MPC8378E datasheet(117/127 Pages) FREESCALE

117 / 127 page

MPC8378E PowerQUICC II Pro Processor Hardware Specifications, Rev. 4

Freescale Semiconductor

117

Thermal

JT = junction to ambient thermal resistance (°C/W)

PD = power dissipation in the package (W)

The thermal characterization parameter is measured per the 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.

23.2.4

Heat Sinks and Junction-to-Case Thermal Resistance

For the power values the device is expected to operate at, it is anticipated that a heat sink will be required.

A preliminary estimate of heat sink performance can be obtained from the following first first-cut

approach. 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

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.

This first-cut approach overestimates the heat sink size required, since heat flow through the board is not

accounted for, which can be as much as one-third to one-half of the power generated in the package.

Accurate thermal design requires thermal modeling of the application environment using computational

fluid dynamics software which can model both the conduction cooling through the package and board and

the convection cooling due to the air moving through the application. Simplified thermal models of the

packages can be assembled using the junction-to-case and junction-to-board thermal resistances listed in

the thermal resistance table. More detailed thermal models can be made available on request.

The thermal performance of devices with heat sinks has been simulated with a few commercially available

heat sinks. The heat sink choice is determined by the application environment (temperature, air flow,

adjacent component power dissipation) and the physical space available. Because of the wide variety of

application environments, a single standard heat sink applicable to all cannot be specified.

MPC8378E Datasheet(PDF) 117 Page - Freescale Semiconductor, Inc