Circuit and method for driving LEDs

An LED driver for driving an LED that is coupled in series with an inductor between a driver output terminal and a first or a second supply terminal, the first supply terminal to carry a supply voltage and the second supply terminal to carry a reference potential, the LED driver comprising:

• a first semiconductor switch and a freewheeling device coupled between the first supply terminal and the second supply terminal, the first semiconductor switch being responsive to a driver signal and a common circuit node between the first semiconductor switch and the freewheeling device being connected to the output terminal;
• a current measurement circuit coupled to the LED, the current measurement circuit configured to provide a load current signal that represents a load current passing through the LED;
• a modulator configured to provide, as reference signal, a modulated signal having a desired duty cycle and an amplitude;
• a first feedback circuit that comprises an on-off controller that is configured to receive load current signal and the reference signal, to compare the load current signal with the reference signal, and to generate the driver signal dependent on the comparison;
• a second feedback circuit that comprises a filter and a regulator,
• wherein the filter is coupled to receive the load current signal and to provide a filtered signal that represents an average load current, and
• wherein the regulator is configured to receive the filtered signal and, as set point value, a reference value, to determine a control signal dependent on the difference between the reference value and the filtered signal in accordance with a predefined control law, and to adjust the amplitude of the reference signal in accordance with the control signal.

A method for driving an LED that is coupled in series to an inductor between an output terminal and a first or a second supply terminal, the first supply terminal to carry a supply voltage and the second supply terminal to carry a reference potential, the method comprises:

• measuring a load current passing through the LED;
• generating a load current signal that represents the load current;
• alternately applying either the supply voltage or the reference potential to the output terminal in accordance with a driver signal;
• comparing the load current signal with a reference signal;
• generating the driver signal dependent on the comparison;
• determining an average load current signal, from the load current signal;
• generating an on/off modulated input signal having a an off-level, an on-level and a duty cycle;
• and
• adjusting the on-level of the input signal dependent on the average load current signal and a reference value and providing the adjusted signal as reference signal.