MPC8378E datasheet(116/127 Pages) FREESCALE

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MPC8378E PowerQUICC II Pro Processor Hardware Specifications, Rev. 4

116

Freescale Semiconductor

Thermal

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. Generally, the value obtained on a single layer board is appropriate for

a 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. Test cases have

demonstrated that errors of a factor of two (in the quantity TJ – TA) are possible.

23.2.2

Estimation of Junction Temperature with Junction-to-Board

Thermal Resistance

NOTE

The heat sink cannot be mounted on the package.

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.

23.2.3

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)

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