MPC852T datasheet(10/80 Pages) FREESCALE

10 / 80 page

MPC852T PowerQUICC™ Hardware Specifications, Rev. 4

Freescale Semiconductor

Thermal Calculation and Measurement

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. Thermal performance of most plastic packages and

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

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