A step-down circuit has a switch connected in series between a direct current power supply and a load. A switch control circuit is configured to turn the switch off when the voltage across the load is higher than a predetermined first threshold and turn the switch on when the voltage across the load is lower than the first threshold. An energy storage unit is charged by the power supply when the switch is on and supply power to the load when the switch is off. An overcurrent protection circuit turns the switch off when the input current of the switch is higher than a predetermined second threshold.