LED driver, lighting system and driving method with prolonged lifetime of luminous output

1. An LED driver, comprising:

a current driver,

a voltage sensor for sensing an LED voltage; and

a controller for controlling the current driver,

wherein the controller is adapted to:

operate a first drive scheme for a first range of sensed voltages up to a threshold voltage, wherein a first constant current is applied during the first drive scheme; and

operate a second drive scheme when the first constant current results in a higher sensed voltage than the threshold voltage, wherein a current lower than the first constant current is applied during the second drive scheme,

wherein during the second drive scheme the voltage is regulated to be constant at the threshold voltage.

2. An LED driver as claimed in claim 1 , wherein during the second drive scheme the current is stepped between discrete values, with the stepping taking place at the threshold voltage.

3. An LED driver as claimed in claim 1 , wherein during the second drive scheme the power is regulated to be constant.

4. An LED driver as claimed in claim 1 , wherein the controller comprises a microprocessor.

5. An LED driver as claimed in claim 1 , wherein the controller comprises an analogue circuit.

6. An LED driver as claimed in claim 1 , wherein the driver comprises an operating window driver having a current-voltage operating window.

7. A lighting system comprising:

an LED driver arrangement as claimed in claim 1 ; and

an LED unit powered by the LED driver.

8. A lighting system as claimed in claim 7 , wherein the LED unit comprises one or more OLEDs.

9. A method of driving an LED using a current driver, comprising:

sensing an LED voltage;

operating a first drive scheme for a first range of sensed voltages up to a threshold voltage, wherein a first constant current is applied during the first drive scheme; and

operating a second drive scheme when the first constant current results in a higher sensed voltage than the threshold voltage, wherein a current lower than the first constant current is applied during the second drive scheme,

wherein during the second drive scheme the voltage is regulated to be constant at the threshold voltage.

10. A method as claimed in claim 9 , wherein during the second drive scheme the current is stepped between discrete values, with the stepping taking place at the threshold voltage.

11. A method as claimed in claim 9 , wherein during the second drive scheme the power is regulated to be constant.

12. A method as claimed in claim 9 , comprising detecting if the voltage is below the threshold or below the threshold by more than a fixed amount when the current setting is below the first constant current, and if so increasing current setting.

13. A method as claimed in claim 9 , wherein the driver comprises an operating window driver having a current-voltage operating window.