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Design Considerations
This section describes various areas to consider when incorporating the DSP56311 device into a system design.
4.1 Thermal Design Considerations
An estimate of the chip junction temperature, T
Equation 1:
T
Where:
T
=
A
R
=
θJA
P
=
D
Historically, thermal resistance has been expressed as the sum of a junction-to-case thermal resistance and a case-
to-ambient thermal resistance, as in this equation:
Equation 2:
Where:
R
=
θJA
R
=
θJC
R
=
θCA
R
is device-related and cannot be influenced by the user. The user controls the thermal environment to change
θJC
the case-to-ambient thermal resistance, R
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 percent 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 estimates obtained from R
performance is adequate, a system-level model may be appropriate.
A complicating factor is the existence of three common ways to determine the junction-to-case thermal resistance
in plastic packages.
Freescale Semiconductor
, in ° C can be obtained from this equation:
J
(
×
)
=
T
+
P
R
θJ A
J
A
D
ambient temperature °C
package junction-to-ambient thermal resistance °C/W
power dissipation in package
R
=
R
+
R
θJA
θJC
θCA
package junction-to-ambient thermal resistance °C/W
package junction-to-case thermal resistance °C/W
package case-to-ambient thermal resistance °C/W
. For example, the user can change the air flow around the device, add
θCA
DSP56311 Technical Data, Rev. 8
do not satisfactorily answer whether the thermal
θJA
4
4-1
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