Abstract:
Provided is a battery device with high convenience, which is capable of setting overcurrent release impedance to be low. The battery device includes: a first comparator circuit for detecting an overcurrent based on a first reference voltage; and a second comparator circuit for outputting an overcurrent release voltage control signal based on a second reference voltage that is higher than the first reference voltage.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application Nos. 2013-256343 filed on Dec. 11, 2013 and 2014-217643 filed on Oct. 24, 2014, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a battery state monitoring circuit and a battery device, and more particularly, to a technology of preventing an overcurrent from flowing through a battery or a device connected to the battery. 
     2. Description of the Related Art 
       FIG. 3  illustrates a circuit diagram of a related-art battery device. The related-art battery device includes a secondary battery  1 , a charge/discharge control switch  2 , a charge/discharge protection circuit  3 , and external terminals  4  and  5 . The charge/discharge protection circuit  3  includes a control circuit  22 , a pull-down circuit  24 , and an overcurrent detection circuit  35 . 
     Next, an operation of the related-art battery device is described. 
     A voltage of the overcurrent detection terminal  14  is determined based on a resistive component of the charge/discharge control switch  2  and a current flowing through a load connected between the external terminal  4  and the external terminal  5 . When abnormality occurs in the load, such as a short-circuit between the external terminal  4  and the external terminal  5 , the load current is increased to raise the voltage of the overcurrent detection terminal  14 . 
     When the voltage of the overcurrent detection terminal  14  exceeds an overcurrent detection voltage of the overcurrent detection circuit  35 , the overcurrent detection circuit  35  outputs a detection signal. When receiving the detection signal, the control circuit  22  controls the charge/discharge control switch  2  so as to interrupt a discharge current. This state is referred to as “discharge overcurrent state”. On the other hand, when the voltage of the overcurrent detection terminal  14  falls below the overcurrent detection voltage of the overcurrent detection circuit  35 , the overcurrent detection circuit  35  outputs a release signal to release the discharge overcurrent state (see, for example, Japanese Patent Application Laid-open No. 2006-101696). 
     In the discharge overcurrent state, the control circuit  22  turns on the pull-down circuit  24  to pull down the overcurrent detection terminal  14 . Specifically, when the load returns to a normal state, the voltage of the overcurrent detection terminal  14  is decreased. Then, when the voltage of the overcurrent detection terminal  14  falls below the overcurrent detection voltage, the overcurrent detection circuit  35  releases the discharge overcurrent state. When receiving the release signal of the current detection circuit  35 , the control circuit  22  controls the charge/discharge control switch  2  to restart the discharge. In this case, load impedance at which the discharge is restarted is referred to as “overcurrent release impedance”. 
     The pull-down circuit  24  is configured to be turned on when the load enters an abnormal state, and hence the discharge current flows via the pull-down circuit  24  in this case. Thus, the pull-down circuit  24  is set so as to have large impedance. 
     Specifically, the voltage of the overcurrent detection terminal  14  does not fall below the overcurrent detection voltage unless the load impedance becomes larger than the impedance of the pull-down circuit  24 , and hence the discharge may not be restarted when the load impedance is not so large. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in order to solve the problem described above, and provides a battery device with high user convenience. 
     In order to solve the related-art problem, a battery state monitoring circuit according to one embodiment of the present invention is configured as follows. 
     The battery state monitoring circuit includes: an overcurrent detection terminal for inputting a voltage generated in a charge/discharge control switch; a pull-down circuit connected to the overcurrent detection terminal, for pulling down the overcurrent detection terminal in an overcurrent state; a first comparator circuit for comparing a voltage of the overcurrent detection terminal with a first reference voltage, and outputting an overcurrent detection signal when the voltage of the overcurrent detection terminal exceeds the first reference voltage; a second comparator circuit for comparing the voltage of the overcurrent detection terminal with a second reference voltage that is higher than the first reference voltage, and outputting an overcurrent release voltage control signal when the voltage of the overcurrent detection terminal exceeds the second reference voltage; and a control circuit for turning off the charge/discharge control switch and on the pull-down circuit when the control circuit receives the overcurrent detection signal, and for switching an overcurrent release voltage to the second reference voltage when the control circuit receives the overcurrent release voltage control signal. 
     The battery device according to one embodiment of the present invention can set overcurrent release impedance to be low, and hence the discharge can be restarted even when a load has not so large load impedance, resulting in improved convenience. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a battery device according to a first embodiment of the present invention. 
         FIG. 2  is a circuit diagram of a battery device according to a second embodiment of the present invention. 
