Patent ID: 8436596

Claim:
A circuit comprising: a first supply rail and a second supply rail for connection to a dc power supply; an inductor; a load; and a controllable switching device, switchable between a conducting state and a non-conducting state, and a switching device current sensing resistor connected in series with the controllable switching device such that a voltage across the switching device current sensing resistor is indicative of a current flowing through the switching device, the inductor, load and controllable switching device being arranged in one of two configurations, the two configurations comprising: a first configuration in which, when the controllable switching device is in the conducting state, current is permitted to flow from the first supply rail to the second supply rail, through the inductor and controllable switching device, bypassing the load and energising the inductor, and when the controllable switching device is in the non-conducting state it blocks current flow through itself, current flows through the load, and the inductor discharges energy into the load; and a second configuration in which, when the controllable switching device is in the conducting state, current is permitted to flow from the first supply rail to the second supply rail through the load, inductor and controllable switching device in series, energising the inductor, and when the controllable switching device is in the non-conducting state it blocks current flow through itself, current flows through the load, and the inductor discharges energy into the load, and wherein the circuit further comprises: control means arranged to control the switching device to alternate between the conducting state and the non-conducting state, and characterised in that the circuit further comprises: a load current sensing resistor connected in series with the load such that a voltage across the load current sensing resistor is indicative of a current flowing through the load; a demand signal input for receiving a demand signal indicative of a desired current through the load; and monitoring means arranged to monitor the demand signal, and the voltages across the sense resistors, and generate a monitor signal having a magnitude dependent upon the demand signal, the voltage across the switching device current sensing resistor and the voltage across the load current sensing resistor, wherein the control means is arranged to receive the monitor signal and to switch the switching device from the conducting state to the non-conducting state in response to the magnitude of the monitor signal exceeding a predetermined threshold, and wherein the control means is arranged to switch the switching device back to the conducting state a delay time after switching the switching device to the non-conducting state in response to the monitor signal, the delay time being determined according to the demand signal.