1. Field of the Invention
The present invention relates to a protection circuit of a secondary battery, such as a lithium battery, and more particularly to a battery voltage detector circuit of a protection circuit that uses a multiplexer.
2. Background Art
In general, a secondary battery, such as a lithium battery, uses a protection circuit to protect the battery from overcharge or overdischarge. A protection circuit has a voltage detecting unit for detecting the voltage of a battery. Protecting a plurality of batteries connected in series requires a plurality of voltage detecting units corresponding to the individual batteries, thus leading to a larger scale and a higher withstand voltage of the circuit.
FIG. 2 is a circuit diagram of a battery voltage detector circuit that uses a conventional multiplexer. The battery voltage detector circuit using the conventional multiplexer includes a battery power supply device 11, a battery 11a, switches 21, 22, 23 and 24, flying capacitors 28 and 29, an amplifier 25, an A/D converter 26, and a controller 30. The switch 21 is composed of normally open contacts 21a and 21b, the switch 23 is composed of normally open contacts 23a and 23b, and the switch 24 is composed of normally open contacts 24a and 24b. 
To detect the voltage of the battery 11a, the switches 21 to 24 are set to an OFF (open) state. In this state, the switch 21 is first set to an ON state and the normally open contacts 21a and 21b are individually set to a closed state. This causes the voltage in the battery 11a to be applied to the flying capacitors 28 and 29, which are connected in series. Thus, electric charges are accumulated in the flying capacitors 28 and 29.
After the switch 21 is held in an ON position for a predetermined period of time, the switch 21 is set to the OFF state and the normally open contacts 21 a and 21b are individually set to the open state. This causes the electric charges corresponding to the voltage of the battery 11a to be accumulated in the flying capacitors 28 and 29.
Thereafter, the switch 22 and the switch 24 are turned on. Turning the switch 22 on causes the connection point of the flying capacitors 28 and 29 to be connected to the ground and fixed to 0V. When the switch 24 is turned on and the normally open contacts 24a and 24b are individually set to the closed state, an inverting input terminal of the amplifier 25 is fixed to the same potential (0V) of an output of the amplifier 25, and the voltage of a non-inverting input terminal is fixed to the ground (0V).
Thereafter, the switch 23 is turned on and the normally open contacts 23a and 23b are individually set to the closed state. This causes the flying capacitors 28 and 29 to be connected to the inverting input terminal and the non-inverting input terminal, respectively, of the amplifier 25. However, the switch 24 is on, so that the voltages of the terminals are fixed and the voltages of the flying capacitors 28 and 29 are not applied to the terminals of the amplifier 25.
Thereafter, the switch 24 is turned off and the fixation of the voltages at the terminals of the amplifier 25 are cleared, thereby causing the voltages accumulated in the flying capacitors 28 and 29 to be applied to the amplifier 25. The voltages of the flying capacitors 28 and 29 are applied in the state wherein the voltages of the input terminals of the amplifier 25 are fixed to 0V. This makes it possible to accurately detect the voltages applied from the flying capacitors 28 and 29 without the risk of the voltages of the input terminals exceeding a permissible range or an output of the amplifier 25 being saturated. In addition, the voltages supplied to the amplifier 25 are unlikely to exceed a permissible range, thus protecting the amplifier 25 from deterioration and damage (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-201548
However, the conventional art has been posing a problem in that, at the instant the switch 22 is turned on, electric charges move to a parasitic capacitance produced between the switch 22 and the flying capacitors 28 and 29 and the holding voltages of the flying capacitors 28 and 29 change, leading to deteriorated voltage detection accuracy.