Overcharge detecting circuit and integrated circuit

This application is a continuation patent application of, and claims the benefit of and priority to U.S. patent application Ser. No. 12/334,674 filed on Dec. 15, 2008, which is based on and claims the benefit of priority of Japanese Patent Application No. 2007-330580 filed on Dec. 21, 2007 with the Japanese Patent Office. The entire contents of U.S. patent application Ser. No. 12/334,674 and Japanese Patent Application No. 2007-330580 are hereby incorporated by reference.

1. Field of the Invention

This invention relates to a battery pack including a protection circuit which detects an overcharge, an overdischarge or an overcurrent of a secondary battery and turns off a switching element arranged in wiring between a load and the secondary battery or between a charging device and the secondary battery.

2. Description of the Related Art

In recent years, a portable device, such as a digital camera, carries lithium ion batteries as secondary batteries. Because lithium ion batteries are prone to overcharging or overdischarging, the lithium ion batteries are arranged with a battery pack containing a protection circuit for protecting the batteries from overcharging or overdischarging.

FIG. 10 shows the composition of a battery pack 1 according to the related art. In the battery pack 1 of FIG. 10 , a lithium ion battery 2 is connected in parallel to a series circuit in which a capacitor C 1 and a resistor R 1 are connected in series. A positive electrode of the lithium ion battery 2 is connected to an external terminal 3 of the battery pack 1 , and a negative electrode of the lithium ion battery 2 is connected to an external terminal 4 of the battery pack 1 via a pair of n channel MOS (metal oxide semiconductor) transistors M 1 and M 2 for current interception.

The drains of the MOS transistors M 1 and M 2 are connected to each other, the source of the MOS transistor M 1 is connected to the negative electrode of the lithium ion battery 2 , and the source of the MOS transistor M 2 is connected to the external terminal 4 . Body diodes D 1 and D 2 are connected between the drain and the source of each of the MOS transistors M 1 and M 2 in an equivalent manner.

A protection IC (integrated circuit) 5 is arranged in the battery pack 1 so that the protection IC 5 includes an overcharge detecting circuit, an overdischarge detecting circuit, and an overcurrent detecting circuit. A power-source voltage Vdd is supplied to the protection IC 5 through the resistor R 1 from the positive electrode of the lithium ion battery 2 , and a power-source voltage Vss is supplied to the protection IC 5 from the negative electrode of the lithium ion battery 2 , so that the protection IC 5 operates.

When an overdischarge or overcurrent is detected by the overdischarge detecting circuit or the overcurrent detecting circuit, the protection IC 5 sets the output signal DOUT to a low level to turn the MOS transistor M 1 to an OFF state. When an overcharge is detected by the overcharge detecting circuit, the protection IC 5 sets the output signal COUT to the low level to turn the MOS transistor M 2 to the OFF state.

FIG. 11 shows the composition of a battery pack 1 according to the related art. In the battery pack 1 of FIG. 11 , a thermistor R 3 is further arranged in the battery pack 1 in addition to the elements shown in FIG. 10 . One end of the thermistor R 3 is connected to a terminal 6 of the battery pack 1 and the other end of the thermistor R 3 is connected to the external terminal 4 .

A predetermined voltage from a charging device is applied to the terminal 6 of the battery pack 1 through a voltage divider resistor in order to charge the battery pack 1 . The resistance of the thermistor R 3 changes in accordance with a temperature of the battery pack 1 , and the voltage of the terminal 6 changes accordingly. The voltage of the terminal 6 is detected and if a temperature of the battery pack 1 derived from the detected voltage exceeds a predetermined value, the charging device is controlled to stop the charging of the battery pack 1 .

Japanese Laid-Open Patent Application No. 2004-152580 discloses a battery pack in which a first diode arranged in a first direction (charging direction) and connected in series to a temperature protection element (PTC element) and a second diode arranged in a second direction (discharging direction) opposite to the first direction and connected in parallel with the first diode and the temperature protection element (PTC element) are connected to a secondary battery. This battery pack is arranged so that, when normal discharging of the secondary battery is performed, a current flows only through a channel including the second diode in the second direction and does not flow through a channel including the first diode in the first direction. Thus, even when a temperature of the battery pack becomes high at the time of normal discharging, no current flows through the channel including the first diode and it is possible to inhibit operation of the temperature protection element (PTC element).

