The nRF52840 DK has a power source switch (SW9) for selecting between three power sources for the nRF52840 System on Chip (SoC).
The three positions of the switch are the following:
- VDD (default)
- Li-Po
- 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, coin cell battery, or external supply (P21). In the Li-Po position, the high voltage regulator of the SoC is supplied directly from the Li-Po 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.
To make sure that the nRF52840 SoC is not powered when the nRF power switch (SW8) is OFF, two load switches are used, one for the high voltage regulator (U23) and one for the USB supply (U22). These switches are controlled by VDD.