DSP56371D Motorola Inc, DSP56371D Datasheet - Page 34

• Manufacturer Part Number: DSP56371D
• Description: high density CMOS device
• Manufacturer: Motorola Inc

Design Considerations
18.0
18.1
An estimation of the chip junction temperature, T
Historically, thermal resistance has been expressed as the sum of a junction-to-case thermal resistance
and a case-to-ambient thermal resistance.
R
change the case-to-ambient thermal resistance, R
around the device, add a heat sink, change the mounting arrangement on the printed circuit board (PCB),
or otherwise change the thermal dissipation capability of the area surrounding the device on a PCB. 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 PCB, analysis of
the device thermal performance may need the additional modeling capability of a system level thermal
simulation tool.
The thermal performance of plastic packages is more dependent on the temperature of the PCB to which
the package is mounted. Again, if the estimations obtained from R
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.
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
34
θJC
Where: T
Where: R
•
•
•
is device-related and cannot be influenced by the user. The user controls the thermal environment to
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
resistance is computed using the value obtained by the equation
(T
J
– T
Design Considerations
Thermal Design Considerations
T J
R θJA
R
P
R
R
A
D
qJA
θJA
θJC
θCA
T
=
)/P
T A
D
=
= ambient temperature °C
= package junction-to-ambient thermal resistance °C/W
= power dissipation in package W
= package junction-to-ambient thermal resistance °C/W
= package junction-to-case thermal resistance °C/W
= package case-to-ambient thermal resistance °C/W
.
R θJC
+
(
P D
Freescale Semiconductor, Inc.
+
×
For More Information On This Product,
R θCA
R θJA
)
DSP56371 Technical Data
Go to: www.freescale.com
J
, in °C can be obtained from the following equation:
T
θCA
) is determined by a thermocouple, the thermal
. For example, the user can change the air flow
θJA
do not satisfactorily answer whether
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