The battery pack according to the related art, shown in FIG. 10 , does not have a temperature protection function to protect the battery pack from high temperature. The battery pack according to the related art, shown in FIG. 11 , has a temperature protection function to protect the battery pack from high temperature. However, in this battery pack, the predetermined voltage from the charging device is supplied through the voltage divider resistor. In a case where the predetermined voltage of the charging device changes remarkably, or in a case where the resistance of the voltage divider resistor has a significant error, it is difficult to detect a temperature of the battery pack accurately, which hinders the battery pack from performing appropriate charging control.

In one aspect of the invention, the present disclosure provides an improved battery pack in which the above-described problems are eliminated.

In one aspect of the invention, the present disclosure provides a battery pack which has a temperature protection function for a secondary battery with good accuracy and is able to perform appropriate charging control.

In an embodiment of the invention which solves or reduces one or more of the above-mentioned problems, the present disclosure provides an overcharge detecting circuit configured to compare a voltage between a positive electrode and a negative electrode of a secondary battery with a threshold voltage to detect an overcharge of the secondary battery, and to turn off a switching element in a wiring between a load and the secondary battery or between a charging device and the secondary battery, the overcharge detecting circuit comprising: a series circuit including a thermistor and a resistor connected in series, the series circuit being arranged in a vicinity of the secondary battery, thermally coupled with the secondary battery, and connected in parallel to the secondary battery; a first comparator arranged to compare a voltage at a junction point of the thermistor and the resistor in the series circuit with a first reference voltage corresponding to a first predetermined temperature; and a changing unit arranged to change the threshold voltage to a first value when a detected temperature of the secondary battery is below the first predetermined temperature, and change the threshold voltage to a second value smaller than the first value when the detected temperature of the secondary battery is above the first predetermined temperature, in response to an output signal of the first comparator; and a non-sensitive time setting circuit arranged to output a high-temperature detect signal to the changing unit when the output signal of the first comparator exceeds the first predetermined temperature for a predetermined period.

The above-mentioned overcharge detecting circuit may include: a voltage divider arranged to output an intermediate voltage from the voltage between the positive electrode and the negative electrode of the secondary battery; and a second comparator arranged to compare the intermediate voltage output from the voltage divider with a fixed threshold voltage, wherein, in response to the output signal of the first comparator, the changing unit changes a division ratio of the voltage divider to a first division ratio when the detected temperature of the secondary battery is below the first predetermined temperature, and changes the division ratio of the voltage divider to a second division ratio smaller than the first division ratio when the detected temperature of the secondary battery is above the first predetermined temperature, in order to change the threshold voltage to the first value or the second value.

The above-mentioned overcharge detecting circuit may be arranged so that the changing unit comprises a third comparator arranged to compare the voltage at the junction point of the thermistor and the resistor in the series circuit with a second reference voltage corresponding to a second predetermined temperature lower than the first predetermined temperature, wherein, in response to an output signal of the third comparator, the changing unit changes the division ratio of the voltage divider to the second division ratio when the detected temperature of the secondary battery is below the second predetermined temperature, in order to change the threshold voltage to the second value.

The above-mentioned overcharge detecting circuit may be arranged so that the thermistor is an NTC thermistor having a negative temperature coefficient.

According to the battery pack of the embodiment of the invention, the temperature protection of the secondary battery can be performed with good accuracy, and appropriate charging control can be performed.

Other objects, features and advantages of the invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.

FIG. 1 is a block diagram showing the composition of a reference example for explaining the battery pack of the invention.

FIG. 2 is a diagram showing the temperature vs. resistance characteristic curves of each of an NTC thermistor and a PTC thermistor.

FIG. 3 is a block diagram showing the composition of a battery pack of a first embodiment of the invention.

FIG. 4 is a diagram showing the composition of an overcharge detecting circuit in the first embodiment.

FIG. 5 is a diagram for explaining operation of a comparator in the first embodiment.

FIG. 6 is a diagram showing the composition of a modification of the overcharge detecting circuit in the first embodiment.

FIG. 7 is a diagram showing the composition of a modification of the overcharge detecting circuit in the first embodiment.

FIG. 8 is a block diagram showing the composition of a battery pack of a second embodiment of the invention.

FIG. 9 is a diagram showing the composition of an overcharge detecting circuit in the second embodiment.

FIG. 10 is a block diagram showing the composition of a battery pack according to the related art.

FIG. 11 is a block diagram showing the composition of a battery pack according to the related art.

Before describing embodiments of the invention, a description will be given of a reference example for explaining the battery pack of the invention.

FIG. 1 is a block diagram showing the composition of a reference example for explaining the battery pack of the invention.

