LED driver

1. A light emitting diode (LED) driver comprising:

a switched-mode power converter for providing power to an LED arrangement connected across output terminals of the converter, and wherein the converter comprises a number of energy storage elements (L, C) and a first semiconductor switch (Q 1 );

a second semiconductor switch (Q 2 ) connected in series with the LED arrangement;

a controller to generate a second control signal (G 2 ) for the second semiconductor switch (Q 2 ), such that a transition of the second control signal (G 2 ) is delayed relative to a corresponding transition of a first control signal (G 1 ) for the first semiconductor switch (Q 1 );

wherein the energy storage elements (L, C) includes an inductor (L) and the controller generates a delay of a trailing edge transition of the second control signal (G 2 ) as a function of a current level of the inductor (L).

2. The LED driver according to claim 1 , wherein the energy storage elements (L, C) comprise capacitor (C), and wherein the controller times the transitions of the second control signal (G 2 ) relative to the first control signal (G 1 ) according to an energy transfer between the energy storage elements (L, C).

3. The LED driver according to claim 2 , wherein the controller generates the delay of the trailing edge transition of the second control signal (G 2 ) as a function of the current level of the inductor (L) which is a threshold inductor current such that the delay make the power of the inductor decrease over time, and eventually drop below a level that is needed to run the LED.

4. The LED driver according to claim 1 , wherein the controller delays a trailing edge transition of the second control signal (G 2 ) relative to a trailing edge transition of the first control signal (G 1 ) by an amount exceeding an interval (t off ) between consecutive pulses of the first control signal (G 1 ).

5. The LED driver according to claim 3 , wherein the controller measures an interval (t off ) between consecutive pulses of the first control signal (G 1 ).

6. The LED driver according to claim 1 , wherein a leading edge transition of the second control signal (G 2 ) is delayed relative to a leading edge transition of the first control signal (G 1 ) by an amount corresponding to a discharge time (t delay ) of the inductor (L).

7. The LED driver according to claim 6 , wherein the controller comprises a discharge monitoring circuit for measuring the discharge time (t delay ) of the inductor (L).

8. The LED driver according to claim 1 , wherein the second control signal (G 2 ) is derived from a strobe signal (S) issued by a processor of a projection arrangement.

9. The LED driver according to claim 1 , comprising a state machine module realized to generate at least the second control signal (G 2 ).

10. A projection arrangement comprising:

a digital micro-mirror device for projecting an image onto a screen;

a light emitting diode (LED) arrangement for directing light at the micro-mirror device;

a processor for controlling the micro-mirror device and for generating control signals (G 1 , S) for a driver of the LED arrangement; and

at least one LED driver according to claim 1 for driving the LED arrangement.

11. The projection arrangement according to claim 10 , comprising a separate LED driver for each of a red LED group, a green LED group and a blue LED group of the LED arrangement.

12. A method of driving a light emitting diode (LED) arrangement, comprising the steps of:

arranging a switched-mode power converter comprising at least a number of energy storage elements (L, C) and a first semiconductor switch (Q 1 ) to provide power to the LED arrangement connected at the output of the converter;

arranging a second semiconductor switch (Q 2 ) in series with the LED arrangement;

providing a control means to generate a second control signal (G 2 ) for the second semiconductor switch (G 2 ), such that a transition of the second control signal (G 2 ) is delayed relative to a corresponding transition (G 1 ) of a first control signal (G 1 ) for the first semiconductor switch (Q 1 ); and

wherein the energy storage elements (L, C) comprise an inductor (L) and the step of providing the control means ( 11 ) comprising generating a delay of a trailing edge transition of the second control signal (G 2 ) as a function of a current level of the inductor (L).

13. The method according to claim 12 , wherein generating the delay of the trailing edge transition of the second control signal (G 2 ) as a function of the current level of the inductor (L) which is a threshold inductor current such that the delay make the power of the inductor decrease over time, and eventually drop below a level that is needed to run the LED, and comprising a step of predetermining a delay (t delay , Δ G2 ) for a leading edge transition of the second control signal (G 2 ) relative to a leading edge transition of the first control signal (G 1 ).

14. The method according to claim 12 , comprising a step of measuring a delay (t delay , Δ G2 ) for a leading edge transition of the second control signal (G 2 ) relative a leading edge transition of the first control signal (G 1 ).

15. The method according to claim 12 , comprising a step of measuring a discharge time (t delay ) of an energy element (L) of the power converter.