Stochastic signal density modulation for optical transducer control

• US 8,129,924 B2
• Filed: 11/13/2006
• Issued: 03/06/2012
• Est. Priority Date: 11/13/2006
• Status: Active Grant

• Current Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Van Ess, David, Prendergast, Patrick N.
• Primary Examiner(s): Owens, Douglas W
• Assistant Examiner(s): A, Minh D
• Application Number:
  US11/598,981
• Publication Number:
  US 20080111503A1
• Time in Patent Office: 1,940 Days
• Field of Search: 315/307, 315/291, 315/312, 362/800, 362/611, 362/612, 362/555, 345/690, 345/691, 345/692, 345/697, 345/82, 345/204
• US Class Current: 315/307
• CPC Class Codes: H05B 45/10 Controlling the intensity of the light
• CPC Class Codes: H05B 45/18 using temperature feedback
• CPC Class Codes: H05B 45/20 Controlling the colour of the light
• CPC Class Codes: H05B 45/30 Driver circuits
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines

Abstract

A controller for optical transducers uses stochastic signal density modulation to reduce electromagnetic interference.

Claims

1. An apparatus, comprising:

a controllable current supply coupled to a light-emitting diode;

a stochastic signal density modulator (SSDM) coupled to the controllable current supply, wherein the SSDM is configured to provide a stochastic control signal to the controllable current supply, to control a light intensity output of the light-emitting diode and to reduce electromagnetic interference, wherein the SSDM comprises;

a signal density register configured to store a signal density value, wherein the stochastic control signal has a signal density corresponding to the stored signal density value.

2. The apparatus of claim 1, wherein the SSDM further comprises:

a comparator comprising a first input, a second input and an output;

a stochastic state machine, coupled to the first input of the comparator, wherein the stochastic state machine is configured to generate a plurality of stochastic values,

wherein the signal density register is coupled to the second input of the comparator, and wherein the output of the comparator is a first output value if a stochastic value of the plurality of stochastic values is greater than the signal density value and wherein the output of the comparator is a second output value if the stochastic value of the plurality of stochastic values is less than or equal to the signal density value.

3. The apparatus of claim 2, wherein the stochastic state machine comprises a stochastic counter.

4. The apparatus of claim 3, wherein the stochastic counter comprises a random number generator.

5. The apparatus of claim 3, wherein the stochastic counter comprises a pseudorandom number generator.

6. The apparatus of claim 2, wherein the stochastic state machine includes a machine-readable medium containing data that, when read by the stochastic state machine, causes the stochastic state machine to perform operations comprising generating a sequence of pseudorandom numbers.

7. The apparatus of claim 2, wherein the signal density register comprises a programmable register.

8. The apparatus of claim 2, wherein the controllable current supply is configured to provide a first current level when the output of the comparator is the first output value and to provide a second current level when the output of the comparator is the second output value.

9. The apparatus of claim 8, wherein the first current level comprises a non-zero current level and the second current level is approximately zero.

10. The apparatus of claim 9, wherein the second current level comprises a non-zero current level and the first current level is approximately zero.

11. The apparatus of claim 1, wherein the light-emitting diode comprises an anode and a cathode, wherein a first terminal of the switched current supply is coupled to the anode, and wherein the current supply is configured to source current to the light-emitting diode.

12. The apparatus of claim 11, wherein the cathode of the light-emitting diode is coupled to a first voltage, wherein a second terminal of the current supply is coupled to a second voltage, and wherein the second voltage is positive with respect to the first voltage.

13. The apparatus of claim 1, wherein the light-emitting diode comprises an anode and a cathode, wherein a first terminal of the switched current supply is coupled to the cathode, and wherein the current supply is configured to sink current from the light-emitting diode.

14. The apparatus of claim 13, wherein the anode of the light-emitting diode is coupled to a first voltage, wherein a second terminal of the current supply is coupled to a second voltage, and wherein the first voltage is positive with respect to the second voltage.

15. A method for controlling a light source with reduced electromagnetic interference, the method comprising:

providing a controllable current for one or more light emitting diodes; and

stochastically controlling the current to select a light intensity output from the one or more light emitting diodes resulting in reduced electromagnetic interference, wherein stochastically controlling the current comprises generating a stochastic signal density modulation signal having a signal density based on a programmed number representing the signal density.

16. The method of claim 15, wherein stochastically controlling the current further comprises:

comparing the state of a stochastic state machine to the programmed number to generate the stochastic signal density modulation signal;

generating the stochastic signal density modulation signal; and

modulating the controllable current with the stochastic signal density modulation signal.

17. The method of claim 16, wherein generating the stochastic signal density modulation signal comprises:

comparing a plurality of stochastic values from the stochastic state machine with the programmed number representing the signal density of the stochastic signal density modulation signal;

generating a pulse train to control the controllable current, the pulse train having a first pulse amplitude if a stochastic value of the plurality of stochastic values is greater than the programmed number and having a second pulse amplitude if the stochastic value of the plurality of stochastic values is less than or equal to the programmed number.

18. The method of claim 17, wherein the stochastic state machine comprises a random number generator, wherein the plurality of stochastic values comprises a plurality of random numbers.

19. The method of claim 17, wherein the stochastic state machine comprises a pseudorandom number generator, wherein the plurality of stochastic values comprises a plurality of pseudorandom numbers.

20. The method of claim 19, further comprising programming the number representing the signal density of the stochastic signal density modulation signal in a programmable register.

21. The method of claim 17, further comprising:

providing a first current level to the light-emitting diode when the pulse train has the first pulse amplitude; and

providing a second current level to the light-emitting diode when the pulse train has the second pulse amplitude.

22. The method of claim 21, wherein the first current level comprises a non-zero current level and the second current level is approximately zero.

23. The method of claim 21, wherein the second current level comprises a non-zero current level and the first current level is approximately zero.

24. An apparatus, comprising;

means for increasing a dimming frequency of a dimming signal for an optical transducer comprising means for generating a stochastic signal density modulation signal; and

means for controlling the optical transducer with the dimming signal comprising means for modulating a controllable current for the transducer with a stochastic signal density modulation signal with a frequency greater than uniform frequency modulation and free of spectral peaks such that the control signal has a reduced electromagnetic interference content relative to uniform frequency modulation.

25. A system, comprising:

a plurality of controllable current supplies coupled to a plurality of optical transducers; and

a plurality of controllers coupled to the plurality of controllable current supplies, wherein each controller of the plurality of controllers is configured to provide a stochastic control signal to one of the controllable current supplies, the stochastic control signal having a selected stochastic signal density to control a light intensity output of one of the plurality of optical transducers.

26. The system of claim 25, wherein the plurality of optical transducers comprises a set of primary color optical transducers and wherein the plurality of controllers is configured to control a color mix of the plurality of optical transducers.

27. The system of claim 25, wherein the plurality of optical transducers comprises a set of secondary color optical transducers and wherein the plurality of controllers is configured to control a color mix of the plurality of optical transducers.

28. The system of claim 25, wherein the plurality of optical transducers comprises a set of complementary color optical transducers and wherein the plurality of controllers is configured to control a color mix of the plurality of optical transducers.