(a) Field of the Invention
The present invention relates to a voltage judging circuit and a battery cell pack having the same, and more in particular to the voltage judging circuit suitably employed in a battery and the battery pack having the same.
(b) Description of the Related Art
With popularization of a mobile telephone and a portable terminal, the demand for realizing a battery type compact cell increases. A lithium ion battery cell is a known compact cells. For example, a battery cell pack is available having therein a plurality of the lithium ion battery cells connected in series to output a desired voltage.
The battery cell may be destroyed due to excessive charging in the lithium battery cell, and such excessive charging must be prevented. After the voltages of the individual lithium cells in the lithium battery cell pack reach to a specified charged voltage, the overall charging is stopped to prevent the excessive charging. For this purpose, voltage judgment circuits for judging terminal voltages of the battery cells exist in each of the battery cells.
Figs. 1 is a circuit diagram showing a conventional voltage judgment circuit for a battery. The voltage judgment circuit has a differential amplifier OP1 to which a voltage Vin to be judged (hereinafter referred to as "judgment voltage") is input and a comparator CMP1 for comparing an output of the differential amplifier OP1 with a reference voltage Vref. The judgment voltage Vin is input to an inverted input (-) and a non-inverted input (+) through resistances R1 and R3, and an output of the differential amplifier OP1 returns to its inverted input (-) through a resistance R2 by means of negative feedback. The non-inverted input (+) is grounded through a resistance R4.
After the voltage value of the judgment voltage Vin is converted in the differential amplifier OP1 at a specified ratio, it is compared with the reference voltage Vref at the comparator CMP1. When the converted voltage value reaches to the reference voltage Vref, the output (out) of the comparator CMP1 is inverted. When an output of one of the voltage judgment circuits becomes "L" during the charging in the lithium battery cell pack, its charging is stopped.
Since, in the above conventional voltage comparison circuit, the non-inverted terminal of the differential amplifier is grounded through the resistance R4 and voltage differences between the non-inverted terminal and each of the negative terminals of the respective cells are present, the current continues to flow. Due to this reason, the voltage judgment circuit has a large consumption current. In order to make the consumption current at 1 micron A or less in the conventional method, many resistances of 10 M ohm (R1 to R4) are required and the accommodation of these resistances in an integrated circuit is impossible.
In order to elevate the accuracy of the voltage judgment, the high accuracy of the resistance values of the respective resistances R1 to R4 is required. Although the resistance values of the high accuracy are obtained if the respective resistances are separately prepared in addition to integrated circuits constituting the voltage judgment circuit, the circuit scale becomes larger and the cost increases.