PXD20 Datasheet, PDF (69/130 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

Electrical characteristics
At a known board temperature, the junction temperature is estimated using the following equation:
T_J = T_B + (R_JB * P_D)
Eqn. 2
where:
T_B= board temperature for the package perimeter (^oC)
R_JB= junction-to-board thermal resistance (^oC/W) per JESD51-8S
P_D= power dissipation in the package (W)
When the heat loss from the package case to the air does not factor into the calculation, an acceptable value for the junction
temperature is predictable. Ensure the application board is similar to the thermal test condition, with the component soldered to
a board with internal planes.
The thermal resistance is expressed as the sum of a junction-to-case thermal resistance plus a case-to-ambient thermal
resistance:
R_JA = R_JC + R_CA
Eqn. 3
where:
R_JA = junction to ambient thermal resistance (^oC/W)
R_JC= junction to case thermal resistance (^oC/W)
R_CA= case to ambient thermal resistance (^oC/W)
R_JC s device related and is not affected by other factors. The thermal environment can be controlled to change the
case-to-ambient thermal resistance, R_CA. For example, change the air flow 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 description is most useful for packages with heat sinks where 90% of the heat flow is through the case to heat
sink to ambient. For most packages, a better model is required.
A more accurate two-resistor thermal model can be constructed from the junction-to-board thermal resistance and the
junction-to-case thermal resistance. The junction-to-case thermal resistance describes when using a heat sink 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. This model can be used to generate simple
estimations and for computational fluid dynamics (CFD) thermal models.
To determine the junction temperature of the device in the application on a prototype board, use the thermal characterization
parameter (_JT) to determine the junction temperature by measuring the temperature at the top center of the package case using
the following equation:
T_J = T_T + (_JT x P_D)
Eqn. 4
where:
T_T= thermocouple temperature on top of the package (^oC)
_JT= thermal characterization parameter (^oC/W)
P_D= power dissipation in the package (W)
The thermal characterization parameter is measured in compliance with the JESD51-2 specification using a 40-gauge type T
thermocouple epoxied to the top center of the package case. Position the thermocouple so that the thermocouple junction rests
on the package. Place a small amount of epoxy on the thermocouple junction and approximately 1 mm of wire extending from
the junction. Place the thermocouple wire flat against the package case to avoid measurement errors caused by the cooling
effects of the thermocouple wire.
References:
PXD20 Microcontroller Data Sheet, Rev. 2
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
69