A method for an electronic power supply applicable to any dual supply rail systems, which require a smooth and uninterrupted output supply, characterized in that it comprises the following steps: 1) providing a supply generator generating an output voltage VDDRTC for one or more loads (POR, Digital, XTAL) having at least a main supply source VSYS and a back-up supply source VBBAT, a local reference voltage source VREF, a means to compare VSYS and VBBAT, additional comparators (4, 11), and a buffer amplifier; 2) providing an input reference selector comprising a local reference voltage generator, wherein the selector selects a reference source for said buffer amplifier; 3) checking if supply generator is enabled and, if so, go to step 4, otherwise go to end in step 9; 4) checking if VBBAT < VSYS and, if so, go to step 5, otherwise go to step 7; 5) sourcing the output voltage VDDRTC using main supply source VSYS, as voltage source via a power path comprising a unity gain buffer, a supply comparator and an output supply switch, wherein a commutation to said power path is only performed after a voltage level of the power path is settled within a defined range - e.g. in the order of magnitude of about 25mVolts, of the actual output voltage (VDDRTC) and therefore preventing larger voltage steps and eliminating oscillations of the output voltage; 6) checking if VBBAT > VSYS + safety margin and, if so, go to step 7, otherwise go to step 5; 7) sourcing from VBBAT, namely powering down all analog components of the supply generator not required for back-up supply and sourcing the output voltage VDDRTC from back up supply VBBAT; 8) checking if VBBAT < VSYS and, if so, go to step 5, otherwise go to step 7; 9) end. 1) providing a supply generator generating an output voltage VDDRTC for one or more loads (POR, Digital, XTAL) having at least a main supply source VSYS and a back-up supply source VBBAT, a local reference voltage source VREF, a means to compare VSYS and VBBAT, additional comparators (4, 11), and a buffer amplifier; 2) providing an input reference selector comprising a local reference voltage generator, wherein the selector selects a reference source for said buffer amplifier; 3) checking if supply generator is enabled and, if so, go to step 4, otherwise go to end in step 9; 4) checking if VBBAT < VSYS and, if so, go to step 5, otherwise go to step 7; 5) sourcing the output voltage VDDRTC using main supply source VSYS, as voltage source via a power path comprising a unity gain buffer, a supply comparator and an output supply switch, wherein a commutation to said power path is only performed after a voltage level of the power path is settled within a defined range - e.g. in the order of magnitude of about 25mVolts, of the actual output voltage (VDDRTC) and therefore preventing larger voltage steps and eliminating oscillations of the output voltage; 6) checking if VBBAT > VSYS + safety margin and, if so, go to step 7, otherwise go to step 5; 7) sourcing from VBBAT, namely powering down all analog components of the supply generator not required for back-up supply and sourcing the output voltage VDDRTC from back up supply VBBAT; 8) checking if VBBAT < VSYS and, if so, go to step 5, otherwise go to step 7; 9) end.