PXD10 Datasheet, PDF (64/130 Pages) Lumins Inc. – 4” Dia.X 6” extruded aluminum step poles

Electrical characteristics
At a known board temperature, the junction temperature is estimated using Equation 2:
T_J = T_B + (R_JB  P_D)
Eqn. 2
where:
T_B = board temperature for the package perimeter (°C)
R_JB = junction-to-board thermal resistance (°C/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 (°C/W)
R_JC = junction to case thermal resistance (°C/W)
R_CA= case to ambient thermal resistance (°C/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 Equation 4:
T_J = T_T + (_JT x P_D)
where:
T_T = thermocouple temperature on top of the package (°C)
_JT = thermal characterization parameter (°C/W)
P_D = power dissipation in the package (W)
Eqn. 4
PXD10 Microcontroller Data Sheet, Rev. 1
64
Freescale Semiconductor