MPC885ZP80 datasheet(13/87 Pages) NXP

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MPC885/MPC880 PowerQUICC Hardware Specifications, Rev. 7

Freescale Semiconductor

Thermal Calculation and Measurement

7.2

Estimation with Junction-to-Case Thermal Resistance

Historically, 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 4.

Figure 4. Effect of Board Temperature Rise on Thermal Behavior

MPC885ZP80 データシート(PDF) 13 Page - NXP Semiconductors