MPC866PCZP100A datasheet(12/96 Pages) FREESCALE

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MPC866/MPC859 Hardware Specifications, Rev. 2

Freescale Semiconductor

Thermal Calculation and Measurement

For the following discussions, PD = (VDDL x IDDL) + PI/O, where PI/O is the power dissipation of the I/O drivers.

The VDDSYN power dissipation is negligible.

7.1

Estimation with Junction-to-Ambient Thermal Resistance

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

TJ = TA +(RθJA x 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 that 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 that 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 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 and especially

PBGA packages is strongly dependent on the board temperature; see Figure 3.

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