PXD10 datasheet(63/130 Pages) FREESCALE

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Electrical characteristics

PXD10 Microcontroller Data Sheet, Rev. 1

Freescale Semiconductor

3.5.1

General notes for specifications at maximum junction temperature

An estimate of the chip junction temperature, TJ, can be obtained from Equation 1:

TJ = TA + (RJA  PD)

Eqn. 1

where:

= ambient temperature for the package (°C)

RJA = junction to ambient thermal resistance (°C/W)

= power dissipation in the package (W)

The thermal resistance values used are based on the JEDEC JESD51 series of standards to provide

consistent values for estimations and comparisons. The difference between the values determined for the

single-layer (1s) board compared to a four-layer board that has two signal layers, a power and a ground

plane (2s2p), demonstrate that the effective thermal resistance is not a constant. The thermal resistance

depends on the:

•

Construction of the application board (number of planes)

•

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.

3 Junction-to-case at the top of the package determined using MIL-STD 883 Method 1012.1. The cold plate

temperature is used for the case temperature. Reported value includes the thermal resistance of the interface layer.

4 Thermal characterization parameter indicating the temperature difference between the package top and the

junction temperature per JEDEC JESD51-2. When Greek letters are not available, the thermal characterization

parameter is written as Psi-JT.

PXD10 Datasheet(PDF) 63 Page - Freescale Semiconductor, Inc

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