Thermal Design Considerations
Part 12 Design Considerations
12.1 Thermal Design Considerations
An estimation of the chip junction temperature, T , can be obtained from the equation:
J
T = T + (R
θJΑ x P )
D
J
A
where:
o
T
R
= Ambient temperature for the package ( C)
A
o
= Junction-to-ambient thermal resistance ( C/W)
θJΑ
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. Unfortunately, there are two values in common usage: the value
determined on a single-layer board and the value obtained on a board with two planes. For packages such
as the PBGA, these values can be different by a factor of two. Which value is closer to the application
depends on the power dissipated by other components on the board. The value obtained on a single-layer
board is appropriate for the 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.
When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-case thermal
resistance and a case-to-ambient thermal resistance:
R
R
R
θJΑ = θJC + θCΑ
where:
R
R
R
= Package junction-to-ambient thermal resistance °C/W
= Package junction-to-case thermal resistance °C/W
= Package case-to-ambient thermal resistance °C/W
θJA
θJC
θCA
R θJC is device-related and cannot be influenced by the user. The user controls the thermal environment to
change the case-to-ambient thermal resistance, R θCA . For instance, the user can change the size of the heat
sink, the air flow around the device, the interface material, the mounting arrangement on printed circuit
board, or change the thermal dissipation on the printed circuit board surrounding the device.
To determine the junction temperature of the device in the application when heat sinks are not used, the
Thermal Characterization Parameter (Ψ ) can be used to determine the junction temperature with a
JT
measurement of the temperature at the top center of the package case using the following equation:
T = T + (Ψ x P )
J
T
JT
D
where:
o
T
Ψ
= Thermocouple temperature on top of package ( C)
= Thermal characterization parameter ( C)/W
T
o
JT
P
= Power dissipation in package (W)
D
56F8367 Technical Data, Rev. 9
Freescale Semiconductor
Preliminary
177
FREESCALE [ Freescale ]
