Method and driver for determining drive values for driving a lighting device

1. Method for determining drive values for driving a lighting device at a desired brightness and color, said lighting device comprising of a plurality of light emitting diodes (LEDs) of at least two different colors, said method comprising the steps of:

determining a first luminous flux weight ratio based on the desired color and a first drive current for driving each of the differently colored LEDs;

determining a first luminous flux for each of the differently colored LEDs based on the desired brightness and the first luminous flux weight ratio;

comparing, for each of the differently colored LEDs, the first luminous flux with a nominal luminous flux for a plurality of different drive currents;

selecting, for each of the differently colored LEDs, a preferred drive current that at least can produce the first luminous flux;

determining a second luminous flux weight ratio based on the desired color and the selected drive currents for each of the differently colored LEDs;

determining a second luminous flux for each of the differently colored LEDs based on the desired brightness and the second luminous flux weight ratio; and

determining a duty cycle for each of the differently colored LEDs at the selected drive currents, wherein the selected currents at the determined duty cycles produces the second luminous flux for each of the differently colored LEDs.

2. Method according to claim 1 , further comprising the step of driving each of the differently colored LEDs with the selected currents at the determined duty cycles.

3. Method according to claim 2 , further comprising the steps of:

acquiring measurement values by means of a temperature sensor mounted in proximity to the differently colored LEDs;

determining a luminous flux and color for each of the differently colored LEDs based on said measurement values;

determining a brightness and color for the lighting device based on said determined luminous fluxes and colors; and

adjusting the drive currents and the duty cycles for each of said differently colored LEDs based on a difference between said desired brightness and color and the determined brightness and color such that the lighting device emits light at the desired brightness and color.

4. Method according to claim 2 , further comprising the steps of:

acquiring measurement values by means of a light sensing unit;

determining a brightness and color for the lighting device based on said measurement values; and

adjusting at least one of the drive currents and the duty cycles for each of said differently colored LEDs based on a difference between the desired brightness and color and the determined brightness and color such that the lighting device emits light at the desired brightness and color.

5. Method according to claim 1 , wherein the plurality of different drive currents for driving each of the differently colored LEDs are provided by:

activating a first current source to generate a first drive signal having a first amplitude;

activating a second current source to generate a second drive having a second amplitude;

adding the first drive signal to the second drive signal, thereby generating a composite drive signal; and

providing the composite drive signal to each of the differently colored LEDs, wherein the composite drive signal can assume one out of four different amplitudes based on if one, both, or none of the current sources are activated.

6. A method according to claim 5 , wherein the second amplitude is lower than the first amplitude.

7. A method according to claim 5 , wherein the first and the second current sources are activated by means of individual pulse width modulated signals.

8. A driver for determining drive values for driving a lighting device at a desired brightness and color, said lighting device comprising of a plurality of light emitting diodes (LEDs) of at least two different colors, said driver comprising:

means for determining a first luminous flux weight ratio based on the desired color and a first drive current for driving each of the differently colored LEDs;

means for determining a first luminous flux for each of the differently colored LEDs based on the desired brightness and the first luminous flux weight ratio;

means for comparing, for each of the differently colored LEDs, the first luminous flux with a nominal luminous flux for a plurality of different drive currents;

means for selecting, for each of the differently colored LEDs, a preferred drive current that at least can produce the first luminous flux;

means for determining a second luminous flux weight ratio based on the desired color and the selected drive currents for each of the differently colored LEDs;

means for determining a second luminous flux for each of the differently colored LEDs based on the desired brightness and the second luminous flux weight ratio; and

means for determining a duty cycle for each of the differently colored LEDs at the selected drive currents, wherein the selected currents at the determined duty cycles produces the second luminous flux for each of the differently colored LEDs.

9. A driver according to claim 8 , further comprising means for driving each of the differently colored LEDs with the selected currents at the determined duty cycles.

10. A driver according to claim 8 , wherein the plurality of different drive currents for driving each of the differently colored LEDs are provided by:

a first current source adapted to receive an activation signal and to generate a first drive signal having a first amplitude;

a second current source adapted to receive an activation signal and to generate a second drive signal having a second amplitude;

an adder for adding the first drive signal to the second drive signal, thereby generating a composite drive signal; and

means for providing the composite drive signal to each of the differently colored LEDs, wherein the composite drive signal can assume one out of four different amplitudes based on if one, both, or none of the current sources are activated.

11. A lighting device comprising:

plurality of LEDs of at least two colors; and

driver according to claim 8 for driving each of the differently colored LEDs such that the lighting device emits light at a desired brightness and color.

12. display unit, comprising:

display panel;

backlight comprising a lighting device comprising of a plurality of differently colored LEDs; and

driver according to claim 8 for driving each of the differently colored LEDs such that the lighting device emits light at a desired brightness and color.