The nRF5340 DK has power source switch SW9 for selecting between VDD (default), lithium-polymer battery, and USB power supplies for the nRF5340 System on Chip (SoC).
The nRF5340 SoC has a high-voltage buck regulator that can support up to 5 V input. In the VDD position, the SoC is powered from the onboard buck regulator, coin cell battery, or external supply P21. In the Li-Po position, the high-voltage regulator of the SoC is supplied directly from the Lithium-polymer (Li-Poly) battery connector J6 or P27. In the USB position, the Universal Serial Bus (USB) high-voltage regulator is powered by the nRF5340 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.
To make sure that the nRF5340 SoC is not powered when the nRF power switch SW8 is in the OFF position, two load switches are used. The U15 switch is for the high-voltage regulator, and the U20 switch is for the USB supply. These switches are controlled by VDD.