Adjusting Color Temperature in a Dimmable LED Lighting System

• US 20140210357A1
• Filed: 01/25/2013
• Published: 07/31/2014
• Est. Priority Date: 01/25/2013
• Status: Active Grant

• Current Assignee: Dialog Semiconductor Incorporated (Renesas Electronics Corporation)
• Original Assignee: iWatt, Inc. (Renesas Electronics Corporation)
• Inventors: Yan, Liang, Zheng, Junjie
• Granted Patent: US 9,844,113 B2
• Time in Patent Office: Days
• US Class Current: 315/186
• CPC Class Codes: H05B 45/20 Controlling the colour of the light
• CPC Class Codes: H05B 45/3577 Emulating the dimming characteristics, brightness or colour temperature of incandescent lamps
• CPC Class Codes: H05B 45/44 with an active control inside an LED matrix
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines

Abstract

A LED lighting system, such as a dimmable LED lamp, that may simulate the performance of an incandescent bulb. LED strings of different colors may be connected to the output of a single LED driver that regulates an overall intensity of light produced by the LED lighting system. The color of the LED lighting system may be controlled by circuitry, such as one or more switches, that allocates current between the LED strings to change the color temperature of light emitted by the LED lighting system as the light intensity changes.

Claims

1. A light emitting diode (LED) lighting system, comprising:

a LED driver configured to generate a regulated current at an output of the LED driver;

a first LED string coupled to the output of the LED driver and configured to emit light of a first color temperature based on a first portion of the regulated current flowing through the first LED string;

a second LED string coupled to the output of the LED driver and configured to emit light of a second color temperature based on a second portion of the regulated current flowing through the second LED string, the second color temperature being different than the first color temperature; and

circuitry configured to control allocation of the regulated current between the first portion of the regulated current flowing through the first LED string and the second portion of the regulated current flowing through the second LED string responsive to a signal indicative of a desired brightness level.

2. The LED system of claim 1, wherein the circuitry is configured to generate a driver control signal responsive to the signal indicative of the desired brightness level, and the LED driver regulates a level of the regulated current responsive to the driver control signal.

3. The LED system of claim 2, wherein the LED driver comprises a switching power regulator that includes a switch to regulate the level of the regulated current, and a duty cycle of ON-times and OFF-times of the switch is controlled by the driver control signal.

4. The LED lighting system of claim 1, wherein the signal indicative of the desired brightness level is controlled by a dimming level set by a dimmer switch.

5. The LED lighting system of claim 1, wherein the first color temperature of the light emitted by the first LED string is lower than the second color temperature of the light emitted by the second LED string.

6. The LED lighting system of claim 1, wherein the first color temperature of the light emitted by the first LED string is substantially red and the second color temperature of the light emitted by the second LED string is substantially white.

7. The LED lighting system of claim 1, wherein the circuitry comprises:

a controller circuit configured to receive the signal indicative of the desired brightness level and to generate at least one switch control signal responsive to the signal indicative of the desired brightness level; and

a first switch coupled in series with the first LED string, wherein a duty cycle of ON and OFF times of the first switch is responsive to a first switch control signal of the at least one switch control signals,

wherein the allocation of the regulated current between the first portion of the regulated current flowing through the first LED string and the second portion of the regulated current flowing through the second LED string is responsive to the duty cycle of the first switch.

8. The LED lighting system of claim 7, wherein the circuitry comprises:

a second switch coupled in series with the first LED string, wherein a duty cycle of ON and OFF times of the second switch is responsive to a second switch control signal of the at least one switch control signals,

wherein the allocation of the regulated current between the first portion of the regulated current flowing through the first LED string and the second portion of the regulated current flowing through the second LED string is responsive to the duty cycle of the second switch.

9. The LED lighting system of claim 7, wherein the second LED string is in parallel with a combination of the first LED string and the first switch.

10. The LED lighting system of claim 7, wherein a number of LEDs in the first LED string is different than a number of LEDs in the second LED string, and wherein substantially all of the regulated current flows through the first LED string when the first switch is ON.

