Load control device for a light-emitting diode light source

1. A circuit for controlling an intensity of a light-emitting diode (LED) light source, the circuit comprising:

an LED drive circuit configured to control a magnitude of a load current conducted through the LED light source to control the intensity of the LED light source;

a current sense circuit configured to generate a feedback signal that indicates the magnitude of the load current; and

a control circuit configured to:

generate at least one drive signal for controlling the LED drive circuit, the at least one drive signal characterized by an operating period and a duty cycle;

adjust an on time of the at least one drive signal in response to the feedback signal to regulate the magnitude of the load current toward a target load current that ranges between a minimum rated current and a maximum rated current, the minimum rated current based on a minimum on time of the LED drive circuit; and

adjust the operating period of the at least one drive signal to a low-end operating period by adjusting the on time of the at least one drive signal to a value less than or equal to the minimum on time while holding the duty cycle of the at least one drive signal constant.

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

operate in a first state and a second state on a periodic basis;

control the LED drive circuit in the first state during a first time period in which the control circuit adjusts the on time of the at least one drive signal in response to the feedback signal to regulate a peak magnitude of the load current toward the target load current;

control the LED drive circuit in the second state during a second time period in which the control circuit ceases adjustment of the on time of the at least one drive signal in response to the feedback signal; and

adjust an average magnitude of the load current by adjusting lengths of the first and second time periods.

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

operate in a normal mode to adjust the average magnitude of the load current above a minimum rated current of the LED drive circuit; and

operate in a burst mode to adjust the average magnitude of the load current below the minimum rated current.

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

during the normal mode, maintain the operating period of the at least one drive signal at a high-end operating period after the target load current reaches a high-end transition value.

5. The circuit of claim 4 , wherein the high-end transition value is equal to approximately a maximum rated current of the LED light source.

6. The circuit of claim 4 , wherein the high-end transition value is less than a maximum rated current of the LED light source.

7. The circuit of claim 4 , wherein the control circuit is configured to adjust the operating period of the at least one drive signal between the low-end operating period and the high-end operating period when the target load current is between a low-end transition value and the high-end transition value.

8. The circuit of claim 4 , wherein the low-end operating period is shorter than the high-end operating period.

9. The circuit of claim 3 , wherein the control circuit is further configured to:

during the burst mode, maintain the peak magnitude of the load current at the minimum rated current.

10. The circuit of claim 3 , wherein the control circuit is further configured to:

during the normal mode, keep the length of the second time period at approximately zero seconds.

11. The circuit of claim 2 , wherein the control circuit is further configured to:

adjust the length of the first time period in first adjustment steps that are dependent on the low-end operating period.

12. The circuit of claim 11 , wherein the control circuit is further configured to:

adjust the length of the first time period in the first adjustment steps while keeping a length of the operating period constant.

13. The circuit of claim 2 , wherein the control circuit is further configured to:

adjust a ratio of the length of the first time period to the length of the second time period to adjust the average magnitude of the load current.

14. The circuit of claim 13 , wherein the control circuit is further configured to:

determine the target load current as a function of a target amount of power to be delivered to the LED light source; and

adjust the ratio of the length of the first time period to the length of the second time period linearly with respect to the target amount of power.

15. The circuit of claim 1 , wherein the control circuit is further configured to:

periodically decrease the operating period of the at least one drive signal by a predetermined amount until the on time of the at least one drive signal is less than or equal to the minimum on time.

16. A method of controlling an intensity of a light-emitting diode (LED) light source, the method comprising:

generating at least one drive signal for controlling an LED drive circuit to adjust a magnitude of a load current conducted through the LED light source to control the intensity of the LED light source, the at least one drive signal characterized by an operating period and a duty cycle;

receiving a load current feedback signal that indicates the magnitude of the load current;

adjusting an on time of the at least one drive signal in response to the feedback signal to regulate the magnitude of the load current toward a target load current that ranges between a minimum rated current and a maximum rated current, the minimum rated current based on a minimum on time of the LED drive circuit; and

adjusting the operating period of the at least one drive signal to a low-end operating period by adjusting the on time of the at least one drive signal to a value less than or equal to the minimum on time while holding the duty cycle of the at least one drive signal constant.

17. The method of claim 16 , further comprising:

operating the LED drive circuit in a first state and a second state on a periodic basis;

controlling the LED drive circuit in the first state during a first time period in which the on time of the at least one drive signal is adjusted in response to the feedback signal to regulate a peak magnitude of the load current toward the target load current;

controlling the LED drive circuit in the second state during a second time period in which adjustment of the on time of the at least one drive signal in response to the feedback signal is ceased; and

adjusting an average magnitude of the load current by adjusting lengths of the first and second time periods.

18. The method of claim 17 , further comprising:

operating in a normal mode to adjust the average magnitude of the load current above a minimum rated current of the LED drive circuit; and

operating in a burst mode to adjust the average magnitude of the load current below the minimum rated current.

19. The method of claim 18 , further comprising:

when operating in the normal mode, maintaining the operating period of the at least one drive signal at a high-end operating period after the target load current reaches a high-end transition value.

20. The method of claim 17 , further comprising:

adjusting the length of the first time period in first adjustment steps that are dependent on the low-end operating period while keeping a length of the operating period constant.

21. The method of claim 17 , wherein adjusting the average magnitude of the load current by adjusting lengths of the first and second time periods comprises:

adjusting a ratio of the lengths of the first and second time periods to adjust the average magnitude of the load current.

22. The method of claim 17 , further comprising:

adjusting a duty cycle defining when the LED drive circuit operates in the first state and the second state to adjust the average magnitude of the load current; and

when the duty cycle defining when the LED drive circuit operates in the first state and the second state is at a maximum duty cycle, adjusting the on time of the at least one drive signal to regulate the average magnitude of the load current toward the target load current.