MPC5634M datasheet(62/118 Pages) FREESCALE

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MPC5634M Microcontroller Data Sheet, Rev. 4

Preliminary—Subject to Change Without Notice

Electrical characteristics

Freescale Semiconductor

•

Effective size of the board which cools the component

•

Quality of the thermal and electrical connections to the planes

•

Power dissipated by adjacent components

Connect all the ground and power balls to the respective planes with one via per ball. Using fewer vias to connect the package

to the planes reduces the thermal performance. Thinner planes also reduce the thermal performance. When the clearance

between the vias leave the planes virtually disconnected, the thermal performance is also greatly reduced.

As a general rule, the value obtained on a single-layer board is within the normal range for the tightly packed printed circuit

board. The value obtained on a board with the internal planes is usually within the normal range if the application board has:

•

One oz. (35 micron nominal thickness) internal planes

•

Components are well separated

•

Overall power dissipation on the board is less than 0.02 W/cm2

The thermal performance of any component depends on the power dissipation of the surrounding components. In addition, the

ambient temperature varies widely within the application. For many natural convection and especially closed box applications,

the board temperature at the perimeter (edge) of the package is approximately the same as the local air temperature near the

device. Specifying the local ambient conditions explicitly as the board temperature provides a more precise description of the

local ambient conditions that determine the temperature of the device.

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

TJ = TB + (RθJB * PD)

Eqn. 2

where:

TB = board temperature for the package perimeter (oC)

RθJB = junction-to-board thermal resistance (oC/W) per JESD51-8S

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

Eqn. 3

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 s 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.

MPC5634M Datasheet(PDF) 62 Page - Freescale Semiconductor, Inc

MPC5634M Datasheet(HTML) 62 Page - Freescale Semiconductor, Inc

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