The nRF21540 DK has a power source switch SW9 for selecting between VDD (default), lithium-polymer battery, and USB power supplies for the nRF52840 System on Chip (SoC).
The nRF52840 SoC has a high voltage buck regulator that can support up to 5 V input. In the VDD position, the SoC is powered either from the onboard buck regulator or external supply P21. In the Li-Po position, the high voltage regulator of the SoC is supplied directly from the Li-poly battery connector J6 or P27. In the USB position, the USB high voltage regulator is powered from the nRF52840 USB connector J3.
When the high voltage regulator is used, the VDD_nRF voltage can be set by SoC firmware. To make sure the rest of the Development Kit (DK) has the same voltage level, the VDD of the DK is sourced by a regulator following the VDD_nRF voltage when the high voltage regulator is used.
Two load switches are used to prevent the nRF21540 DK from receiving power when the nRF power switch SW8 is in the OFF position. The U23 switch is for the high voltage regulator, and the U22 is for the USB supply. Both switches are controlled by VDD.