SAM L21 can operate from one of three different internal voltage regulators, refer to Figure 1. The main voltage regulator (Main VREG) supplies the core domain (VDDCORE) when the device is in Active or Idle sleep mode. In Standby sleep mode, VDDCORE is either powered by Main VREG or the low power voltage regulator (LP VREG). VDDBU is powered by the Backup voltage regulator (Backup VREG).