Load control device for a light-emitting diode light source

1 . A load control device for controlling an amount of power delivered to a light source having a first emitter circuit configured to emit light having a first color temperature and a second emitter circuit configured to emit light having a second color temperature, the load control device comprising:

a first drive circuit configured to control a first intensity level of the first emitter circuit;

a second drive circuit configured to control a second intensity level of the second emitter circuit; and

a control circuit configured to control the first drive circuit and the second drive circuit to adjust the first intensity level of the first emitter circuit and the second intensity level of the second emitter circuit to cause a present intensity level and a present color temperature of a cumulative light emitted by the light source to be controlled towards a target intensity level and a target color temperature, respectively;

wherein, when the target intensity level is less than a threshold intensity level, the control circuit is configured to limit the target color temperature according to at least one limit curve that causes the target color temperature to converge towards at least one of a first color temperature or a second color temperature as the target intensity level decreases towards a low-end intensity level.

2 . The load control device of claim 1 , wherein the control circuit is configured to:

receive a commanded color temperature for controlling a cumulative light emitted by the light source;

when the commanded color temperature is greater than a threshold color temperature, limit the target color temperature according to a first limit curve that causes the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level; and

when the commanded color temperature is less than the threshold color temperature, limit the target color temperature according to a second limit curve that causes the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level.

3 . The load control device of claim 2 , wherein the control circuit is configured to:

cause the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to a value of a first limited color temperature on the first limit curve at the target intensity level when the commanded color temperature exceeds the first limit curve; and

cause the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to a value of a second limited color temperature on the second limit curve at the target intensity level when the commanded color temperature exceeds the second limit curve.

4 . The load control device of claim 3 , wherein the control circuit is configured to:

set the target color temperature equal to the commanded color temperature when the commanded color temperature is greater than the threshold color temperature and the commanded color temperature does not exceed the first limit curve; and

set the target color temperature equal to the commanded color temperature when the commanded color temperature is less than the threshold color temperature and the commanded color temperature does not exceed the second limit curve.

5 . The load control device of claim 3 , wherein the control circuit is configured to:

determine that the commanded color temperature exceeds the first limit curve when the commanded color temperature is greater than the threshold color temperature and the commanded color temperature is less than the first limit curve at the target intensity level; and

determine that the commanded color temperature exceeds the second limit curve when the commanded color temperature is less than the threshold color temperature and the commanded color temperature is less than the second limit curve at the target intensity level.

6 . The load control device of claim 2 , further comprising:

a communication circuit configured to receive messages;

wherein the control circuit is configured to receive a message including the commanded color temperature.

7 . The load control device of claim 1 , wherein the control circuit is configured to:

receive a commanded color temperature for controlling a cumulative light emitted by the light source;

when the commanded color temperature is greater than a first threshold color temperature, limit the target color temperature according to a first limit curve that causes the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level;

when the commanded color temperature is less than a second threshold color temperature, limit the target color temperature according to a second limit curve that causes the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level; and

when the commanded color temperature is between the first and second threshold color temperatures, limit the target color temperature according to a third and fourth limit curves that cause the target color temperature to converge towards a third color temperature as the target intensity level decreases towards the low-end intensity level, wherein the third color temperature is between the first and second color temperatures.

8 . The load control device of claim 7 , wherein the control circuit is configured to:

cause the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to the value of the first limited color temperature on the first limit curve at the target intensity level when the commanded color temperature exceeds the first limit curve;

cause the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to the value of the second limited color temperature on the second limit curve at the target intensity level when the commanded color temperature exceeds the second limit curve; and

cause the target color temperature to converge towards the third color temperature as the target intensity level decreases towards the low-end intensity level by setting the target color temperature equal to a value of a third limited color temperature on the third limit curve at the target intensity level when the commanded color temperature is greater than the third color temperature and the commanded color temperature exceeds the third limit curve, and setting the target color temperature equal to a value of a fourth limited color temperature on the fourth limit curve at the target intensity level when the commanded color temperature is less than the third color temperature and the commanded color temperature exceeds the fourth limit curve.

9 . The load control device of claim 7 , wherein the control circuit is configured to cause the target color temperature to converge to the first, second, and third color temperatures when the target intensity level is equal to an intermediate intensity level that is less than the threshold intensity level and greater than the low-end intensity level.

