The nRF21540 DK has a power source switch (SW9) for selecting between three power sources for the nRF52840 System on Chip (SoC).
The three power sources on the switch are:
- VDD (default)
- Li-Po
- Universal Serial Bus (USB)
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 connectors (J6 or P27). In the USB position, the USB high voltage regulator gets power from the nRF52840 USB connector (J3).
When the high voltage regulator is used, the VDD_nRF voltage can be set by the firmware of the SoC. 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 OFF. One switch is for the high voltage regulator (U23) and the other for the USB supply (U22). Both switches are controlled by VDD.