Input current conditioner for precision coulomb counting

1. A coulomb counter for counting coulombs drawn from a power source by a load, the coulomb counter comprising:

a reference current source (I REF ) coupled between an input node and an output node of the coulomb counter and configured to regulate a current drawn by the load from the power source;

a switch coupled between the reference current source and the output node of the coulomb counter;

a comparator having a first input node coupled to the input node of the coulomb counter, a second input node coupled to the output node of the coulomb counter, and an output node coupled to a control node of the switch, wherein the comparator is configured to control a time the switch is closed and a time the switch is open;

an output capacitor coupled to the output node of the coulomb counter;

an oscillator coupled to the output node of the comparator; and

a counter having an input coupled to an output node of the oscillator and configured to count a number of oscillator cycles during the time the switch is closed and provide the count at a second output node.

2. The coulomb counter of claim 1 , wherein the reference current source is configured to regulate the current drawn by the load at any given time to either be zero or a predetermined fixed amount.

3. The coulomb counter of claim 1 , wherein one or more circuits of the coulomb counter are each powered by a voltage at the output node of the coulomb counter.

4. The coulomb counter of claim 1 , further comprising a voltage source coupled between the second input node of the comparator and the output node of the coulomb counter, wherein the voltage source is configured to provide a predetermined offset for the comparator.

5. The coulomb counter of claim 1 , wherein the comparator includes hysteresis.

6. The coulomb counter of claim 1 , wherein the comparator is configured, upon determining that a voltage at the output node of the coulomb counter is below a voltage at the input node of the coulomb counter by over a first predetermined threshold, to close the switch and allow a charge to be drawn by the load from the power source at a constant rate controlled by the current source.

7. The coulomb counter of claim 6 , wherein the comparator is configured, upon determining that the voltage at the output node of the coulomb counter is within a second predetermined threshold of the voltage at the input node of the coulomb counter, to open the switch and prevent a charge to be drawn by the load from the power source.

8. The coulomb counter of claim 7 , wherein the second predetermined threshold has a magnitude that is less than a magnitude of the first predetermined threshold.

9. The coulomb counter of claim 1 , wherein no coulomb counting errors are associated with the coulomb counter when the reference current is zero.

10. The coulomb counter of claim 1 , wherein the number of coulombs drawn by the power source is based on a product of the regulated current of the current source and the time the switch is closed.

11. The coulomb counter of claim 1 , wherein an accuracy of a number of counted coulombs by the coulomb counter is based mainly on an accuracy of the current of the current source and an accuracy of the determination of the time the switch is closed.

12. A method of counting coulombs drawn from a power source by a load, in a coulomb counter having a reference current source (I REF ), a switch coupled between the reference current source and a first output node of the coulomb counter; a comparator coupled between an input node and the first output node of the coulomb counter, an oscillator coupled to an output node of the comparator, a counter coupled to an output of the oscillator, and a second output of the coulomb counter, the method comprising:

regulating, by the reference current source, the current drawn by the load at any given time to either be zero or a predetermined fixed amount;

controlling, by the comparator, a time the switch is closed and a time the switch is open;

allowing, by the switch, coulombs to be drawn from the power source when the switch is closed and preventing coulombs from being drawn from the power source when the switch is open;

generating, by the oscillator, a clock signal only during the time the switch is closed;

counting a number of clock cycles from the clock signal during the time the switch is closed; and

providing a signal at the second output of the coulomb counter that indicates the number of counted clock cycles.

13. The method of claim 12 , further comprising powering the coulomb counter by a voltage at the first output node of the coulomb counter.

14. The method of claim 12 , further comprising, upon determining by the comparator that a voltage at the output node of the coulomb counter is below a voltage at the input node of the coulomb counter by over a first predetermined threshold, closing the switch and allowing a charge to be drawn by the load from the power source at a constant rate controlled by the current source.

15. The method of claim 14 , further comprising, upon determining by the comparator that the voltage at the output node of the coulomb counter is within a second predetermined threshold of the voltage at the input node of the coulomb counter, opening the switch and preventing a charge to be drawn by the load from the power source.

16. The method of claim 14 , wherein the second predetermined threshold has a magnitude that is less than the first predetermined threshold.

17. The method of claim 12 , further comprising determining a number of coulombs drawn by the power source based on a product of the regulated current of the current source and the time the switch is closed.

18. The method of claim 12 , wherein an accuracy of a number of counted coulombs by the coulomb counter is based on an accuracy of the current of the current source and an accuracy of the determination of the time the switch is closed.