Charging device, electronic equipment including same, and control method of charging device

1. A charging device to charge a secondary battery, the charging device comprising:

a DC/DC circuit to generate a charging current supplied to the secondary battery;

an impedance measurement circuit to measure an impedance of the secondary battery;

a first control circuit to output a duty-cycle decrease signal in accordance with the measured impedance:

a charging-current monitor circuit to detect the charging current outputted from the DC/DC circuit when the duty cycle of the pulses generated by the DC/DC circuit is decreased by the duty-cycle decrease signal; and

a second control circuit to compare the charging current detected by the charging-current monitor circuit and a charging-current threshold and to output to the DC/DC circuit a frequency change signal that increases a switching operation frequency in the DC/DC circuit,

wherein the second control circuit determines from the comparison of the charging current detected by the charging-current monitor circuit and a charging-current threshold that the power supply for the secondary battery has decreased and in response outputs a frequency change signal to the DC/DC circuit to cause the switching operation frequency in the DC/DC circuit to change.

2. The charging device according to claim 1 , wherein the first control circuit compares the impedance measured by the impedance measurement circuit to a predetermined impedance threshold and outputs the duty-cycle decrease signal to the DC/DC circuit to decrease the duty cycle of pulses generated in the DC/DC circuit when the measured impedance is greater than the threshold.

3. The charging device according to claim 1 , wherein the first control circuit classifies the impedance measured by the impedance measurement circuit into levels and outputs the duty-cycle decrease signal to the DC/DC circuit to reduce the duty cycle of pulses generated in the DC/DC circuit in accordance with the level of impedance.

4. The charging device according to claim 1 , further comprising:

a battery-voltage detection circuit to detect a secondary battery voltage charge in the secondary battery,

wherein the charging device performs pre-charging with a predetermined first constant current when the secondary battery voltage detected by the battery-voltage detection circuit is lower than a first predetermined voltage, performs constant-current charging with a predetermined second constant current higher than the predetermined first constant current when the detected secondary battery voltage is higher than the first predetermined voltage, and performs constant-voltage charging when the detected secondary battery voltage reaches a second predetermined voltage greater than the first predetermined voltage.

5. The charging device according to claim 4 , wherein the charging device stops the charging when the charging current detected by the charging-current monitor circuit becomes equal to or lower than a predetermined completion current value.

6. The charging device according to claim 5 , further comprising

a third control circuit to output a pre-charging current change signal, a pre-charging a current at constant-current changing signal, and a charging completion current control signal to the DC/DC circuit.

7. The charging device according to claim 6 , further comprising:

a fourth control circuit to output a charging stop signal during pre-charging, during constant-current chagrining, and a constant voltage charging, to the DC/DC circuit.

8. The charging device according to claim 7 , wherein the charging device stops charging when the pre-charging time exceeds the pre-charging stop time, when the constant-current charging time exceeds the constant-current stop time, and when the constant-voltage charging time exceeds the constant-voltage charging stop time before the charging current detected by the charging-current monitor circuit becomes equal to or lower than a predetermined completion current value.

9. An electronic device that employs the charging device according to claim 1 .

10. A method for controlling a charging device to charge a secondary battery, the method comprising:

generating a charging current supplied to the secondary battery with a DC/DC circuit;

measuring impedance of the secondary battery;

adjusting a duty cycle of pulses generated in the DC/DC circuit in accordance with the measured impedance;

detecting a charging current supplied to the secondary battery;

comparing the detected charging current and a first charging-current threshold when the duty cycle of the pulses generated in the DC/DC circuit is adjusted;

increasing a clock frequency of the DC/DC circuit; and

determining from the comparison of the charging current and the charging-current threshold that the power supply for the secondary battery has decreased and in response outputting a frequency change signal to the DC/DC circuit to cause the switching operation frequency in the DC/DC circuit to change.

11. The method for controlling the charging device according to claim 10 , further comprising:

comparing the measured impedance to a predetermined impedance threshold; and

decreasing the duty cycle of pulses generated in the DC/DC circuit when the measured impedance is greater than the threshold.

12. The method for controlling the charging device according to claim 10 , further comprising:

classifying the measured impedance into levels; and

decreasing the duty cycle of pulses generated in the DC/DC circuit in accordance with the classified leveled of impedance.

13. The method for controlling the charging device according to claim 10 , further comprising:

detecting a secondary battery voltage charged in the secondary battery;

performing pre-charging with a predetermined first constant current when the detected secondary battery voltage is lower than a first predetermined voltage;

charging the secondary battery with a predetermined second constant current higher than the first constant current when the detected secondary battery voltage is higher than the first predetermined voltage; and

charging the secondary battery with a constant voltage when the battery voltage of the detected secondary battery reaches a second predetermined voltage higher than the first predetermined voltage.

14. The method for controlling the charging device according to claim 13 ; further comprising:

stopping the charging when the charging current detected by the charging-current monitor circuit becomes equal to or lower than a predetermined completion current value.

15. The method for controlling the charging device according to claim 14 , further comprising:

changing a charging current at the pre-charging;

changing a charging current at the constant-current charging; and

setting and changing a charging completion current value.

16. The method for controlling the charging device according to claim 13 , further comprising:

setting and changing a stop time of the pre-charging current;

setting and changing a stop time of the constant current charging; and

setting and changing a stop time of the constant-voltage charging.

17. The method for controlling the charging device according to claim 16 , further comprising:

stopping charging when the pre-charging time is over the pre-charging stop time, when the constant-current charging time is over the constant-current charging stop time, and when the constant-voltage charging time is over the constant-voltage charging stop time before the charging current detected by the charging-current monitor circuit becomes equal to or lower than a predetermined completion current value.