Adjusting color temperature in a dimmable LED lighting system
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.
1. A light emitting diode (LED) controller for a switching power converter, comprising:
a dimming detection module configured to receive a rectified input voltage, to detect a desired brightness level from a magnitude and/or a shape of the rectified input voltage, and to thereby generate a target current level signal that varies responsive to a variation of the desired brightness level;
a regulator configured to regulate a cycling of a power switch to generate a regulated current at an output of the switching power converter responsive to the target current level signal; and
circuitry configured to allocate the regulated current between a first portion of the regulated current flowing through a first LED string and a second portion of the regulated current flowing through a second LED string responsive to changes in the desired brightness level such that a light output from the first LED string and the second LED string increases in redness responsive to a decrease in the desired brightness level, the circuitry comprising:
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; and
a color control module configured to generate the first switch control signal responsive to the target current level signal.
2. The LED controller of claim 1 , wherein a duty cycle of ON-times and OFF-times of the power switch is controlled responsive to the target current level signal.
3. The LED controller of claim 1 , wherein the circuitry further comprises:
a second switch coupled in series with the second LED string, wherein a duty cycle of ON and OFF times of the second switch is responsive to a second switch control signal, wherein the color control module is configured to generate the second switch control signal responsive to the target current level signal.
4. The LED controller of claim 1 , wherein substantially all of the regulated current flows through the first LED string when the first switch is ON.
5. The LED controller of claim 1 , wherein the regulated current is split between the first LED string and the second LED string when the first switch is ON.
6. The LED controller 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.
7. The LED controller 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.
8. A method of operation for a light emitting diode (LED) controller for a switching power converter, the method comprising:
detecting a desired brightness level from a magnitude and/or a shape of a rectified input voltage from a phase cut dimmer switch;
generating a target current level signal that varies responsive to a variation in the desired brightness level;
cycling a power switch to drive a regulated current through an output of the switching power converter; and
controlling an allocation of the regulated current between a first LED string and a second LED string, the allocation of the regulated current being controlled such that an overall light emitted by the first LED string and the second LED string increases in redness responsive to a decrease in the desired brightness level.
9. The method of claim 8 , further comprising controlling a duty cycle of ON-times and OFF-times of the power switch responsive to the target current level signal.
10. The method of claim 8 , wherein a first color temperature of light emitted by the first LED string is lower than a second color temperature of light emitted by the second LED string.
11. The method of claim 10 , 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.
12. The method of claim 8 , wherein controlling the allocation of the regulated current comprises:
generating at least one switch control signal responsive to the target current level signal; and
controlling a duty cycle of ON times and OFF times of a first switch coupled in series with the first LED string.
13. The method of claim 12 , wherein the controlling of the allocation of the regulated current further comprises:
controlling a duty cycle of ON times and OFF times of a second switch coupled in series with the second LED string.
14. The method of claim 12 , 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.
15. The method of claim 12 , 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.
16. The method of claim 12 , wherein, as a level of the regulated current decreases, a first portion of the regulated current flowing through the first LED string increases relative to a second portion of the regulated current flowing through the second LED string.