Current sense apparatus for battery charger systems
A current sense apparatus comprises a first switch having a first terminal connected to a power source and a second terminal connected to a battery, a second switch comprising a gate connected to a gate of the first switch and a first terminal connected to the first terminal of the first switch, a first operational amplifier having a first input connected to the second terminal of the first switch and a second input connected to a second terminal of the second switch, 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 and a reference voltage source connected to one input of the second operational amplifier.
1. A system comprising:
a first switch electrically connected between a power source and a load;
a second switch comprising a gate connected to a gate of the first switch and a first terminal connected to a first terminal of the first switch;
a first operational amplifier having a first input connected to a second terminal of the first switch and a second input connected to a second terminal of the second switch;
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, wherein the second operational amplifier is configured to maintain a minimum voltage across the first switch; and
a reference voltage source connected to one input of the second operational amplifier.
2. The system of claim 1 , wherein:
the first switch is a first N-type transistor having a drain connected to the power source and a source connected to the first input of the first operational amplifier; and
the second switch is a second N-type transistor having a drain connected to the power source and a source connected to the second input of the first operational amplifier.
3. The system of claim 1 , wherein:
the first input of the first operational amplifier is a non-inverting input; and
the second input of the first operational amplifier is an inverting input.
4. The system of claim 1 , further comprising:
a 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.
5. The system of claim 4 , wherein:
a drain of the third switch is connected to the second terminal of the second switch;
a source of the third switch is connected to ground;
a source of the fourth switch is connected to the source of the third switch; and
a drain of the fourth switch is configured to generate a current sense signal proportional to a current flowing through the first switch.
6. The system of claim 1 , wherein:
the reference voltage source is connected between the first terminal of the first switch and a non-inverting input of the second operational amplifier.
7. The system of claim 6 , further comprising:
a fifth switch connected between an inverting input of the second operational amplifier and the second terminal of the first switch; and
a sixth switch connected between the inverting input of the second operational amplifier and ground.
8. The system of claim 7 , wherein:
the fifth switch and the sixth switch are configured to be controlled by complementary drive signals.
9. The system of claim 1 , wherein:
the reference voltage source is connected between the second terminal of the first switch and an inverting input of the second operational amplifier.
10. The system of claim 1 , wherein:
an output voltage of the reference voltage source is equal to 50 mV.
11. The system of claim 1 , wherein:
the power source is a direct current power source; and
the load is a rechargeable battery.
12. The system of claim 1 , wherein:
a current flowing through the second switch is a fraction of a current flowing through the first switch.
13. A system comprising:
a first switch electrically connected between a power source and a load;
a second switch comprising a gate connected to a gate of the first switch and a first terminal connected to a first terminal of the first switch, wherein:
the first switch is a first N-type transistor having a drain connected to the power source and a source connected to a first input of a first operational amplifier; and
the second switch is a second N-type transistor having a drain connected to the power source and a source connected to a second input of the first operational amplifier;
the first operational amplifier having the first input connected to a second terminal of the first switch and the second input connected to a second terminal of the second switch;
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; and
a reference voltage source connected to one input of the second operational amplifier.
14. A system comprising:
a first switch electrically connected between a power source and a load;
a second switch comprising a gate connected to a gate of the first switch and a first terminal connected to a first terminal of the first switch;
a first operational amplifier having a first input connected to a second terminal of the first switch and a second input connected to a second terminal of the second switch;
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;
a reference voltage source connected to one input of the second operational amplifier; and
a 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.