Granted Patent B2

US 7,960,864 · App. 12/513,690 · Granted Jun 14, 2011

Abstract

Claims (39)

1. An arrangement for providing power to a circuit by selecting between a first supply and a second supply, the arrangement comprising:

a first circuit that charges a first capacitive element using the first supply and generates a first reference voltage by distributing charge between the first capacitive element and a second capacitive element;

a first comparator that compares the first reference voltage to a second reference voltage derived from the second supply;

a second comparator that compares the first reference voltage to a third reference voltage; and

a power control circuit that selects one of the first supply and the second supply based on the comparisons performed by the first and the second comparators.

2. The arrangement of claim 1 , further comprising a second circuit that charges a third capacitive element using the second supply and generates the second reference voltage by distributing charge between the third capacitive element and a fourth capacitive element.

3. The arrangement of claim 2 , wherein the second circuit includes:

a first switching element coupled to the second supply and the third capacitive element;

a second switching element coupled to the third capacitive element and the fourth capacitive element; and

a third switching element coupled to the fourth capacitive element.

4. The arrangement of claim 3 , wherein the first switching element is used to charge the third capacitive element using the second supply, the second switching element is used to distribute charge between the third capacitive element and the fourth captive element, and the third switching element is used to discharge the fourth capacitive element.

5. The arrangement of claim 2 , wherein the third capacitive element and the fourth capacitive element are capacitors having approximately equal capacitance.

6. The arrangement of claim 1 , wherein the first capacitive element and the second capacitive element are capacitors having approximately equal capacitance.

7. The arrangement of claim 1 , wherein the first circuit includes:

a first switching element coupled to the first supply and the first capacitive element;

a second switching element coupled to the first capacitive element and the second capacitive element; and

a third switching element coupled to the second capacitive element.

8. The arrangement of claim 7 , wherein the first switching element is used to charge the first capacitive element using the first supply, the second switching element is used to distribute charge between the first capacitive element and the second capacitive element, and the third switching element is used to discharge the second capacitive element.

9. The arrangement of claim 1 , wherein the first supply is a main power supply and the second supply is a backup battery.

10. The arrangement of claim 1 , wherein the arrangement is implemented on an integrated circuit using CMOS technology.

11. The arrangement of claim 1 , wherein the power control circuit selects the second supply when the first reference voltage is less than both the second reference voltage and the third reference voltage, otherwise the power control circuit selects the first supply.

12. The arrangement of claim 1 , wherein the third reference voltage is derived from a threshold value.

13. A method for providing power to a circuit by selecting between a first supply and a second supply, the method comprising;

charging a first capacitive element using the first supply;

generating a first reference voltage by distributing charge between the first capacitive element and a second capacitive element;

comparing the first reference voltage to a second reference voltage derived from the second supply;

comparing the first reference voltage to a third reference voltage; and

selecting one of the first supply and the second supply based on the comparisons.

14. The method of claim 13 , further comprising charging a third capacitive element using the second supply, and generating the second reference voltage by distributing charge between the third capacitive element and a fourth capacitive element.

15. The method of claim 13 , wherein the power is provided from the second supply when the first reference voltage is less than both the second reference voltage and the third reference voltage, otherwise the power is provided from the first supply.

16. The method of claim 13 , wherein the first supply is a main power supply and the second supply is a backup battery, and the power is provided to the circuit by automatically switching between the main power supply and the backup battery.

17. The method of claim 13 , wherein the circuit is a CMOS circuit.

18. The method of claim 13 , wherein the first capacitive element and the second capacitive element are capacitors having approximately equal capacitance.

19. The method of claim 13 , further comprising discharging the second capacitive element, and wherein a first switching element is used to charge the first capacitive element, a second switching element is used to distribute charge between the first capacitive element and the second captive element, and a third switching element is used to discharge the second capacitive element.

20. An arrangement for providing power to a circuit by selecting between a first supply and a second supply, the arrangement comprising:

means for generating a first reference voltage by charging a first capacitive element using the first supply and generating the first reference voltage by distributing charge between the first capacitive element and a second capacitive element;

means for comparing the first reference voltage to a second reference voltage derived from the second supply;

means for comparing the first reference voltage to a third reference voltage; and

means for selecting one of the first supply and the second supply based on the comparisons.

Assignments (20)