MPC8280 datasheet(13/83 Pages) FREESCALE

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MPC8280 PowerQUICC II Family Hardware Specifications, Rev. 2

Freescale Semiconductor

Thermal Characteristics

4.2

Estimation with Junction-to-Case Thermal Resistance

Historically, the thermal resistance has frequently been 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 (ºC/W)

RθJC = junction-to-case thermal resistance (ºC/W)

RθCA = case-to-ambient thermal resistance (ºC/W)

RθJC is device related and cannot be influenced by the user. The user adjusts the thermal environment to

affect the case-to-ambient thermal resistance, RθCA. For instance, the user can change the air flow around

the device, add a heat sink, change the mounting arrangement on the printed circuit board, or change the

thermal dissipation on the printed circuit board surrounding the device. This thermal model is most useful

for ceramic packages with heat sinks where some 90% of the heat flows through the case and the heat sink

to the ambient environment. For most packages, a better model is required.

4.3

Estimation with Junction-to-Board Thermal Resistance

A simple package thermal model which has demonstrated reasonable accuracy (about 20%) is a

two-resistor model consisting of a junction-to-board and a junction-to-case thermal resistance. The

junction-to-case thermal resistance covers the situation where a heat sink is used or where a substantial

amount of heat is dissipated from the top of the package. The junction-to-board thermal resistance

describes the thermal performance when most of the heat is conducted to the printed circuit board. It has

been observed that the thermal performance of most plastic packages, especially PBGA packages, is

strongly dependent on the board temperature.

If the board temperature is known, an estimate of the junction temperature in the environment can be made

using the following equation:

TJ = TB + (RθJB × PD)

where:

RθJB = junction-to-board thermal resistance (ºC/W)

TB = board temperature (ºC)

PD = power dissipation in package

If the board temperature is known and the heat loss from the package case to the air can be ignored,

acceptable predictions of junction temperature can be made. For this method to work, the board and board

mounting must be similar to the test board used to determine the junction-to-board thermal resistance,

namely a 2s2p (board with a power and a ground plane) and by attaching the thermal balls to the ground

plane.

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