DSP96002UMSLASHAD datasheet(84/111 Pages) NXP

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Design Considerations

Thermal Design Considerations

4-2

DSP96002/D, Rev. 2

MOTOROLA

The thermal performance of plastic packages is more dependent on the

temperature of the printed circuit board to which the package is mounted. Again,

if the estimations 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 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) is 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, that 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

JT, 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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

DSP96002UM/AD Datasheet(PDF) 84 Page - NXP Semiconductors

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