MC68181 Datasheet, PDF (24/119 Pages) Freescale Semiconductor, Inc – Advance Information ROAMING FLEX chip SIGNAL PROCESSOR

MC68181
Freescale Semiconductor, Inc.
Thermal Design Considerations
estimations obtained from R_θJA 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 (T_T) is determined by a thermocouple,
the thermal resistance is computed using the value obtained by the equation
(T_J – T_T)/P_D.
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 Ψ_JT, has been defined to be
(T_J – T_T)/P_D. 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.
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MC68181 Technical Data Sheet
MOTOROLA
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