A circuit for providing back-up power, comprising: a switching circuit (112, 118) configured to be coupled to a first power source and a second power source to provide a client voltage to its output from one of said power sources (104; 102); a client voltage level monitor circuit coupled with the first power source and the second power source and with the output of the switching circuit (112, 118), the client voltage level monitor being configured to generate a first current and comprising an output according to said current such that when the client voltage is lower than the highest voltage of the first and second power sources (104; 102), the client voltage level monitor is arranged to increase the first current; a dynamic level shifter circuit coupled with the first power source and the second power source and the output of the client voltage level monitor, the dynamic level shifter circuit being configured to receive a second current as a mirrored version of the first current via the output of the client voltage level monitor circuit to provide a differential output according to a difference between the voltage of the first and the second power sources (104, 102) and to create higher output voltage drops at said output as the second current is higher; and a double controlled latch circuit coupled with the first power source and the second power source and driven by the differential output of the dynamic level shifter circuit, wherein the double controlled latch circuit is configured to provide control signals to the switching circuit (112, 118) such that the switching circuit (112, 118) provides the highest voltage of the first power source and the second power source to its output. a switching circuit (112, 118) configured to be coupled to a first power source and a second power source to provide a client voltage to its output from one of said power sources (104; 102); a client voltage level monitor circuit coupled with the first power source and the second power source and with the output of the switching circuit (112, 118), the client voltage level monitor being configured to generate a first current and comprising an output according to said current such that when the client voltage is lower than the highest voltage of the first and second power sources (104; 102), the client voltage level monitor is arranged to increase the first current; a dynamic level shifter circuit coupled with the first power source and the second power source and the output of the client voltage level monitor, the dynamic level shifter circuit being configured to receive a second current as a mirrored version of the first current via the output of the client voltage level monitor circuit to provide a differential output according to a difference between the voltage of the first and the second power sources (104, 102) and to create higher output voltage drops at said output as the second current is higher; and a double controlled latch circuit coupled with the first power source and the second power source and driven by the differential output of the dynamic level shifter circuit, wherein the double controlled latch circuit is configured to provide control signals to the switching circuit (112, 118) such that the switching circuit (112, 118) provides the highest voltage of the first power source and the second power source to its output.