Low dropout light emitting diode (LED) ballast circuit and method therefor
View Patent ↗A ballast circuit for a Light Emitting Diode (LED) has a regulator element coupled to the LED and to an input voltage source. A control circuit is coupled to the LED and to an input voltage source. A first switching device is coupled in series with the regulator element. A second switching device is coupled to the input voltage and the control circuit.
1. A ballast circuit for a Light Emitting Diode (LED) comprising:
a regulator element coupled to the LED and to an input voltage source;
a control circuit coupled to the LED and to the input voltage source;
a first switching device coupled in series with the regulator element;
a second switching device coupled to the input voltage source and the control circuit; and
a switching control circuit coupled to the first switching device,
wherein the switching control circuit comprises a comparator having one input coupled to a reference voltage, a second input coupled to the second switching element, and an output coupled to the first switching device.
2. The ballast circuit for a Light Emitting Diode (LED) in accordance with claim 1 , further comprising a capacitive element to power the control circuit when the second switching device is turned off.
3. The ballast circuit for a Light Emitting Diode (LED) in accordance with claim 1 , wherein the first switching device is a field effect transistor.
4. The ballast circuit for a Light Emitting Diode (LED) in accordance with claim 1 , wherein the second switching element is a diode.
5. The ballast circuit for a Light Emitting Diode (LED) in accordance with claim 4 , further comprising a voltage regulator coupled to the diode.
6. A ballast circuit for a Light Emitting Diode (LED) comprising:
a regulator element coupled to the LED and to an input voltage source;
a control circuit coupled to the LED and to the input voltage source;
a first switching device coupled in series with the regulator element, wherein said first switching device is a transistor;
a second switching device coupled to the input voltage source and the control circuit; and
a switching control circuit for controlling the first switching device, wherein the switching control circuit comprises a comparator having one input coupled to a reference voltage, a second input coupled to the second switching element, and an output coupled to the first switching device.
7. The ballast circuit for a Light Emitting Diode (LED) in accordance with claim 6 , further comprising a capacitive element to power the control circuit when the second switching device is turned off.
8. The ballast circuit for a Light Emitting Diode (LED) in accordance with claim 6 , wherein the second switching element is a diode.
9. The ballast circuit for a Light Emitting Diode (LED) in accordance with claim 8 , further comprising a voltage regulator coupled to the diode.
10. A method for providing a lower voltage dropout for a ballast circuit for a Light Emitting Diode (LED) comprising:
providing a capacitive element to power a control circuit of the ballast circuit;
coupling a first switching device coupled in series with a regulator element of the ballast circuit;
coupling a second switching device to an input voltage of the ballast circuit and the control circuit;
closing the second switching device to charge the capacitive element to a voltage higher than a dropout voltage of the regulator element of the ballast circuit; and
closing the first switching device while the second switching device is open to discharge the capacitive element.