DSP56011 Просмотр технического описания (PDF) - Motorola => Freescale

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DSP56011

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Motorola => Freescale 

Design Considerations

Thermal Design Considerations

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.

P R E L I M I N A R Y 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 ΨJT, 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.

Preliminary Information

4-2

DSP56011 Technical Data Sheet, Rev. 1

MOTOROLA

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