An LDO with high efficiency receives an input voltage from an input power line and outputs an output voltage at an output power line. The LDO includes a first active device, a second active device, an operational amplifier and a protection circuit. The first and second active devices are connected in series between the input and output power lines via a connecting node. The operational amplifier controls the second active device according to the output voltage and a core power voltage of a core power line to cause the output voltage to stabilize at a target voltage value. The protection circuit is connected to the input and output power lines, the connecting node and a control node of the first active device, and controls a voltage of the connecting node and the control node of the first active device according to the input and output voltages.