MPC8347ECVRADDB datasheet(86/99 Pages) FREESCALE

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MPC8347EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 12

Freescale Semiconductor

Thermal

20.2.1

Estimation of Junction Temperature with Junction-to-Ambient

Thermal Resistance

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

TJ = TA + (RθJA × PD)

where:

TJ = junction temperature (°C)

TA = ambient temperature for the package (°C)

RθJA = junction-to-ambient thermal resistance (°C/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. 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.

20.2.2

Estimation of Junction Temperature with Junction-to-Board

Thermal Resistance

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 + (RθJA × PD)

where:

TJ = junction temperature (°C)

TA = ambient temperature for the package (°C)

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

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.

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