MPC8248 Freescale Semiconductor, Inc, MPC8248 Datasheet - Page 12

• Manufacturer Part Number: MPC8248
• Description: Mpc8248 Powerquicc Ii Integrated Communications Processor
• Manufacturer: Freescale Semiconductor, Inc

Thermal Characteristics
4.2
Historically, the thermal resistance has frequently been expressed as the sum of a junction-to-case thermal
resistance and a case-to-ambient thermal resistance:
where:
R
affect the case-to-ambient thermal resistance, R
the device, add a heat sink, change the mounting arrangement on the printed circuit board, or change the
thermal dissipation on the printed circuit board surrounding the device. This thermal model is most useful
for ceramic packages with heat sinks where some 90% of the heat flows through the case and the heat sink
to the ambient environment. For most packages, a better model is required.
4.3
A simple package thermal model which has demonstrated reasonable accuracy (about 20%) is a
two-resistor model consisting of a junction-to-board and a junction-to-case thermal resistance. The
junction-to-case thermal resistance covers the situation where a heat sink is used or where a substantial
amount of heat is dissipated from the top of the package. The junction-to-board thermal resistance
describes the thermal performance when most of the heat is conducted to the printed circuit board. It has
been observed that the thermal performance of most plastic packages, especially PBGA packages, is
strongly dependent on the board temperature.
If the board temperature is known, an estimate of the junction temperature in the environment can be made
using the following equation:
where:
If the board temperature is known and the heat loss from the package case to the air can be ignored,
acceptable predictions of junction temperature can be made. For this method to work, the board and board
mounting must be similar to the test board used to determine the junction-to-board thermal resistance,
namely a 2s2p (board with a power and a ground plane) and by attaching the thermal balls to the ground
plane.
12
θJC
is device related and cannot be influenced by the user. The user adjusts the thermal environment to
R
R
R
R
T
P
D
B
θJA
θJC
θCA
θJB
Estimation with Junction-to-Case Thermal Resistance
Estimation with Junction-to-Board Thermal Resistance
= power dissipation in package
= board temperature (ºC)
= junction-to-case thermal resistance (ºC/W)
= junction-to-board thermal resistance (ºC/W)
= junction-to-ambient thermal resistance (ºC/W)
= case-to-ambient thermal resistance (ºC/W)
R
T
J
θJA
= T
= R
B
MPC8272 PowerQUICC II™ Family Hardware Specifications, Rev. 1.5
+ (R
θJC
θJB
+ R
θCA
× P
D
)
θCA
. For instance, the user can change the air flow around
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