MPC857T Freescale Semiconductor, Inc, MPC857T Datasheet - Page 14

• Manufacturer Part Number: MPC857T
• Description: Mpc857t Powerquicc Integrated Communications Processor
• Manufacturer: Freescale Semiconductor, Inc

Thermal Calculation and Measurement
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 vias attaching the thermal balls to the ground
plane.
7.4
When the board temperature is not known, a thermal simulation of the application is needed. The simple
two resistor model can be used with the thermal simulation of the application [2], or a more accurate and
complex model of the package can be used in the thermal simulation.
7.5
To determine the junction temperature of the device in the application after prototypes are available, the
thermal characterization parameter (Ψ
measurement of the temperature at the top center of the package case using the following equation:
where:
The thermal characterization parameter is measured per JESD51-2 specification published by JEDEC
using a 40-gauge type T thermocouple epoxied to the top center of the package case. The thermocouple
should be positioned so that the thermocouple junction rests on the package. A small amount of epoxy is
placed over the thermocouple junction and over about 1 mm of wire extending from the junction. The
thermocouple wire is placed flat against the package case to avoid measurement errors caused by cooling
effects of the thermocouple wire.
14
T
T
J
J
Estimation Using Simulation
Experimental Determination
= T
= T
R
T
P
Ψ
T
P
D
D
B
T
θJB
JT
B
T
= thermocouple temperature on top of package
= board temperature (ºC)
= power dissipation in package
= power dissipation in package
+(Ψ
= thermal characterization parameter
+(R
= junction-to-board thermal resistance (ºC/W)
MPC862/857T/857DSL PowerQUICC™ Family Hardware Specifications, Rev. 3
θJB
JT
x P
x P
D
D
)
)
JT
) can be used to determine the junction temperature with a
Freescale Semiconductor