MPC5674F datasheet(23/132 Pages) FREESCALE

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Electrical Characteristics

MPC5674F Microcontroller Data Sheet, Rev. 6

Freescale Semiconductor

4.2.1

General Notes for Specifications at Maximum Junction Temperature

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

TJ =TA + (RθJA * PD)

Eqn. 1

where:

TA = ambient temperature for the package (

oC)

RθJA = junction to ambient thermal resistance (

oC/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, there are two values 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 TEPBGA, 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

Eqn. 2

where:

RθJA = junction to ambient thermal resistance (

oC/W)

RθJC = junction to case thermal resistance (

oC/W)

RθCA = case to ambient thermal resistance (

oC/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 air flow around the device, the

interface material, the mounting arrangement on printed circuit board, or change 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 (

JT) 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 x PD)

Eqn. 3

2 Junction temperature is a function of on-chip power dissipation, package thermal resistance, mounting

site (board) temperature, ambient temperature, air flow, power dissipation of other components on the

board, and board thermal resistance.

3 Per SEMI G38-87 and JEDEC JESD51-2 with the single-layer board horizontal.

4 Per JEDEC JESD51-6 with the board horizontal.

5 Thermal resistance between the die and the printed circuit board per JEDEC JESD51-8. Board

temperature is measured on the top surface of the board near the package.

6 Indicates the average thermal resistance between the die and the case top surface as measured by the

cold plate method (MIL SPEC-883 Method 1012.1) with the cold plate temperature used for the case

temperature.

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

junction temperature per JEDEC JESD51-2.

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