10 . The load control device of claim 1 , wherein control circuit configured to:

generate first and second drive signals for controlling the first and second drive circuits, respectively;

determine respective on-times for the first and second drive signals in response to the target intensity level and the target color temperature; and

round the respective on-times for the first and second drive signals to multiples of a minimum step size at which the control circuit may adjust the respective on-times of the first and second drive signals.

11 . A method for controlling an amount of power delivered to a light source having a first emitter circuit configured to emit light having a first color temperature and a second emitter circuit configured to emit light having a second color temperature, the method comprising:

controlling a first drive circuit to control a first intensity level of the first emitter circuit;

controlling a second drive circuit to control a second intensity level of the second emitter circuit;

controlling the first and second drive circuits to cause a present intensity level and a present color temperature of a cumulative light emitted by the light source to be controlled towards a target intensity level and a target color temperature, respectively; and

when the target intensity level is less than a threshold intensity level, limiting the target color temperature according to at least one limit curve that causes the target color temperature to converge towards at least one of a first color temperature or a second color temperature as the target intensity level decreases towards a low-end intensity level.

12 . The method of claim 11 , further comprising:

receiving a commanded color temperature for controlling a cumulative light emitted by the light source.

13 . The method of claim 12 , wherein limiting the target color temperature further comprises:

when the commanded color temperature is greater than a threshold color temperature, limiting the target color temperature according to a first limit curve that causes the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level; and

when the commanded color temperature is less than the threshold color temperature, limiting the target color temperature according to a second limit curve that causes the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level.

14 . The method of claim 13 , wherein limiting the target color temperature according to the first limit curve further comprises setting the target color temperature equal to a value of a first limited color temperature on the first limit curve at the target intensity level when the commanded color temperature exceeds the first limit curve; and

wherein limiting the target color temperature according to the second limit curve further comprises setting the target color temperature equal to a value of a second limited color temperature on the second limit curve at the target intensity level when the commanded color temperature exceeds the second limit curve.

15 . The method of claim 14 , further comprising:

setting the target color temperature equal to the commanded color temperature when the commanded color temperature is greater than the threshold color temperature and the commanded color temperature does not exceed the first limit curve; and

setting the target color temperature equal to the commanded color temperature when the commanded color temperature is less than the threshold color temperature and the commanded color temperature does not exceed the second limit curve.

16 . The method of claim 14 , further comprising:

determining that the commanded color temperature exceeds the first limit curve when the commanded color temperature is greater than the threshold color temperature and the commanded color temperature is less than the first limit curve at the target intensity level; and

determining that the commanded color temperature exceeds the second limit curve when the commanded color temperature is less than the threshold color temperature and the commanded color temperature is less than the second limit curve at the target intensity level.

17 . The method of claim 12 , wherein limiting the target color temperature further comprises:

when the commanded color temperature is greater than a first threshold color temperature, limiting the target color temperature according to a first limit curve that causes the target color temperature to converge towards the first color temperature as the target intensity level decreases towards the low-end intensity level;

when the commanded color temperature is less than a second threshold color temperature, limiting the target color temperature according to a second limit curve that causes the target color temperature to converge towards the second color temperature as the target intensity level decreases towards the low-end intensity level; and

when the commanded color temperature is between the first and second threshold color temperatures, limiting the target color temperature according to a third and fourth limit curves that cause the target color temperature to converge towards a third color temperature as the target intensity level decreases towards the low-end intensity level, wherein the third color temperature is between the first and second color temperatures.

18 . The method of claim 17 , further comprising:

causing the target color temperature to converge to the first, second, and third color temperatures when the target intensity level is equal to an intermediate intensity level that is less than the threshold intensity level and greater than the low-end intensity level.

19 . The method of claim 12 , further comprising:

receiving a message including the commanded color temperature.

20 . The method of claim 11 , further comprising:

generating first and second drive signals for controlling the first and second drive circuits, respectively;

determining respective on-times for the first and second drive signals in response to the target intensity level and the target color temperature; and

rounding the respective on-times for the first and second drive signals to multiples of a minimum step size at which the control circuit may adjust the respective on-times of the first and second drive signals.