Thermal
20.2.1 Estimation of Junction Temperature with Junction-to-Ambient
Thermal Resistance
An estimation of the chip junction temperature, T , can be obtained from the equation:
J
T = T + (R
× P )
D
J
A
θJA
where:
T = junction temperature (°C)
J
T = ambient temperature for the package (°C)
A
R
= junction-to-ambient thermal resistance (°C/W)
θJA
P = power dissipation in the package (W)
D
The junction-to-ambient thermal resistance is an industry-standard value that provides a quick and easy
estimation of thermal performance. Generally, 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 – T ) are possible.
J
A
20.2.2 Estimation of Junction Temperature with Junction-to-Board
Thermal Resistance
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:
T = T + (R
× P )
D
J
A
θJA
where:
T = junction temperature (°C)
J
T = ambient temperature for the package (°C)
A
R
= junction-to-ambient thermal resistance (°C/W)
θJA
P = power dissipation in the package (W)
D
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.
MPC8347EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 12
86
Freescale Semiconductor
FREESCALE [ Freescale ]