In the battery pack 10 shown in FIG. 1 , a series circuit in which a capacitor C 11 and a resistor R 11 are connected in series is connected in parallel to a lithium ion battery 12 . A positive electrode of the lithium ion battery 12 is connected to an external terminal 13 (Pack+) of the battery pack 10 by wiring, and a negative electrode of the lithium ion battery 12 is connected to an external terminal 14 (Pack-) of the battery pack 10 by wiring via a pair of n channel MOS transistors M 11 and M 12 for current interception.

The drains of the MOS transistors M 11 and M 12 are connected to each other, the source of the MOS transistor M 11 is connected to the negative electrode of the lithium ion battery 12 , and the source of the MOS transistor M 12 is connected to the external terminal 14 of the battery pack 10 . Body diodes D 11 and D 12 are connected between the drain and the source of each of the MOS transistors M 11 and M 12 respectively in an equivalent manner.

A series circuit in which a thermistor R 13 and a resistor R 14 are connected in series is connected in parallel to the lithium ion battery 12 . The thermistor R 13 is arranged in the vicinity of the lithium ion battery 12 within the battery pack 10 , so that the thermistor R 13 is thermally coupled with the lithium ion battery 12 . The thermistor R 13 used in the battery pack 10 is an NTC (Negative Temperature Coefficient) thermistor which has a negative temperature coefficient.

In this regard, FIG. 2 shows the temperature vs. resistance characteristic curves of each of an NTC thermistor having a negative temperature coefficient and a PTC (Positive Temperature Coefficient) thermistor having a positive temperature coefficient.

A protection IC 15 is arranged in the battery pack 10 so that the protection IC 15 includes an overcharge detecting circuit 16 , an overdischarge detecting circuit 17 , and an overcurrent detecting circuit 18 . A power-source voltage Vdd from the positive electrode of the lithium ion battery 12 is supplied to a terminal 15 a of the protection IC 15 through the resistor R 11 and a power-source voltage Vss from the negative electrode of the lithium ion battery 12 is supplied to a terminal 15 c of the protection IC 15 so that the protection IC 15 operates.

The overcharge detecting circuit 16 detects an overcharge of the lithium ion battery 12 from the voltages of the terminals 15 a and 15 c , and supplies an overcharge detect signal to a logic circuit 19 . The overdischarge detecting circuit 17 detects an overdischarge of the lithium ion battery 12 from the voltages of the terminals 15 a and 15 c , and supplies an overdischarge detect signal to the logic circuit 19 . The overcurrent detecting circuit 18 detects an overcurrent, which means an excessively large current flowing through a resistor R 12 connected between a terminal 15 f of the protection IC 15 and the external terminal 14 of the battery pack 10 , from the voltages of the terminals 15 c and 15 f , and supplies an overcurrent detect signal to the logic circuit 19 .

The protection IC 15 is arranged so that a junction point A of the thermistor R 13 and the resistor R 14 is connected to a terminal 15 b of the protection IC 15 , one end of the resistor R 12 is connected to the terminal 15 f , and the other end of the resistor R 12 is connected to the external terminal 14 .

The protection IC 15 includes a terminal 15 d to which an output signal DOUT of the logic circuit 19 is supplied is connected to the gate of the MOS transistor M 11 , and a terminal 15 e to which an output signal COUT of the logic circuit 19 is supplied is connected to the gate of the MOS transistor M 12 .

In the protection IC 15 , the terminal 15 b is connected to the non-inverted input terminal of a comparator 21 , the terminal 15 c is connected to the negative electrode of a constant voltage source 20 (which is formed of, for example, a Zener diode), and the positive electrode of the constant voltage source 20 is connected to the inverted input terminal of the comparator 21 .

Because the thermistor R 13 is an NTC thermistor having a negative temperature coefficient as shown in FIG. 2 , the resistance of the thermistor R 13 decreases as the temperature of the thermistor R 13 rises, and accordingly the voltage at the junction point A increases as the temperature of the thermistor R 13 rises.

The comparator 21 has a hysteresis characteristic. The comparator 21 compares the voltage at the junction point A (which voltage is supplied to the non-inverted input terminal of the comparator 21 ) with a constant voltage V 1 generated by the constant voltage source 20 (which voltage is supplied to the inverted input terminal of the comparator 21 ). When the voltage at the junction point A is higher than the constant voltage V 1 , the comparator 21 outputs a high-level signal. Namely, when the detected temperature from the thermistor R 13 exceeds a predetermined temperature (for example, 45 degrees C.) corresponding to the constant voltage V 1 , the comparator 21 outputs a high-level high-temperature detect signal.

The high-temperature detect signal output from the comparator 21 is supplied to a non-sensitive time setting circuit 22 . The non-sensitive time setting circuit 22 outputs a high-level high-temperature detect signal to the logic circuit 19 , when the high-level period of the high-temperature detect signal exceeds a predetermined value (for example, 0.5 seconds).

