DSP56321 datasheet(61/84 Pages) FREESCALE

61 / 84 page

DSP56321 Technical Data, Rev. 11

Freescale Semiconductor

4-1

Design Considerations

This section describes various areas to consider when incorporating the DSP56321 device into a system design.

4.1 Thermal Design Considerations

An estimate of the chip junction temperature, TJ, in ° C can be obtained from this equation:

Equation 1:

Where:

ambient temperature °C

RθJA

package junction-to-ambient thermal resistance °C/W

power dissipation in package

Historically, thermal resistance has been expressed as the sum of a junction-to-case thermal resistance and a case-

to-ambient thermal resistance, as in this equation:

Equation 2:

Where:

RθJA

package junction-to-ambient thermal resistance °C/W

RθJC

package junction-to-case thermal resistance °C/W

RθCA

package case-to-ambient thermal resistance °C/W

RθJC is device-related and cannot be influenced by the user. The user controls the thermal environment to change

the case-to-ambient thermal resistance, RθCA. For example, the user can change the air flow around the device, add

a heat sink, change the mounting arrangement on the printed circuit board (PCB) or otherwise change the thermal

dissipation capability of the area surrounding the device on a PCB. This model is most useful for ceramic packages

with heat sinks; some 90 percent of the heat flow is dissipated through the case to the heat sink and out to the

ambient environment. For ceramic packages, in situations where the heat flow is split between a path to the case

and an alternate path through the PCB, analysis of the device thermal performance may need the additional

modeling capability of a system-level thermal simulation tool.

The thermal performance of plastic packages is more dependent on the temperature of the PCB to which the

package is mounted. Again, if the estimates obtained from RθJA do not satisfactorily answer whether the thermal

performance is adequate, a system-level model may be appropriate.

A complicating factor is the existence of three common ways to determine the junction-to-case thermal resistance

in plastic packages.

•

To minimize temperature variation across the surface, the thermal resistance is measured from the junction

to the outside surface of the package (case) closest to the chip mounting area when that surface has a

proper heat sink.

RθJA

()

RθJA

RθJC RθCA

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