Thermal Calculation and Measurement
7
Thermal Calculation and Measurement
For the following discussions, P
D
= (V
DD
×
I
DD
) + PI/O, where PI/O is the power dissipation of the I/O
drivers.
7.1
Estimation with Junction-to-Ambient Thermal Resistance
An estimation of the chip junction temperature, T
J
, in ºC can be obtained from the equation:
T
J
= T
A
+ (R
θJA
×
P
D
)
where:
T
A
= ambient temperature (ºC)
R
θJA
= package junction-to-ambient thermal resistance (ºC/W)
P
D
= power dissipation in package
The junction-to-ambient thermal resistance is an industry standard value which provides a quick and easy
estimation of thermal performance. However, the answer is only an estimate; test cases have demonstrated
that errors of a factor of two (in the quantity T
J
–
T
A
) are possible.
7.2
Estimation with Junction-to-Case Thermal Resistance
Historically, the thermal resistance has frequently been expressed as the sum of a junction-to-case thermal
resistance and a case-to-ambient thermal resistance:
R
θJA
= R
θJC
+ R
θCA
where:
R
θJA
= junction-to-ambient thermal resistance (ºC/W)
R
θJC
= junction-to-case thermal resistance (ºC/W)
R
θCA
= case-to-ambient thermal resistance (ºC/W)
R
θJC
is device related and cannot be influenced by the user. The user adjusts the thermal environment to
affect the case-to-ambient thermal resistance, R
θCA
. For instance, the user can change the airflow around
the device, add a heat sink, change the mounting arrangement on the printed-circuit board, or change the
thermal dissipation on the printed-circuit board surrounding the device. This thermal model is most useful
for ceramic packages with heat sinks where some 90% of the heat flows through the case and the heat sink
to the ambient environment. For most packages, a better model is required.
7.3
Estimation with Junction-to-Board Thermal Resistance
A simple package thermal model which has demonstrated reasonable accuracy (about 20%) is a
two-resistor model consisting of a junction-to-board and a junction-to-case thermal resistance. The
junction-to-case thermal resistance covers the situation where a heat sink is used or where a substantial
amount of heat is dissipated from the top of the package. The junction-to-board thermal resistance
describes the thermal performance when most of the heat is conducted to the printed-circuit board. It has
been observed that the thermal performance of most plastic packages, especially PBGA packages, is
strongly dependent on the board temperature; see
MPC860 PowerQUICC™ Family Hardware Specifications, Rev. 8
12
Freescale Semiconductor
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
