MPC8377E datasheet(113/124 Pages) FREESCALE

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MPC8377E PowerQUICC™ II Pro Processor Hardware Specifications, Rev. 2

Freescale Semiconductor

113

Thermal

The thermal performance of a device cannot be adequately predicted from the junction to ambient thermal

resistance. The thermal performance of any component is strongly dependent on the power dissipation of

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 = TA + (RθJB × PD)

where:

TA = ambient temperature for the package (°C)

RθJB = junction to board thermal resistance (°C/W) per JESD51-8

PD = power dissipation in the package (W)

When the heat loss from the package case to the air can be ignored, acceptable predictions of junction

temperature can be made. The application board should be similar to the thermal test condition: the

component is soldered to a board with internal planes.

24.2.3

Experimental Determination of Junction Temperature

NOTE

The heat sink cannot be mounted on the package.

To determine the junction temperature of the device in the application after prototypes are available, use

the thermal characterization parameter (

JT) to determine the junction temperature and a measure of the

temperature at the top center of the package case using the following equation:

TJ = TT + (ΨJT × PD)

where:

TJ = junction temperature (°C)

TT = thermocouple temperature on top of package (°C)

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.

24.2.4

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

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