MPC5553_07 datasheet(7/66 Pages) FREESCALE

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Electrical Characteristics

MPC5553 Microcontroller Data Sheet, Rev. 2.0

Freescale Semiconductor

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Construction of the application board (number of planes)

•

Effective size of the board which cools the component

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Quality of the thermal and electrical connections to the planes

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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

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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)

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)

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