DSP56004 datasheet(75/82 Pages) FREESCALE

75 / 82 page

MOTOROLA

DSP56004/D, Rev. 3

4-1

SECTION

DESIGN CONSIDERATIONS

THERMAL DESIGN CONSIDERATIONS

An estimation of the chip junction temperature, TJ, in °C can be obtained from the

equation:

Equation 1:

Where:

TA = ambient temperature ˚C

RθJA = package junction-to-ambient thermal resistance ˚C/W

PD = 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:

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, or otherwise change the

thermal dissipation capability of the area surrounding the device on a Printed Circuit

Board. This model is most useful for ceramic packages with heat sinks; some 90% 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 Printed Circuit Board, analysis of

the device thermal performance may need the additional modeling capability of a

system level thermal simulation tool.

RθJA

()

RθJA

RθJC RθCA

Freescale Semiconductor, Inc.

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DSP56004 Datasheet(PDF) 75 Page - Freescale Semiconductor, Inc