4.1.2 In-Application RJ-A Measurement

The junction-to-air thermal resistance is a physical constant depending on the mechanical characteristics of the IC package mounted on its PCB. It does not vary with temperature or power. Thus, by measuring the junction temperature and the power consumed by the product in a controlled ambient temperature, the thermal resistance can be computed with the formula:

RJ-A = (TJ-TA)/PTOT

Note:
  • The SAMA7G54 device junction temperature can be read from the internal calibrated temperature sensor. The Linux kernel reports this junction temperature in thermal zone 0 (see Appendix A). For other implementations, refer to the “SAMA7G5 Series Temperature Sensor Calibration” application note (see Reference Documents).
  • For power measurements, we recommend to make provision for one or more Microchip PAC1934 footprints on the application PCB as implemented on the SAMA7G54-EK board. For more information, refer to the PAC1934 data sheet (see Reference Documents).