mpc8378 Freescale Semiconductor, Inc, mpc8378 Datasheet - Page 116

• Manufacturer Part Number: mpc8378
• Description: Powerquicc Ii Pro Processor
• Manufacturer: Freescale Semiconductor, Inc

Thermal
The thermal performance of a device cannot be adequately predicted from the junction to ambient thermal
resistance. The thermal performance of any component is strongly dependent on the power dissipation of
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:
When the heat loss from the package case to the air can be ignored, acceptable predictions of junction
temperature can be made. The application board should be similar to the thermal test condition: the
component is soldered to a board with internal planes.
24.2.3
To determine the junction temperature of the device in the application after prototypes are available, use
the thermal characterization parameter (
temperature at the top center of the package case using the following equation:
The thermal characterization parameter is measured per the JESD51-2 specification 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.
24.2.4
For the power values the device is expected to operate at, it is anticipated that a heat sink will be required.
A preliminary estimate of heat sink performance can be obtained from the following first first-cut
116
Experimental Determination of Junction Temperature
Heat Sinks and Junction-to-Case Thermal Resistance
The heat sink cannot be mounted on the package.
T
where:
T
where:
MPC8377E PowerQUICC II Pro Processor Hardware Specifications, Rev. 3
J
J
= T
= T
T
R
P
T
T
Ψ
P
A
T
A
J
T
θ
D
D
JT
+ (R
+ (
JB
= junction temperature (°C)
= ambient temperature for the package (°C)
= thermocouple temperature on top of package (°C)
= power dissipation in the package (W)
= power dissipation in the package (W)
= junction to ambient thermal resistance (°C/W)
Ψ
= junction to board thermal resistance (°C/W) per JESD51-8
θ
JT
JB
× P
× P
D
D
)
)
Ψ
JT
) to determine the junction temperature and a measure of the
NOTE
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