Answer

Using the 1.8V output of the internal DC-DC converter of EFR32 for DC supply of the PA provides better efficiency (i.e. lower PA current for the same output power) compared to the case when the PA is running from the main 3.3V. The recommendation for applying the DC-DC output to PAVDD:

• <= 13dBm for 2.4GHz
• <= 14dBm for sub-GHz

Above these power levels, using the main 3.3V supply for DC supply of the PA results better efficiency.