11. The LED lighting system of claim 7, wherein a number of LEDs in the first LED string is equal to a number of LEDs in the second LED string, and wherein the regulated current is split between the first LED string and the second LED string when the first switch is ON.

12. The LED lighting system of claim 7, further comprising:

a third LED string coupled between the output of the LED driver and both the first LED string and the second LED string.

13. The LED lighting system of claim 1, further comprising:

a first capacitor coupled in parallel with the first LED string; and

a second capacitor coupled in parallel with the second LED string.

14. The LED lighting system of claim 1, wherein as the desired brightness level decreases, an overall color temperature of the light emitted by the first string and the light emitted by the second LED string decreases.

15. The LED lighting system of claim 1, wherein, as a level of the regulated current decreases, the first portion of the regulated current flowing through the first LED string increases relative to the second portion of the regulated current flowing through the second LED string.

16. A method of operation in a light emitting diode (LED) lighting system, the method comprising:

receiving a signal indicative of a desired brightness level;

generating a regulated current at an output of a LED driver, wherein a first LED string is configured to emit light of a first color temperature based on a first portion of the regulated current flowing through the first LED string and the second LED string is configured to emit a light of a second color temperature based on a second portion of the regulated current flowing through the second LED string, the second color temperature being different than the first color temperature; and

controlling an allocation of the regulated current between the first portion of the regulated current flowing through the first LED string and the second portion of the regulated current flowing through the second LED string responsive to the signal indicative of the desired brightness level.

17. The method of claim 16, further comprising:

generating a driver control signal responsive to the signal indicative of the desired brightness level,

wherein a level of the regulated current is regulated responsive to the driver control signal.

18. The method of claim 17, wherein the LED driver comprises a switching power regulator that includes a switch to regulate the level of the regulated current, and a duty cycle of ON-times and OFF-times of the switch is controlled by the driver control signal.

19. The method of claim 16, wherein the signal indicative of a desired brightness level is controlled by a dimming level set by a dimmer switch.

20. The method of claim 16, wherein the first color temperature of light emitted by the first LED string is lower than the second color temperature of light emitted by the second LED string.

21. The method of claim 20, wherein the first color temperature of emitted by the first LED string is substantially red and the second color temperature of light emitted by the second LED string is substantially white.

22. The method of claim 16, wherein controlling an allocation of the regulated current comprises:

generating at least one switch control signal responsive to the signal indicative of the desired brightness level; and

controlling a duty cycle of ON times and OFF times of a first switch coupled in series with the first LED string, the ON times and OFF times of the first switch responsive to a first control signal of the at least one control signal,

wherein the allocation of the regulated current between the first portion of the regulated current flowing through the first LED string and the second portion of the regulated current flowing through the second LED string is responsive to the duty cycle of the first switch.

23. The method of claim 22, wherein controlling an allocation of the regulated current comprises:

controlling a duty cycle of ON times and OFF times of a second switch coupled in series with the second LED string, the ON times and OFF times of the first switch responsive to a second control signal of the at least one control signal,

wherein the allocation of the regulated current between the first portion of the regulated current flowing through the first LED string and the second portion of the regulated current flowing through the second LED string is responsive to the duty cycle of the second switch.

24. The method of claim 22, wherein a number of LEDs in the first LED string is different than a number of LEDs in the second LED string, and wherein substantially all of the regulated current flows through the first LED string when the first switch is ON.

25. The method of claim 22, wherein a number of LEDs in the first LED string is equal to a number of LEDs in the second LED string, and wherein the regulated current is split between the first LED string and the second LED string when the first switch is ON.

26. The method of claim 16, wherein as the desired brightness level decreases, an overall color temperature of the light emitted by the first string and the light emitted by the second LED string decreases.

27. The method of claim 16, wherein, as a level of the regulated current decreases, the first portion of the regulated current flowing through the first LED string increases relative to the second portion of the regulated current flowing through the second LED string.