MPC5565 datasheet(7/54 Pages) FREESCALE

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Electrical Characteristics

MPC5565 Microcontroller Data Sheet, Rev. 2.0

Freescale Semiconductor

At a known board temperature, the junction temperature is estimated using the following equation:

TJ = TB + (RθJB × PD)

where:

TJ = junction temperature (

oC)

TB = board temperature at the package perimeter (

oC/W)

RθJB = junction-to-board thermal resistance (

oC/W) per JESD51-8

PD = power dissipation in the package (W)

When the heat loss from the package case to the air does not factor into the calculation, an acceptable value

for the junction temperature is predictable. Ensure the application board is similar to the thermal test

condition, with the component soldered to a board with internal planes.

The thermal resistance is expressed as the sum of a junction-to-case thermal resistance plus 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 is not affected by other factors. The thermal environment can be controlled to

change the case-to-ambient thermal resistance, RθCA. For example, 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 description is most useful for

packages with heat sinks where 90% of the heat flow is through the case to heat sink to ambient.

For most packages, a better model is required.

A more accurate two-resistor thermal model can be constructed from the junction-to-board thermal

resistance and the junction-to-case thermal resistance. The junction-to-case thermal resistance describes

when using a heat sink 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. This model can be used to generate simple estimations and for

computational fluid dynamics (CFD) thermal models.

To determine the junction temperature of the device in the application on a prototype board, use the

thermal characterization parameter (

JT) to determine the junction temperature by measuring the

temperature at the top center of the package case using the following equation:

TJ = TT + (ΨJT × PD)

where:

TT = thermocouple temperature on top of the package (

oC)

JT = thermal characterization parameter (

oC/W)

PD = power dissipation in the package (W)

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