Charge/discharge control circuit and secondary battery

1. A charge/discharge control circuit for controlling charging and discharging of a secondary battery, wherein the secondary battery includes a cell, the control circuit comprising:

an overdischarge detection circuit for comparing a voltage of the cell with a predetermined discharge reference voltage, determining whether the secondary battery is in an overdischarged state, and generating an overdischarge determination signal;

an overcharge detection circuit for comparing the voltage of the cell with a predetermined charge reference voltage, determining whether the secondary battery is in an overcharged state, and generating an overcharge determination signal;

a discharge control switch connected to the overdischarge detection circuit and deactivated in the overdischarged state based on the overdischarge determination signal;

a first charge control switch deactivated in the overdischarged state based on the overdischarge determination signal and the overcharge determination signal;

a second charge control switch activated in the overdischarged state based on the overdischarge determination signal and the overcharge determination signal; and

a current-limiting circuit connected in series with the second charge control switch for limiting a charging current when charging is performed.

2. The charge/discharge control circuit according to claim 1 , further comprising:

an output circuit connected to the overdischarge detection circuit and the overcharge detection circuit for receiving the overdischarge determination signal and the overcharge determination signal and generating first and second control signals to control activation and deactivation of the first and second control switches.

3. The charge/discharge control circuit according to claim 2 , wherein the output circuit generates the first and second control signals so as to deactivate the first and second charge control switches when the cell is overcharged, wherein the output circuit generates the first and second control signals so as to activate at least the second charge control switch when the cell is overdischarged, and wherein the output circuit generates the first and second control signals so as to activate only the first charge control switch when the cell is neither overcharged nor overdischarged.

4. The charge/discharge control circuit according to claim 1 , wherein the second charge control switch is connected in parallel with the first charge control switch.

5. The charge/discharge control circuit according to claim 1 , wherein the current-limiting circuit includes a resistor having a resistance set such that a voltage drop across the resistor is greater than a predetermined target charging voltage when performing charging in the overdischarged state.

6. The charge/discharge control circuit according to claim 1 , wherein a constant voltage charging is performed by the current-limiting circuit and the second charge control switch when the second charge control switch is activated, and a target charging voltage for the constant voltage charging is substantially equal to the voltage between a source and gate of the second charge control switch.

7. A charge/discharge control circuit for controlling charging and discharging of a secondary battery, wherein the secondary battery includes a cell, the control circuit comprising:

an overdischarge detection circuit for comparing a voltage of the cell with a predetermined discharge reference voltage, determining whether the secondary battery is in an overdischarged state, and generating an overdischarge determination signal;

an overcharge detection circuit for comparing the voltage of the cell with a predetermined charge reference voltage, determining whether the secondary battery is in an overcharged state, and generating an overcharge determination signal;

a discharge control switch connected to the overdischarge detection circuit and deactivated in the overdischarged state based on the overdischarge determination signal;

a first charge control switch deactivated in the overdischarged state based on the overdischarge determination signal and the overcharge determination signal;

a second charge control switch activated in the overdischarged state based on the overdischarge determination signal and the overcharge determination signal;

a current-limiting circuit connected in series with the second charge control switch for limiting a charging current when charging is performed; and

an erroneous functioning prevention circuit for controlling activation and deactivation of the first charge control switch in the overdischarged state based on a level of a voltage supplied by the secondary battery.

8. The charge/discharge control circuit according to claim 7 , wherein the erroneous functioning prevention circuit has a threshold voltage, and wherein the erroneous functioning prevention circuit compares the voltage of the second battery to the threshold voltage, generates a comparison signal, and controls the activation and deactivation of the first charge control switch based on the comparison signal.

9. The charge/discharge control circuit according to claim 8 , wherein the threshold voltage of the erroneous functioning circuit is variable.

10. The charge/discharge control circuit according to claim 7 , further comprising:

an output circuit connected to the overdischarge detection circuit, the overcharge detection circuit and the erroneous functioning prevention circuit, for receiving the overdischarge determination signal, the overcharge determination signal, and the comparison signal and generating first and second control signals to control the activation and deactivation of the first and second control switches.

11. The charge/discharge control circuit according to claim 10 , wherein the output circuit generates the first and second control signals so as to deactivate the first and second charge control switches when the cell is overcharged, wherein the output circuit generates the first and second control signals so as to activate at least the second charge control switch when the cell is overdischarged, and wherein the output circuit generates the first and second control signals so as to activate only the first charge control switch when the cell is neither overcharged nor overdischarged.

12. The charge/discharge control circuit according to claim 7 , wherein the second charge control switch is connected in parallel with the first charge control switch.

13. The charge/discharge control circuit according to claim 7 , wherein the current-limiting circuit includes a resistor having a resistance set such that a voltage drop across the resistor is greater than a predetermined target charging voltage when performing charging in the overdischarged stage.