The respective detection signals of the overcharge detecting circuit 16 , the overdischarge detecting circuit 17 , and the overcurrent detecting circuit 18 are supplied to the logic circuit 19 , and the high-temperature detect signal output from the non-sensitive time setting circuit 22 is also supplied to the logic circuit 19 .

When an overcharge detect signal from the overcharge detecting circuit 16 is supplied to the logic circuit 19 , the logic circuit 19 sets the output signal COUT of the terminal 15 e to the low level to turn the MOS transistor M 12 to an OFF state. When an overdischarge detect signal from the overdischarge detecting circuit 17 is supplied to the logic circuit 19 , the logic circuit 19 sets the output signal DOUT of the terminal 15 d to the low level to turn the MOS transistor M 11 to an OFF state. When an overcurrent detect signal from the overcurrent detecting circuit 18 is supplied to the logic circuit 19 , the logic circuit 19 sets the output signal DOUT of the terminal 15 d to the low level to turn the MOS transistor M 11 to an OFF state.

Moreover, when a high-level high-temperature detect signal from the non-sensitive time setting circuit 22 is supplied to the logic circuit 19 , the logic circuit 19 sets the output signal COUT of the terminal 15 e to a low level to turn the MOS transistor M 12 to an OFF state. Thereby, the temperature of the lithium ion battery 12 can be detected with good accuracy, and when the temperature of the lithium ion battery 12 is excessively high, the charging of the lithium ion battery 12 can be stopped to protect the lithium ion battery 12 from overcharging.

Because the thermistor R 13 used is the NTC thermistor, the resistance of which changes almost linearly to the temperature as shown in FIG. 2 , and the thermistor R 13 is arranged in the vicinity of the lithium ion battery 12 in the battery pack 10 , the temperature of the lithium ion battery 12 can be detected with good accuracy.

In this regard, if the thermistor R 13 used is the PTC thermistor having a positive temperature coefficient, the resistance of the PTC thermistor increases rapidly if the temperature of the PTC thermistor exceeds a certain temperature as shown in FIG. 2 . For this reason, when the PTC thermistor is used, it is difficult to detect the temperature of the lithium ion battery 12 with good accuracy.

Generally, the lithium ion battery 12 is easily damaged when it is in the fully charged state at a high temperature. For this reason, it is preferred that the charging control of the lithium ion battery 12 in the battery pack 10 is performed while the temperature protection thereof is performed such that, when the temperature of the lithium ion battery 12 is high, the lithium ion battery 12 may not be in the fully charged state.

Accordingly, the battery pack of an embodiment of the invention is arranged to perform appropriate charging control of the lithium ion battery 12 in the battery pack while performing the temperature protection of the lithium ion battery 12 with good accuracy.

A description will now be given of embodiments of the invention with reference to the accompanying drawings.

FIG. 3 is a block diagram showing the composition of a battery pack 10 A of a first embodiment of the invention. In FIG. 3 , the elements which are the same as corresponding elements in FIG. 1 are designated by the same reference numerals and a description thereof will be omitted.

In this embodiment, when a temperature of the lithium ion battery 12 is higher than a predetermined temperature, a relative threshold voltage used for detecting an overcharge in an overcharge detecting circuit 16 A is changed to a lower voltage, so that an overcharge may be detected before the lithium ion battery 12 is in the fully charged state.

In the battery pack 10 A of this embodiment shown in FIG. 3 , the series circuit in which the capacitor C 11 and the resistor R 11 are connected in series is connected in parallel to the lithium ion battery 12 . The positive electrode of the lithium ion battery 12 is connected to the external terminal 13 (Pack+) of the battery pack 10 A by wiring, and the negative electrode of the battery pack 10 A is connected to the external terminal 14 (Pack-) of the battery pack 10 A by wiring via the pair of n channel MOS transistors M 11 and M 12 for current interception.

The drains of the MOS transistors M 11 and M 12 are connected to each other, the source of the MOS transistor M 11 is connected to the negative electrode of the lithium ion battery 12 , and the source of the MOS transistor M 12 is connected to the external terminal 14 of the battery pack 10 A. The body diodes D 11 and D 12 are connected between the drain and the source of each of the MOS transistors M 11 and M 12 respectively in an equivalent manner.

The series circuit in which the thermistor R 13 and the resistor R 14 are connected in series is connected in parallel to the lithium ion battery 12 . The thermistor R 13 is arranged in the vicinity of the lithium ion battery 12 within the battery pack 10 A, so that the thermistor R 13 is thermally coupled with the lithium ion battery 12 . The thermistor R 13 used in the battery pack 10 A is an NTC thermistor having a negative temperature coefficient.

