After completing a system-level thermal simulation and obtaining a Theta Ja (ƟJA) value, power design manager estimation can be further refined using this information. When doing an initial thermal simulation, the worst case Tj should be assumed and the estimation can be refined once ƟJA is calculated. For more information on this flow, see UltraFast Design Methodology Guide for FPGAs and SOCs (UG949).
ΘJA is a measure of how the junction temperature (Tj) raises above the ambient temperature (Ta) for every watt of power dissipated (Pd) in the device. The units are oC/W and it is calculated using the following equation:
ΘJA = (Tj – Ta)/ Pd
In the Environment table, override the default ƟJA with the ƟJA from the thermal simulation results and specify the ambient temperature. PDM uses these values to calculate junction temperature and improve power estimation accuracy.
In the following example, Ta = 40oC with a ƟJA of 2.0 oC/W is entered when User Override OFF is selected. Power is estimated at 7.2W which results in an estimated Tj of 54.4oC. This ensures a more accurate static power estimate of the device.