Voltage switching circuit, secondary battery protection circuit, and battery pack

A secondary battery protection circuit installed in a battery pack having a negative power supply terminal, a current-detection resistor, and a switching element connected in series to the current-detection resistor, the battery pack connected to a secondary battery and one of a charger and a load,

• the secondary battery protection circuit comprising:
• a sensing terminal, connected between the current-detection resistor and the switching element, to which a voltage obtained by converting a current flowing through the current-detection resistor to voltage is supplied;
• a charger negative power supply terminal connected to the negative power supply terminal of the battery pack;
• a voltage switching circuit to cause an output voltage to switch,
• the voltage switching circuit comprising:
• a first input terminal to which the voltage at the sensing terminal is supplied;
• a second input terminal to which a voltage at the negative power supply terminal of the battery pack and a voltage at the charger negative power supply terminal of the secondary battery protection circuit are supplied;
• an output terminal to output the output voltage;
• and
• a selection terminal to select either the voltage from the first input terminal or the voltage from the second input terminal for output through the output terminal as the output voltage;
• and
• a detection circuit to detect an excess current and output a detection result to the selection terminal of the voltage switching circuit to switch off the switching element and shut off the current flowing through the switching element and the current-detection resistor when the excess current is detected.

A battery pack to charge and discharge a secondary battery, connectable to one of a charger and a load,

• the battery pack comprising:
• a negative power supply terminal connected to one of negative terminals of the charger and the load;
• a switching element to connect and disconnect the secondary battery and one of the charger and the load;
• a current-detection resistor to detect a charging current and a discharging current, connected in series to the switching element;
• and
• a secondary battery protection circuit to protect the secondary battery from an excess current,
• the secondary battery protection circuit comprising:
• a sensing terminal connected between the current-detection resistor and the switching element, to which a voltage obtained by converting a current flowing through the current-detection resistor to voltage is supplied;
• a charger negative power supply terminal, connected to the negative power supply terminal of the battery pack, to which a voltage at the negative power supply terminal of the battery pack is supplied;
• a detection circuit to detect the excess current;
• and
• a voltage switching circuit, having a first input terminal to which the voltage at the sensing terminal is supplied, a second input terminal to which a voltage at the negative power supply terminal of the battery pack and a voltage at the charger negative power supply terminal of the secondary battery protection circuit are supplied, an output terminal to output an output voltage to the detection circuit, and a selection terminal to which an output signal from the detection circuit is supplied and to select either the voltage from the first input terminal or the voltage from the second input terminal for output through the output terminal as the output voltage,
• wherein the voltage switching circuit switches between the voltage from the first input terminal and the voltage from the second input terminal for output as the output voltage when either excess current is detected while the battery pack is in a chargeable-dischargeable state or the battery pack returns to the chargeable-dischargeable state from a charge-discharge inhibit state.