MPC855T datasheet(12/77 Pages) NXP

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MPC860 PowerQUICC Family Hardware Specifications, Rev. 10

Freescale Semiconductor

Thermal Calculation and Measurement

For the following discussions, PD = (VDD × IDD) + PI/O, where PI/O is the power dissipation of the I/O

drivers.

7.1

Estimation with Junction-to-Ambient Thermal Resistance

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

TJ = TA + (RθJA × PD)

where:

TA = ambient temperature (ºC)

RθJA = package junction-to-ambient thermal resistance (ºC/W)

PD = power dissipation in package

The junction-to-ambient thermal resistance is an industry standard value which provides a quick and easy

estimation of thermal performance. However, the answer is only an estimate; test cases have demonstrated

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

7.2

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 airflow 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.

7.3

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; see Figure 2.

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