MPC8358E_11 FREESCALE [Freescale Semiconductor, Inc], MPC8358E_11 Datasheet - Page 85

• Manufacturer Part Number: MPC8358E_11
• Description: PowerQUICC II Pro Processor Revision 2.1 PBGA Silicon
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

23.2.1
An estimation of the chip junction temperature, T
where:
The junction-to-ambient thermal resistance is an industry standard value that provides a quick and easy
estimation of thermal performance. As a general statement, the value obtained on a single-layer board is
appropriate for a tightly packed printed-circuit board. The value obtained on the board with the internal
planes is usually appropriate if the board has low power dissipation and the components are well separated.
Test cases have demonstrated that errors of a factor of two (in the quantity T
23.2.2
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 will be 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:
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.
Freescale Semiconductor
T
T
R
P
T
T
R
P
MPC8358E PowerQUICC II Pro Processor Revision 2.1 PBGA Silicon Hardware Specifications, Rev. 3
A
J
J
B
θ
D
θ
D
JA
JA
= junction temperature (°C)
= junction temperature (°C)
= ambient temperature for the package (°C)
= board temperature at the package perimeter (°C)
= power dissipation in the package (W)
= power dissipation in the package (W)
Estimation of Junction Temperature with Junction-to-Ambient
Thermal Resistance
Estimation of Junction Temperature with Junction-to-Board
Thermal Resistance
= junction-to-ambient thermal resistance (°C/W)
= junction to board thermal resistance (°C/W) per JESD51-8
T
T
J
J
= T
= T
A
B
+ (R
+ (R
θ
θ
JA
JB
× P
× P
D
D
)
)
J
, can be obtained from the equation:
J
– T
A
) are possible.
Thermal
85