In this regard, the temperature vs. resistance characteristic curves of each of the NTC thermistor having a negative temperature coefficient and the PTC thermistor having a positive temperature coefficient are shown in FIG. 2 .

A protection IC 15 A is arranged in the battery pack 10 A so that the protection IC 15 A includes an overcharge detecting circuit 16 A, an overdischarge detecting circuit 17 , and an overcurrent detecting circuit 18 . The power-source voltage Vdd from the positive electrode of the lithium ion battery 12 is supplied to the terminal 15 a of the protection IC 15 A through the resistor R 11 , and the power-source voltage Vss from the negative electrode of the lithium ion battery 12 is supplied to the terminal 15 c of the protection IC 15 A, so that the protection IC 15 A operates.

The overcharge detecting circuit 16 A detects an overcharge of the lithium ion battery 12 from the voltages of the terminals 15 a and 15 c and supplies an overcharge detect signal to the logic circuit 19 . The overcharge detecting circuit 16 A is arranged to receive a high-temperature detect signal output from the non-sensitive time setting circuit 22 . A detailed structure of the overcharge detecting circuit 16 A will be described later.

The overdischarge detecting circuit 17 detects an overdischarge of the lithium ion battery 12 from the voltages of the terminals 15 a and 15 c , and supplies an overdischarge detect signal to the logic circuit 19 . The overcurrent detecting circuit 18 detects an overcurrent, which means an excessively large current flowing through the resistor R 12 , from the voltages of the terminals 15 c and 15 f , and supplies an overcurrent detect signal to the logic circuit 19 .

The protection IC 15 A is arranged so that the junction point A of the thermistor R 13 and the resistor R 14 is connected to the terminal 15 b , one end of the resistor R 12 is connected to the terminal 15 f , and the other end of the resistor R 12 is connected to the external terminal 14 .

In the protection IC 15 A, the terminal 15 d to which an output signal DOUT of the logic circuit 19 is supplied is connected to the gate of the MOS transistor M 11 , and the terminal 15 e to which an output signal COUT of the logic circuit 19 is supplied is connected to the gate of the MOS transistor M 12 .

In the protection IC 15 A, the terminal 15 b is connected to the non-inverted input terminal of the comparator 21 , the terminal 15 c is connected to the negative electrode of the constant voltage source 20 (which is formed of, for example, a Zener diode), and the positive electrode of the constant voltage source 20 is connected to the inverted input terminal of the comparator 21 .

Because the thermistor R 13 is an NTC thermistor having a negative temperature coefficient as shown in FIG. 2 , the resistance of the thermistor R 13 decreases as the temperature of the thermistor R 13 rises, and accordingly the voltage at the junction point A increases as the temperature of the thermistor R 13 rises.

The comparator 21 has a hysteresis characteristic. The comparator 21 compares the voltage at the junction point A with the constant voltage V 1 generated by the constant voltage source 20 . When the voltage at the junction point A is higher than the constant voltage V 1 , the comparator 21 outputs a high-level signal. Namely, when the detected temperature from the thermistor R 13 exceeds a predetermined temperature (for example, 45 degrees C.) corresponding to the constant voltage V 1 , the comparator 21 outputs a high-level high-temperature detect signal.

The high-temperature detect signal output from the comparator 21 is supplied to the non-sensitive time setting circuit 22 . The non-sensitive time setting circuit 22 outputs a low-level high-temperature detect signal to the logic circuit 19 , when the high-level period of the high-temperature detect signal exceeds a predetermined value (for example, 0.5 seconds).

The respective detection signals of the overcharge detecting circuit 16 A, the overdischarge detecting circuit 17 , and the overcurrent detecting circuit 18 are supplied to the logic circuit 19 , and the high-temperature detect signal output from the non-sensitive time setting circuit 22 is also supplied to the logic circuit 19 .

When an overcharge detect signal from the overcharge detecting circuit 16 A is supplied to the logic circuit 19 , the logic circuit 19 sets the output signal COUT of the terminal 15 e to the low level to turn the MOS transistor M 12 to an OFF state. When an overdischarge detect signal from the overdischarge detecting circuit 17 is supplied to the logic circuit 19 , the logic circuit 19 sets the output signal DOUT of the terminal 15 d to the low level to turn the MOS transistor M 11 to an OFF state. When an overcurrent detect signal from the overcurrent detecting circuit 18 is supplied to the logic circuit 19 , the logic circuit 19 sets the output signal DOUT of the terminal 15 d to the low level to turn the MOS transistor M 11 to an OFF state.

