MPC5744P datasheet(66/114 Pages) FREESCALE

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MPC5744P Data Sheet, Rev. 0.3

Preliminary—Subject to Change Without Notice

Electrical characteristics

Freescale Semiconductor

3.4.1

General notes for specifications at maximum junction temperature

An estimation of the chip junction temperature, TJ, can be obtained from Equation 1:

TJ =TA +(RθJA ×PD)

Eqn. 1

where:

= ambient temperature for the package (oC)

RθJA = junction to ambient thermal resistance (

oC/W)

= 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 PBGA, 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 in Equation 2 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 (°C/W)

RθJC = junction to case thermal resistance (°C/W)

RθCA = case to ambient thermal resistance (°C/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 Equation 3:

TJ =TT +(ΨJT ×PD)

Eqn. 3

where:

4 Junction-to-Case at the top of the package determined using MIL-STD 883 Method 1012.1. The cold plate

temperature is used for the case temperature. Reported value includes the thermal resistance of the interface layer.

5 Thermal characterization parameter indicating the temperature difference between the 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.

MPC5744P Datasheet(PDF) 66 Page - Freescale Semiconductor, Inc

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