Load control system operable under different power supply conditions

1. A load control system responsive to electric power from either a first power source or a second power source operable during first and second time periods, respectively, comprising:

a first circuit responsive to the first and second power sources to develop power waveforms comprising different characteristics during the first and second time periods;

a second circuit coupled to the first circuit and responsive to the characteristics of the power developed by the first circuit for developing first and second different outputs during the first and second time periods, respectively, wherein the first output operates at least one light emitting diode (LED) in a first operating mode, and the second output operates the at least one LED in a second operating mode different from the first operating mode; and

wherein the first circuit switches between the first and second power sources in response to at least one of the characteristics of the power waveform developed by the first power source crossing a predetermined threshold.

2. The load control system of claim 1 , wherein the first circuit develops power at first and second frequencies during the first and second time periods, respectively.

3. The load control system of claim 2 , wherein the first output operates the LEDs in a fully energized mode and the second output operates the LEDs in a dimmed mode.

4. The load control system of claim 1 , wherein the first power source comprises utility power and the second power source comprises a back-up source of power.

5. The load control system of claim 4 , wherein the first and second outputs result in first and second load magnitude conditions, respectively, wherein the first load magnitude condition is greater than the second load magnitude condition.

6. The load control system of claim 1 , wherein the second circuit is disposed in a luminaire comprising a plurality of LEDs operated by the second circuit.

7. The load control system of claim 6 , wherein the second circuit comprises a frequency detector responsive to the power developed by the first circuit.

8. The load control system of claim 7 , wherein the frequency detector comprises an analog phase locked loop.

9. The load control system of claim 8 , wherein the frequency detector comprises a digital phase locked loop.

10. A load control system responsive to electric power from either a utility power source or a back-up power source operable during first and second time periods, respectively, comprising:

a first circuit responsive to the utility and back-up power sources to develop power at first and second different frequencies during the first and second time periods, respectively; and

a second circuit coupled to the first circuit and responsive to the different frequencies of the power developed by the first circuit for developing first and second outputs during the first and second time periods, respectively; and

wherein the first circuit switches between the utility and back-up power sources in response to the second circuit detecting that the first frequency developed by the utility power source crosses a predetermined threshold.

11. The load control system of claim 10 , wherein the first and second outputs operate a plurality of LEDs.

12. The load control system of claim 11 , wherein the first and second outputs command first and second different LED operating conditions, respectively, wherein the first LED operating condition is greater than the second LED operating condition.

13. The load control system of claim 12 , wherein the first circuit comprises a power converter and the second circuit operates the plurality of LEDs.

14. The load control system of claim 11 , wherein the second circuit comprises a frequency detector responsive to the power developed by the first circuit.

15. The load control system of claim 14 , wherein the frequency detector comprises an analog phase locked loop.

16. The load control system of claim 15 , wherein the frequency detector comprises a digital phase locked loop.

17. A combination of a control circuit and an LED load operated by the control circuit, wherein the combination is responsive to electric power from either a utility power source or a back-up power source operable during first and second time periods, respectively, comprising:

a first circuit responsive to the utility and back-up power sources to develop power at first and second different frequencies during the first and second time periods, respectively;

a second circuit coupled to the first circuit and responsive to the frequency of the power developed by the first circuit for developing first and second outputs, respectively; and

a plurality of LEDs coupled to the second circuit, energized by the first and second outputs, and developing a brightness dependent upon the outputs; and

wherein the first circuit switches between the utility and back-up power sources in response to the second circuit detecting that the first frequency developed by the utility power source crosses a predetermined threshold.

18. The combination of claim 17 , wherein the first circuit comprises a power converter.

19. The combination of claim 18 , wherein the second circuit is disposed in a luminaire that comprises the plurality of LEDs.

20. The combination of claim 19 , wherein the second circuit comprises a frequency detector responsive to the power developed by the first circuit.

21. The combination of claim 20 , wherein the frequency detector comprises an analog phase locked loop.

22. The combination of claim 21 , wherein the frequency detector comprises a digital phase locked loop.