Next, a description will be given of the overcharge detecting circuit 16 A in this embodiment. FIG. 4 is a diagram showing the composition of an overcharge detecting circuit 16 A in this embodiment.

As shown in FIG. 4 , the overcharge detecting circuit 16 A includes a voltage divider 160 , a constant voltage source 161 , a comparator 162 , and an MOS transistor M 13 .

The voltage divider 160 includes resistors R 41 , R 42 , and R 43 which are connected in series, and this voltage divider 160 outputs an intermediate voltage Vin between the power-source voltage Vdd and the power-source voltage Vss from a junction point B of the resistor R 41 and the resistor R 42 . One end of the resistor R 41 is connected to the terminal 15 a and the power-source voltage Vdd from the terminal 15 a is supplied to the voltage divider 160 . One end of the resistor R 43 is connected to the terminal 15 c and the power-source voltage Vss from the terminal 15 c is supplied to the voltage divider 160 .

The MOS transistor M 13 is a changing unit which changes a division ratio of the voltage divider 160 . The division ratio of the voltage divider 160 means the ratio of the output voltage to the input voltage (Vdd−Vss) of the voltage divider 160 . The source of the MOS transistor M 13 is connected to the junction point B of the resistor R 41 and the resistor R 42 , and the drain of the MOS transistor M 13 is connected to a junction point C of the resistor R 42 and the resistor R 43 . A high-temperature detect signal output from the non-sensitive time setting circuit 22 is supplied to the gate of the MOS transistor M 13 . The MOS transistor M 13 used in this embodiment is an n channel MOS transistor, similar to the MOS transistor M 11 and the MOS transistor M 12 .

The constant voltage source 161 is a constant voltage source made of a Zener diode or the like, and this constant voltage source 161 generates a constant voltage Voc.

The comparator 162 is arranged so that the non-inverted input terminal 162 a of the comparator 162 is connected to the junction point B and the inverted input terminal 162 b of the comparator 162 is connected to the positive electrode of the constant voltage source 161 . An output signal of the comparator 162 is supplied to the logic circuit 19 .

Next, operation of the overcharge detecting circuit 16 A will be described.

When no high-temperature detect signal is supplied, a high-level signal is supplied to the gate of the MOS transistor M 13 in the overcharge detecting circuit 16 A and the MOS transistor M 13 is turned to an ON state.

In this case, the comparator 162 compares the voltage Vin at the junction point B with the constant voltage Voc from the constant voltage source 161 . The voltage Vin is an intermediate voltage between the power-source voltage Vdd and the power-source voltage Vss, which is output from the voltage divider 160 according to the division ratio of the resistor R 41 and the resistors R 42 and R 43 .

When the voltage Vin exceeds the constant voltage Voc, the comparator 162 outputs a high-level overcharge detect signal which indicates that an overcharge is occurring. This overcharge detect signal is supplied to the logic circuit 19 .

When the high-level overcharge detect signal is received, the logic circuit 19 sets the COUT output of the terminal 15 e to the low level to turn the MOS transistor M 12 to an OFF state, so that the charging of the secondary battery is stopped.

Therefore, when the MOS transistor M 13 is turned ON, the intermediate voltage Vin between the power-source voltage Vdd and the power-source voltage Vss, which is produced according to the division ratio of the resistor R 41 and the resistor R 43 , is used as a relative threshold voltage (a first value) for detecting an overcharge of the lithium ion battery 12 . The division ratio of Vin/(Vdd−Vss) at this time is equal to the ratio of (R 41 +R 43 ) (a first division ratio).

When the detected temperature from the thermistor R 13 is higher than the predetermined temperature, the non-sensitive time setting circuit 22 outputs the high-temperature detect signal. This high-temperature detect signal is supplied to the gate of the MOS transistor M 13 and the MOS transistor M 13 is turned to an OFF state.

Therefore, when the MOS transistor M 13 is turned OFF, the voltage Yin at the junction point B is equal to an intermediate voltage Vin between the power-source voltage Vdd and the power-source voltage Vss, which is produced according to the ratio of the resistor R 41 and the resistors R 42 and R 43 .

When the voltage Vin exceeds the constant voltage Voc at this time, the comparator 162 outputs a high-level overcharge detect signal, which indicates that an overcharge is occurring Therefore, when the MOS transistor M 13 is turned OFF, the intermediate voltage Vin between the power-source voltage supply Vdd and the power-source voltage Vss, which is produced according to the ratio of the resistor R 41 and the resistors R 42 and R 43 , is used as a relative threshold voltage (a second value) for detecting an overcharge of the lithium ion battery 12 . The division ratio of Vin/(Vdd−Vss) at this time is equal to the ratio of (R 42 +R 43 )/(R 41 +R 42 +R 43 ) (a second division ratio).

