Current sense apparatus for battery charger systems

1. A method comprising:

in a current sense mode, applying a minimum drain-to-source voltage limiter to a first current mirror of a current sense apparatus, wherein, as a result of applying the minimum drain-to-source voltage limiter to the first current mirror, a voltage across the first current mirror is clamped to a predetermined value; and

in a power saving mode, disabling the minimum drain-to-source voltage limiter;

wherein the current sense apparatus comprises:

the first current mirror comprising a first switch coupled between a power source and a load, and a second switch comprising a gate connected to a gate of the first switch;

a first operational amplifier having a first input connected to the first switch and a second input connected to the second switch; and

a second operational amplifier having an output connected to the gate of the first switch, a first input coupled to a first drain/source terminal of the first switch and a second input coupled to a second drain/source terminal of the first switch.

2. The method of claim 1 , wherein the current sense apparatus further comprises:

a reference voltage source connected to one input of the second operational amplifier.

3. The method of claim 2 , wherein the current sense apparatus further comprises:

a second current mirror comprising a third switch and a fourth switch, wherein a gate of the third switch is connected to a gate of the fourth switch and further connected to an output of the first operational amplifier.

4. The method of claim 2 , further comprising:

dynamically adjusting an output voltage of the reference voltage source based upon a sensed current generated by the current sense apparatus.

5. The method of claim 2 , further comprising:

in the current sense mode, injecting a minimum voltage into a voltage across the first switch so that the voltage across the first switch is substantially equal to an output voltage of the reference voltage source.

6. The method of claim 5 , wherein:

the reference voltage source is connected between the first drain/source terminal of the first switch and a non-inverting input of the second operational amplifier.

7. The method of claim 6 , wherein:

a fifth switch is connected between an inverting input of the second operational amplifier and the second drain/source terminal of the first switch;

a sixth switch is connected between the inverting input of the second operational amplifier and ground;

the current sense apparatus operates in the current sense mode when the fifth switch is turned on and the sixth switch is turned off; and

the current sense apparatus operates in the power saving mode when the fifth switch is turned off and the sixth switch is turned on.

8. A method comprising:

generating a voltage across a first current mirror of a current sense apparatus, the voltage being proportional to a current flowing through the current sense apparatus when the current is greater than a predetermined current value; and

applying a minimum drain-to-source voltage limiter to the first current mirror of the current sense apparatus when the current is less than the predetermined current value, wherein, as a result of applying the minimum drain-to-source voltage limiter to the first current mirror, the voltage across the first current mirror is clamped to a predetermined voltage value;

wherein the current sense apparatus comprises:

the first current mirror comprising a first switch and a second switch, wherein a gate of the first switch is connected to a gate of the second switch;

a first operational amplifier having a first input connected to the first switch and a second input connected to the second switch; and

a second operational amplifier having an output connected to the gate of the first switch, a non-inverting input connected to a first drain/source terminal of the first switch and an inverting input connected to a second drain/source terminal of the first switch.

9. The method of claim 8 , wherein the current sense apparatus further comprises:

a second current mirror comprising a third switch and a fourth switch, wherein a gate of the third switch is connected to a gate of the fourth switch;

wherein the non-inverting input of the second operational amplifier is connected to the first drain/source terminal of the first switch through a reference voltage source.

10. The method of claim 9 , wherein:

a voltage across the fourth switch is proportional to a current flowing through the first switch when the current flowing through the first switch is greater than the predetermined current value.

11. The method of claim 9 , wherein:

the first operational amplifier is configured such that a drain-to-source voltage of the first switch is substantially equal to a drain-to-source voltage of the second switch.

12. The method of claim 9 , wherein:

a drain of the third switch is connected to a source of the second switch; and

a source of the third switch is connected to ground, and wherein the first operational amplifier adjusts a drain-to-source voltage of the second switch through adjusting a gate voltage of the third switch.

13. The method of claim 9 , wherein:

an output voltage of the reference voltage source is approximately equal to 50 mV.

14. The method of claim 9 , wherein:

the voltage across the first current mirror is clamped to a predetermined voltage through reducing a gate voltage of the first switch.

15. The method of claim 8 , further comprising:

configuring the current sense apparatus to operate in a power saving mode by disabling the minimum drain-to-source voltage limiter.

16. A method comprising:

passing a current from a power source to a load through a first switch;

mirroring in a second switch the current passing through the first switch, wherein the second switch comprises a gate connected to a gate of the first switch and a first terminal connected to a first terminal of the first switch;

driving a voltage at a second terminal of the first switch equal to a voltage at a second terminal of the second switch through a first operational amplifier;

maintaining a minimum voltage across the first switch through a second operational amplifier having an output connected to the gate of the first switch, a first input coupled to the first terminal of the first switch and a second input coupled to the second terminal of the first switch;

enabling a reference voltage source connected to one input of the second operational amplifier so that a voltage across the first switch is approximately equal to an output voltage of the reference voltage source during a current sense operation; and

disabling the reference voltage source so that the voltage across the first switch is equal to the current flowing through the first switch times an on-resistance value of the first switch during a power saving operation.

17. The method of claim 16 , further comprising:

dynamically adjusting the output voltage of the reference voltage source based upon the current flowing through the first switch.

18. The method of claim 16 , further comprising:

coupling a third switch between the second terminal of the first switch and the second input of the second operational amplifier; and

coupling a fourth switch between the second input of the second operational amplifier and ground.

19. The method of claim 18 , further comprising:

in response to the current sense operation, turning on the third switch and turning off the fourth switch; and

in response to the power saving operation, turning on the fourth switch and turning off the third switch.

20. The method of claim 18 , wherein:

the reference voltage source is connected to the first input of the second operational amplifier.