An apparatus for protecting a circuit from an input voltage, comprising: a switchable element arranged to couple the input voltage (V IN) to the circuit in response to a first control signal (V DRV1) having a first value and to decouple the input voltage (V IN) from the circuit in response to the first control signal (V DRV1) having a second value; a monitor stage arranged to compare a monitored voltage (V MON) to a threshold (V TH); and a controller arranged to provide the first control signal (V DRV1) to the switchable element, the first control signal (V DRV1) having the first value when the monitored voltage (V MON) is on one side of the threshold (V TH) and the second value when the monitored voltage (V MON) is on the other side of the threshold (V TH); wherein the first value is independent of the input voltage (V IN) and the second value is equal to the input voltage (V IN); and wherein the monitor stage is arranged to generate an indication (V IND) of whether the monitored voltage (V MON) is above or below the threshold (V TH), and the controller is arranged to switch the first control signal (V DRV1) between the first value and second value in response to the indication (V IND); characterised in that the controller comprises a controller first transistor (T1 c) and a controller second transistor (T2 c), the controller first transistor (T1 c) having a source and a substrate coupled to an auxiliary input for receiving the input voltage (V IN), and the controller second transistor (T2 c) having a source and a substrate coupled to a voltage rail (V ss) at 0V, that is ground, a gate of the controller first transistor (T1 c) and a gate of the controller second transistor (T2 c) coupled to an indication input for receiving the indication (V IND), and a drain of the controller first transistor (T1 c) and a drain of the controller second transistor (T2 c) coupled to a control output for providing the first control signal (V DRV1), and wherein, dependent on a value of the indication (V IND), one of the controller first and second transistors (T1 c, T2 c) is switched on and the other of the controller first and second transistors (T1 c, T2 c) is switched off. a switchable element arranged to couple the input voltage (V IN) to the circuit in response to a first control signal (V DRV1) having a first value and to decouple the input voltage (V IN) from the circuit in response to the first control signal (V DRV1) having a second value; a monitor stage arranged to compare a monitored voltage (V MON) to a threshold (V TH); and a controller arranged to provide the first control signal (V DRV1) to the switchable element, the first control signal (V DRV1) having the first value when the monitored voltage (V MON) is on one side of the threshold (V TH) and the second value when the monitored voltage (V MON) is on the other side of the threshold (V TH); wherein the first value is independent of the input voltage (V IN) and the second value is equal to the input voltage (V IN); and wherein the monitor stage is arranged to generate an indication (V IND) of whether the monitored voltage (V MON) is above or below the threshold (V TH), and the controller is arranged to switch the first control signal (V DRV1) between the first value and second value in response to the indication (V IND); characterised in that the controller comprises a controller first transistor (T1 c) and a controller second transistor (T2 c), the controller first transistor (T1 c) having a source and a substrate coupled to an auxiliary input for receiving the input voltage (V IN), and the controller second transistor (T2 c) having a source and a substrate coupled to a voltage rail (V ss) at 0V, that is ground, a gate of the controller first transistor (T1 c) and a gate of the controller second transistor (T2 c) coupled to an indication input for receiving the indication (V IND), and a drain of the controller first transistor (T1 c) and a drain of the controller second transistor (T2 c) coupled to a control output for providing the first control signal (V DRV1), and wherein, dependent on a value of the indication (V IND), one of the controller first and second transistors (T1 c, T2 c) is switched on and the other of the controller first and second transistors (T1 c, T2 c) is switched off.