In this embodiment, it is preferred that the resistances of the resistors R 41 , R 42 and R 43 are selected so that the second division ratio is smaller than the first division ratio.

Specifically, in this embodiment, when the detected temperature from the thermistor R 13 is higher than 45 degrees C., the comparator 21 outputs a high-temperature detect signal. In this embodiment, when the MOS transistor M 13 is in the ON state and the voltage Vdd is larger than 4.3 V (the first value), it is detected that an overcharge of the lithium ion battery 12 is occurring. And, in this embodiment, when the MOS transistor M 13 is in the OFF state and the voltage Vdd is larger than 4.1 V (the second value), it is detected that an overcharge of the lithium ion battery 12 is occurring.

Next, the switching of the relative threshold voltage in this embodiment will be described. FIG. 5 is a diagram for explaining operation of the comparator 162 of the overcharge detecting circuit 16 A in this embodiment.

In FIG. 5 , the vertical axis indicates the voltage Vin of the non-inverted input terminal of the comparator 162 and the horizontal axis indicates the power-source voltage Vdd.

In this embodiment, if charging of the lithium ion battery 12 is started when the detected temperature from the thermistor R 13 is less than 45 degrees C. (the MOS transistor M 13 is turned ON), the voltage Vin increases along the curve indicated by the graph Vin 1 in FIG. 5 . In this case, the comparator 162 outputs a high-level overcharge detect signal when the voltage Vdd reaches 4.3 V (the first value) exceeding the constant voltage Voc.

In this embodiment, when the detected temperature from the thermistor R 13 is above 45 degrees C., the MOS transistor M 13 is turned OFF and the division ratio of the power-source voltage Vdd and the power-source voltage Vss is changed. If charging of the lithium ion battery 12 is started in this state, the voltage Vin increases along the inclination indicated by the graph Vin 2 in FIG. 5 . In this case, the comparator 162 outputs a high-level overcharge detect signal when the voltage Vdd reaches 4.1 V (the second value) exceeding the constant voltage Voc.

According to this embodiment, the relative threshold voltage used for detecting an overcharge of the lithium ion battery 12 is changed by controlling the ON or OFF state of the MOS transistor M 13 based on the detected temperature from the thermistor R 13 , and the division ratio between the power-source voltage Vdd and the power-source voltage Vss is changed.

Specifically, in this embodiment, when the detected temperature from the thermistor R 13 is higher than the predetermined temperature, the division ratio between the power-source voltage Vdd and the power-source voltage Vss is changed from the first division ratio to the second division ratio. Thereby, the relative threshold voltage used for detecting an overcharge of the lithium ion battery 12 is changed from the first value corresponding to the first division ratio to the second value corresponding to the second division ratio. Suppose that the second value is smaller than the first value in this embodiment.

Accordingly, in this embodiment, before the lithium ion battery 12 is in the fully charged state at a high temperature, an overcharge of the lithium ion battery 12 can be detected in order to stop the charging of the lithium ion battery 12 . It is possible to prevent the lithium ion battery 12 from being fully charged at a high temperature. Therefore, the temperature protection of the lithium ion battery 12 can be performed with good accuracy and appropriate charging control can be performed.

FIG. 6 and FIG. 7 show modifications of the overcharge detecting circuit 16 A in this embodiment. FIG. 6 shows the composition of a modification of the overcharge detecting circuit in the first embodiment. FIG. 7 shows the composition of another modification of the overcharge detecting circuit in the first embodiment.

In the overcharge detecting circuit 16 a shown in FIG. 6 , the voltage divider 160 A includes a resistor R 41 , a resistor R 43 , and a variable resistor R 44 . In the voltage divider 160 A, the resistor R 41 , the variable resistor R 44 , and the resistor R 43 are connected in series between the power-source voltage Vdd and the power-source voltage Vss. This voltage divider 160 A outputs an intermediate voltage Vin between the power-source voltage Vdd and the power-source voltage Vss.

In this modification, the variable resistor R 44 serves as a changing unit which changes the division ratio of the voltage divider 160 A. A junction point D of the resistor R 41 and the variable resistor R 44 is connected to the non-inverted input terminal of the comparator 162 .

