mpc5645s Freescale Semiconductor, Inc, mpc5645s Datasheet - Page 69

• Manufacturer Part Number: mpc5645s
• Description: Mpc5645s Microcontroller Data Sheet
• Manufacturer: Freescale Semiconductor, Inc

The thermal resistance values used are based on the JEDEC JESD51 series of standards to provide consistent values for
estimations and comparisons. The difference between the values determined for the single-layer (1s) board compared to a
four-layer board that has two signal layers, a power and a ground plane (2s2p), demonstrate that the effective thermal resistance
is not a constant. The thermal resistance depends on the:
Connect all the ground and power balls to the respective planes with one via per ball. Using fewer vias to connect the package
to the planes reduces the thermal performance. Thinner planes also reduce the thermal performance. When the clearance
between the vias leave the planes virtually disconnected, the thermal performance is also greatly reduced.
As a general rule, the value obtained on a single-layer board is within the normal range for the tightly packed printed circuit
board. The value obtained on a board with the internal planes is usually within the normal range if the application board has:
The thermal performance of any component depends on the power dissipation of the surrounding components. In addition, the
ambient temperature varies widely within the application. For many natural convection and especially closed box applications,
the board temperature at the perimeter (edge) of the package is approximately the same as the local air temperature near the
device. Specifying the local ambient conditions explicitly as the board temperature provides a more precise description of the
local ambient conditions that determine the temperature of the device.
At a known board temperature, the junction temperature is estimated using the following equation:
where:
T
R
P
When the heat loss from the package case to the air does not factor into the calculation, an acceptable value for the junction
temperature is predictable. Ensure the application board is similar to the thermal test condition, with the component soldered to
a board with internal planes.
The thermal resistance is expressed as the sum of a junction-to-case thermal resistance plus a case-to-ambient thermal
resistance:
where:
R
R
R
R
case-to-ambient thermal resistance, R
mounting arrangement on the printed circuit board, or change the thermal dissipation on the printed circuit board surrounding
the device. This description is most useful for packages with heat sinks where 90% of the heat flow is through the case to heat
sink to ambient. For most packages, a better model is required.
Freescale Semiconductor
D
B
JB
JA
JC
CA
JC
= board temperature for the package perimeter (
= power dissipation in the package (W)
•
•
•
•
•
•
•
= junction-to-board thermal resistance (
= junction to case thermal resistance (
s device related and is not affected by other factors. The thermal environment can be controlled to change the
= junction to ambient thermal resistance (
= case to ambient thermal resistance (
Construction of the application board (number of planes)
Effective size of the board which cools the component
Quality of the thermal and electrical connections to the planes
Power dissipated by adjacent components
One oz. (35 micron nominal thickness) internal planes
Components are well separated
Overall power dissipation on the board is less than 0.02 W/cm2
CA
MPC5645S Microcontroller Data Sheet, Rev. 6
. For example, change the air flow around the device, add a heat sink, change the
o
o
C/W)
C/W)
o
C/W) per JESD51-8S
o
T
R
C/W)
J
JA
= T
o
C)
= R
B
+ (R
JC
JB
+ R
* P
CA
D
)
Electrical characteristics
Eqn. 2
Eqn. 3
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