the package (W)

When the heat loss from the package case to the air does not factor into the calculation, an acceptable value for the junction

temperature is predictable. Ensure the application board is similar to the thermal test condition, with the component soldered to

a board with internal planes.

7 Thermal characterization parameter indicating the temperature difference between package top and the junction

temperature per JEDEC JESD51-2. When Greek letters are not available, the thermal characterization parameter

is written as Psi-JT.

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