In the voltage divider 160 A, when a high-temperature detect signal is supplied, the resistance of the variable resistor R 44 is increased so that the intermediate voltage Vin output from the voltage divider 160 A is increased. Therefore, in this modification, when a high-temperature detect signal is supplied, the relative threshold voltage used for detecting an overcharge of the lithium ion battery 12 can be changed according to the changed division ratio between the power-source voltage Vdd and the power-source voltage Vss.

The overcharge detecting circuit 16 b , shown in FIG. 7 , includes a switch 45 which serves as a changing unit which changes the division ratio of the voltage divider 160 B. In the voltage divider 160 B, when no high level high-temperature detect signal is supplied, the moving contact of the switch 45 is turned to the junction point C of the resistor R 42 and the resistor R 43 so that the junction point C is connected to the non-inverted input terminal of the comparator 162 through the switch 45 . When a high-temperature detect signal is supplied, the moving contact of the switch 45 is turned to the junction point B of the resistor R 41 and the resistor R 42 so that the junction point B is connected to the non-inverted input terminal of the comparator 162 through the switch 45 . Normally, the voltage Vinc output from the junction point C is lower than the voltage Yin output from the junction point B.

Therefore, also in this modification, when a high-level high-temperature detect signal is supplied, the relative threshold voltage used for detecting an overcharge of the lithium ion battery 12 can be changed according to the changed division ratio between the power-source voltage Vdd and the power-source voltage Vss.

Next, a battery pack 10 B of a second embodiment of the invention will be described.

In the second embodiment, when a temperature detected by the thermistor R 13 is higher than a first predetermined temperature or lower than a second predetermined temperature, the threshold voltage for detecting an overcharge of the lithium ion battery 12 is changed by changing the division ratio between the power-source voltage Vdd and the power-source voltage Vss, so that an overcharge may be detected before the lithium ion battery 12 is in the fully charged state.

FIG. 8 is a block diagram showing the composition of the battery pack 10 B of the second embodiment of the invention. In FIG. 8 , the elements which are the same as corresponding elements in FIG. 3 are designated by the same reference numerals, and a description thereof will be omitted.

In the battery pack 10 B shown in FIG. 8 , the protection IC 15 B includes a comparator 21 arranged to detect whether the detected temperature from the thermistor R 13 is higher than the first predetermined temperature, and a comparator 21 A arranged to detect whether the detected temperature from the thermistor R 13 is lower than the second predetermined temperature.

In this embodiment, the first predetermined temperature is equal to, for example, 45 degrees C., and the second predetermined temperature is equal to, for example, 0 degrees C.

The inverted input terminal of the comparator 21 A is connected to the terminal 15 b . The non-inverted input terminal of the comparator 21 A is connected to the positive electrode of a constant voltage source 20 A (which is made of a Zener diode or the like). The negative electrode of the constant voltage source 20 A is connected to the terminal 15 c.

The comparator 21 A outputs a high-level low-temperature detect signal, when the detected temperature from the thermistor R 13 is lower than the second predetermined temperature (for example, 0 degrees C.) corresponding to a constant voltage V 2 generated by the constant voltage source 20 A.

The low-temperature detect signal output from the comparator 21 A is supplied to the non-sensitive time setting circuit 22 . The non-sensitive time setting circuit 22 outputs a low-level low-temperature detect signal to the overcharge detecting circuit 16 B, when the high-level period of the low-temperature detect signal exceeds a predetermined value (for example, 0.5 seconds).

FIG. 9 shows the composition of the overcharge detecting circuit 16 B in the second embodiment.

The composition of the overcharge detecting circuit 16 B of this embodiment is basically the same as that of the overcharge detecting circuit 16 A of the first embodiment. The overcharge detecting circuit 16 B differs from the overcharge detecting circuit 16 A in that either a high-temperature detect signal or a low-temperature detect signal is supplied to the gate of the MOS transistor M 13 in the overcharge detecting circuit 16 B.

When either a low-level high-temperature detect signal or a low-level low-temperature detect signal is supplied, the overcharge detecting circuit 16 B of this embodiment performs the operation that is the same as the operation of the overcharge detecting circuit 16 A of the first embodiment. Therefore, according to this embodiment, when the detected temperature from the thermistor R 13 is higher than the first predetermined temperature, or when the detected temperature from the thermistor R 13 is lower than the second predetermined temperature, the relative threshold voltage used for detecting an overcharge of the lithium ion battery 12 can be reduced. For this reason, it is possible to prevent the lithium ion battery 12 from being fully charged when the lithium ion battery 12 is at a high temperature or a low temperature, and the temperature protection of the lithium ion battery 12 can be performed with good accuracy and appropriate charging control can be performed.

The present invention is not limited to the above-described embodiments, and variations and modifications may be made without departing from the scope of the invention.