dsp56853 Freescale Semiconductor, Inc, dsp56853 Datasheet - Page 54

• Manufacturer Part Number: dsp56853
• Description: 16-bit Digital Signal Controllers
• Manufacturer: Freescale Semiconductor, Inc

Part 6 Design Considerations
6.1 Thermal Design Considerations
An estimation of the chip junction temperature, T
Where:
Historically, thermal resistance has been expressed as the sum of a junction-to-case thermal resistance and a
case-to-ambient thermal resistance:
Where:
R
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 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
performance is adequate, a system level model may be appropriate.
A complicating factor is the existence of three common definitions 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
54
θJC
•
•
•
is device-related and cannot be influenced by the user. The user controls the thermal environment to change
Equation 1:
T
R
P
Equation 2:
R
R
R
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
determined by a thermocouple.
A
D
θJA
θJA
θJC
θCA
= ambient temperature °C
= power dissipation in package
= package junction-to-case thermal resistance °C/W
= package junction-to-ambient thermal resistance °C/W
= package junction-to-ambient thermal resistance °C/W
= package case-to-ambient thermal resistance °C/W
T
R
J
θJA
= T
= R
A
+ (P
θJC
D
+ R
x R
θCA
θCA
θJA
56853 Technical Data, Rev. 6
. For example, the user can change the air flow around the device, add
)
J
J
, in °C can be obtained from the equation:
– T
T
)/P
θJA
D
where T
do not satisfactorily answer whether the thermal
T
is the temperature of the package case
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