14. The charge/discharge control circuit according to claim 7 , wherein a constant voltage charging is performed by the current limiting circuit and the second charge control switch when the second control switch is activated, and a target charging voltage for the constant voltage charging is substantially equal to the voltage between a source and gate of the second charge control switch.

15. A secondary battery including:

a cell; and

a charge/discharge control circuit connected to the cell for controlling charging and discharging of the cell, the charge/discharge control circuit comprising:

an overdischarge detection circuit for comparing a voltage of the cell with a predetermined discharge reference voltage, determining whether the secondary battery is in an overdischarged state, and generating an overdischarge determination signal;

an overcharge detection circuit for comparing the voltage of the cell with a predetermined charge reference voltage, determining whether the secondary battery is in an overcharged state, and generating an overcharge determination signal;

a discharge control switch connected to the overdischarge detection circuit and deactivated in the overdischarged state based on the overdischarge determination signal;

a first charge control switch deactivated in the overdischarged state based on the overdischarge determination signal and the overcharge determination signal;

a second charge control switch activated in the overdischarged state based on the overdischarge determination signal and the overcharge determination signal; and

a current-limiting circuit connected in series with the second charge control switch for limiting a charging current when charging is performed.

16. The secondary battery according to claim 15 , wherein the charge/discharge control circuit further includes:

an erroneous functioning prevention circuit for controlling activation and deactivation of the first charge control switch in the overdischarged state based on a level of a voltage supplied by the secondary battery.

17. A charge/discharge control circuit for charging a secondary battery via first and second charge paths connected in parallel and for discharging the secondary battery, the first charge path comprising a first charge control switch, and the second charge path comprising a second charge control switch and bypassing the first charge path, wherein the conductance of the first charge path is higher than that of the second charge path, and wherein the secondary battery includes a cell, the control circuit comprising:

an overdischarge detection circuit for determining that the secondary battery is in an overdischarged state by comparing a voltage of the cell with a predetermined discharge reference voltage to generate an overdischarge determination signal which deactivates the first charge control switch and activates the second charge control switch.

18. The charge/discharge control circuit according to claim 17 , further comprising an output circuit connected to the overdischarge detection circuit for receiving the overdischarge determination signal and generating first and second control signals to control the activation and deactivation of the first and second control switches.

19. The charge/discharge control circuit according to claim 18 , wherein the output circuit generates the first and second control signals to activate at least the second charge control switch when the cell is overdischarged.

20. The charge/discharge control circuit according to claim 17 , further comprising a current- limiting circuit connected in series to the second charge control switch for limiting charging current when charging is performed, wherein the current - limiting circuit includes a resistor having a resistance set so that a voltage drop at the resistor is greater than a predetermined target charging voltage when performing charging in the overdischarged state.

21. The charge/discharge control circuit according to claim 17 , wherein constant voltage charging is performed by the second charge control switch when the second control switch is activated, and the target charging voltage for the constant voltage charging is substantially equal to the voltage between a source and gate of the second charge control switch.

22. A charge/discharge control circuit for charging a secondary battery via first and second charge paths connected in parallel and for discharging the secondary battery, the first charge path comprising a first charge control switch, and the second charge path comprising a second charge control switch and bypassing the first charge path, wherein the conductance of the first charge path is higher than that of the second charge path, and wherein the secondary battery includes a plurality of cells, the control circuit comprising:

an overdischarge detection circuit for receiving voltages of the plurality of cells and determining that the secondary battery is in an overdischarged state when one of the voltages of the plurality of cells is lower than a predetermined discharge reference voltage to generate an overdischarge determination signal, in the overdischarge determination signal, in the overdischarged state, which deactivates the first charge control switch and activates the second charge control switch.

23. The charge/discharge control circuit according to claim 22 , further comprising an output circuit connected to the overdischarged detection circuit for receiving the overdischarge determination signal and generating first and second control signals to control the activation and deactivation of the first and second control switches.

24. The charge/discharge control circuit according to claim 23 , wherein the output circuit generates the first and second control signals to activate at least the second charge control switch when the cell is overdischarged.

25. The charge/discharge control circuit according to claim 22 , further comprising a current- limiting circuit connected in series to the second charge control switch for limiting charging current when charging is performed, wherein the current - limiting circuit includes a resistor having a resistance set so that a voltage drop at the resistor is greater than a predetermined target charging voltage when performing charging in the overdischarged state.

26. The charge/discharge control circuit according to claim 22 , wherein constant voltage charging is performed by the second charge control switch when the second control switch is activated, and a target charging voltage for the constant voltage charging is substantially equal to the voltage between a source and gate of the second charge control switch.