Control circuit and method for controlling light emitting diodes

• US 20080007497A1
• Filed: 06/28/2006
• Published: 01/10/2008
• Est. Priority Date: 06/28/2006
• Status: Active Grant

• Current Assignee: ams AG (Ams-Osram AG)
• Original Assignee: ams AG (Ams-Osram AG)
• Inventors: Pauritsch, Manfred
• Granted Patent: US 7,768,216 B2
• Time in Patent Office: Days
• US Class Current: 345/82
• CPC Class Codes: G09G 2310/08 Details of timing specific for flat panels, other than clock recovery
• CPC Class Codes: G09G 2320/0233 Improving the luminance or brightness uniformity across the screen
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2330/06 Handling electromagnetic interferences [EMI], covering emitted as well as received electromagnetic radiation
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: H05B 45/10 Controlling the intensity of the light
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines
• CPC Class Codes: Y02B 20/30 Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]

Abstract

A control circuit for controlling light emitting diodes comprises a switch for turning on or off a string of light emitting diodes. A combiner generates a control signal from a data signal and a noise signal. A sigma delta modulator receives the control signal and a clock signal with a clock period and generates a switching signal for controlling the switch.

Claims

1. A control circuit for controlling light emitting diodes, comprising:

a first switch to turn a first string of light emitting diodes on or off, the first switch comprising a control input;

a first sigma delta modulator comprising;

a signal input to receive a first control signal;

a signal output electrically connected to the control input of the first switch; and

a clock input to receive a clock signal having a clock period; and

a first combiner circuit to generate the first control signal based on a data signal and a first noise signal.

2. The control circuit of claim 1, further comprising:

a second switch to turn a second string of light emitting diodes on or off; and

a delay element electrically connected between the signal output of the first sigma delta modulator and a control input of the second switch, the delay element for generating a second switching signal to control the second switch based on the output of the first sigma delta modulator.

3. The control circuit of claim 2, wherein the delay element has a delay time that correspond to the clock period of the clock signal.

4. The control circuit of claim 1, further comprising:

N(N) switches to turn N strings of light emitting diodes on or off, wherein each of the N switches comprises;

a sigma delta modulator comprising;

a signal input to receive an N_th control signal,

a signal output coupled to a control input of an N_th switch; and

a clock input to receive a clock signal; and

a combiner circuit to generate the N_th control signal from the data signal and an N_th noise signal.

5. The control circuit of claim 1, wherein the first string of light emitting diodes comprises a plurality of light emitting diodes electrically connected to the first switch.

6. The control circuit of claim 1, further comprising a first noise generator to generate the first noise signal.

7. The control circuit of claim 6, wherein the first noise generator comprises an input to receive the clock signal.

8. The control circuit of claim 6, wherein the first noise generator is configured to generate the first noise signal as a pseudo-random noise sequence.

9. The control circuit of claim 1, wherein the first noise signal comprises a signal having a time averaged mean value equal to about zero.

10. The control circuit of claim 1, wherein the data signal has a word length that is greater than a word length of the first noise signal.

19. The control circuit of claim 1, wherein the first sigma delta modulator comprises an adder and a delay element, the adder for receiving the first control signal and a feedback signal from the delay element.

11. A method of controlling light emitting diodes, comprising:

generating a first control signal based on a data signal and a first noise signal;

generating a first switching signal based on the first control signal using sigma delta modulation; and

switching a first string of light emitting diodes on or off based on the first switching signal.

12. The method of claim 11, further comprising:

generating a second switching signal by delaying the first switching signal; and

switching a second string of light emitting diodes on or off based on the second switching signal.

13. The method of claim 12, wherein a delay time for generating the second switching signal corresponds to a clock period of a clock signal used for the sigma delta modulation.

14. The method of claim 11, further comprising, for each N(N) strings of light emitting diodes:

generating an N^th control signal based on the data signal and an N^th noise signal;

generating an N^th switching signal from the N^th control signal using sigma delta modulation; and

switching an N^th string of light emitting diodes on or off based on the N^th switching signal.

15. The method of claim 11, wherein the first noise signal comprises a signal having a time averaged mean value equal to about zero.

16. The method of claim 11, wherein the data signal has a word length that is greater than a word length of the first noise signal.

17. The method of claim 11, wherein the data signal corresponds to a desired brightness of light emitting diodes.

18. The method of claim 11, wherein the first noise signal comprises a pseudo random noise sequence.

20. A control circuit for controling light emitting diodes, comprising:

means for generating a first control signal based on a data signal and a first noise signal;

means for generating s first switching signal based on the first control signal using sigma delta modulation; and

means for switching a first string of light emitting diodes on or off based on the first switching signal.