Design Considerations
4
4.1 Thermal Design Considerations
An estimate of the chip junction temperature, T , in °C can be obtained from this equation:
J
Equation 1: TJ = TA + (PD × RθJA
)
Where:
T
=
=
=
ambient temperature °C
A
R
package junction-to-ambient thermal resistance °C/W
power dissipation in package
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: RθJA = RθJC + RθCA
Where:
R
R
R
=
=
=
package junction-to-ambient thermal resistance °C/W
package junction-to-case thermal resistance °C/W
package case-to-ambient thermal resistance °C/W
θJA
θJC
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
. For example, the user can change the air flow around the device, add
θCA
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
do not satisfactorily answer whether the thermal
θJA
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.
•
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 the point at which the leads attach to the case.
DSP56301 Technical Data, Rev. 10
Freescale Semiconductor
4-1
FREESCALE [ Freescale ]
