DSP56600FMSLASHAD datasheet(68/76 Pages) NXP

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Preliminary

4-2

DSP56652 Technical Data Sheet

MOTOROLA

Design Considerations

Heat Dissipation

estimations obtained from RqJA 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 for determining 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.

To define a value approximately equal to a junction-to-board thermal resistance, the

thermal resistance is measured from the junction to where the leads are attached to

the case.

If the temperature of the package case (TT) as determined by a thermocouple, the

thermal resistance is computed using the value obtained by the equation (TJ - TT)/

PD.

As noted above, the junction-to-case thermal resistances quoted in this data sheet are

determined using the first definition. From a practical standpoint, this value is also

suitable for determining the junction temperature from a case thermocouple reading

in forced convection environments. In natural convection, using the junction-to-case

thermal resistance to estimate junction temperature from a thermocouple reading on

the case of the package will estimate a junction temperature slightly hotter than

actual temperature. Hence, the new thermal metric, thermal characterization

parameter or

YJT, has been defined to be (TJ - TT)/PD. This value gives a better

estimate of the junction temperature in natural convection when using the surface

temperature of the package. Remember that surface temperature readings of

packages are subject to significant errors caused by inadequate attachment of the

sensor to the surface and to errors caused by heat loss to the sensor. The

recommended technique is to attach a 40-gauge thermocouple wire and bead to the

top center of the package with thermally conductive epoxy.

Note: Table 2-2

on page 2-2 of this document contains the package thermal values

for this chip.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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