Thermal Design Considerations
56F801 Technical Data
39
Part 5 Design Considerations
5.1 Thermal Design Considerations
An estimation of the chip junction temperature, T
J
, in
°
C can be obtained from the equation:
Equation 1:
Where:
T
A
= ambient temperature °C
R
θ
JA
= package junction-to-ambient thermal resistance °C/W
P
D
= power dissipation in package
Historically, thermal resistance has been expressed as the sum of a junction-to-case thermal resistance and
a case-to-ambient thermal resistance:
Equation 2:
Where:
R
θ
JA
= package junction-to-ambient thermal resistance °C/W
R
θ
JC
= package junction-to-case thermal resistance °C/W
R
θ
CA
= package case-to-ambient thermal resistance °C/W
R
θ
JC
is device-related and cannot be influenced by the user. The user controls the thermal environment to
change the case-to-ambient thermal resistance, R
θ
CA
. For example, the user can change the air flow 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 the 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
θ
JA
do not satisfactorily answer whether the
thermal performance is adequate, a system level model may be appropriate.
Definitions:
A complicating factor is the existence of three common definitions for determining the junction-to-case
thermal resistance in plastic packages:
Measure the thermal resistance 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. This is done to minimize
temperature variation across the surface.
Measure the thermal resistance from the junction to where the leads are attached to the case. This
definition is approximately equal to a junction to board thermal resistance.
Use the value obtained by the equation (T
J
– T
T
)/P
D
where T
T
is the temperature of the package
case determined by a thermocouple.
T
J
T
A
P
D
R
θ
JA
×
(
)
+
=
R
θ
JA
R
θ
JC
R
θ
CA
+
=
F
Freescale Semiconductor, Inc.
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Go to: www.freescale.com
n
.