Thermal Design Considerations
Part 5 Design Considerations
5.1 Thermal Design Considerations
An estimation of the chip junction temperature, T , in °C can be obtained from the equation:
J
Equation 1:TJ = TA + (PD × RθJA
)
Where:
TA = ambient temperature °C
RθJA = package junction-to-ambient thermal resistance °C/W
PD = 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:RθJA = RθJC + RθCA
Where:
RθJA = package junction-to-ambient thermal resistance °C/W
RθJC = package junction-to-case thermal resistance °C/W
RθCA = package case-to-ambient thermal resistance °C/W
R
is device-related and cannot be influenced by the user. The user controls the thermal environment to
θJC
change the case-to-ambient thermal resistance, R
. For example, the user can change the air flow around
θCA
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
the thermal performance is adequate, a system level model may be appropriate.
do not satisfactorily answer whether
θJA
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.
56F801 Technical Data, Rev. 17
Freescale Semiconductor
43
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