         FIG. 3  is a circuit diagram of a related-art battery device. 
         FIG. 4  is a circuit diagram of a battery device according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, embodiments of the present invention are described with reference to the accompanying drawings. 
     &lt;First Embodiment&gt; 
       FIG. 1  is a circuit diagram of a battery device according to a first embodiment of the present invention. 
     The battery device according to the first embodiment includes a secondary battery  1 , a charge/discharge control switch  2 , a battery state monitoring circuit  3 , and external terminals  4  and  5 . 
     The battery state monitoring circuit  3  includes a control circuit  22 , a pull-down circuit  24 , an overcurrent detection comparator circuit  25 , an overcurrent release comparator circuit  27 , an overcurrent detection reference voltage circuit  26 , an overcurrent release reference voltage circuit  28 , a positive power supply terminal  11 , a negative power supply terminal  12 , a charge/discharge control signal output terminal  13 , and an overcurrent detection terminal  14 . 
     The secondary battery  1  has a positive terminal connected to the external terminal  4  and the positive power supply terminal  11  and a negative terminal connected to the negative power supply terminal  12  and one terminal of the charge/discharge control switch  2 . The charge/discharge control switch  2  has a control terminal connected to the charge/discharge control signal output terminal  13 . The other terminal of the charge/discharge control switch  2  is connected to the overcurrent detection terminal  14  and the external terminal  5 . 
     The control circuit  22  has a first input terminal connected to the positive power supply terminal  11 , a second input terminal connected to the negative power supply terminal  12 , a third input terminal connected to an output terminal of the comparator circuit  25 , a fourth input terminal connected to an output terminal of the comparator circuit  27 , a first output terminal connected to the charge/discharge control signal output terminal  13 , and a second output terminal connected to a first input terminal of the pull-down circuit  24 . 
     The comparator circuit  25  has a first input terminal connected to the overcurrent detection terminal  14  and a second input terminal connected to an output terminal of the reference voltage circuit  26 . The comparator circuit  27  has a first input terminal connected to the overcurrent detection terminal  14  and a second input terminal connected to an output terminal of the reference voltage circuit  28 . The pull-down circuit  24  has a second input terminal connected to the negative power supply terminal  12  and an output terminal connected to the overcurrent detection terminal  14 . 
     Next, an operation of the battery device according to the first embodiment is described. 
     When a load is connected between the external terminal  4  and the external terminal  5 , a potential difference is generated between the external terminal  4  and the external terminal  5  depending on a current flowing through the load and a resistive component of the charge/discharge control switch  2 . The battery state monitoring circuit  3  monitors the potential difference through the overcurrent detection terminal  14 , to thereby protect the battery device from a discharge overcurrent. In this case, a voltage of the reference voltage circuit  26  is an overcurrent detection voltage, and a voltage of the reference voltage circuit  28  is an overcurrent release voltage. Then, the voltage of the reference voltage circuit  28  is set to be higher than the voltage of the reference voltage circuit  26 . 
     When abnormality such as a short-circuit occurs in the load to increase a load current so that the voltage of the overcurrent detection terminal  14  exceeds the voltage of the reference voltage circuit  26 , the comparator circuit  25  outputs a detection signal. When receiving the detection signal, the control circuit  22  controls the charge/discharge control switch  2  so as to interrupt a discharge current. In addition, the control circuit  22  turns on the pull-down circuit  24  to pull down the overcurrent detection terminal  14 . This state is referred to as “discharge overcurrent state”. 
     In the discharge overcurrent state, the voltage of the overcurrent detection terminal  14  is raised due to the load. When the load returns to a normal state, that is, when load impedance increases, the voltage of the overcurrent detection terminal  14  is decreased by the pull-down circuit  24 . Then, when the voltage of the overcurrent detection terminal  14  falls below the overcurrent release voltage, the control circuit  22  releases the discharge overcurrent state, and controls the charge/discharge control switch  2  so as to restart the discharge. In this case, load impedance at which the discharge is restarted is referred to as “overcurrent release impedance”. 
     At this time, the control circuit  22  switches the overcurrent release voltage depending on the voltage of the overcurrent detection terminal  14 . The overcurrent release voltage is set to be the voltage of the reference voltage circuit  26  when the voltage of the overcurrent detection terminal  14  is lower than the output voltage of the reference voltage circuit  28 , and to be the voltage of the reference voltage circuit  28  when the voltage of the overcurrent detection terminal  14  is equal to or higher than the voltage of the reference voltage circuit  28 . 
     When the load impedance is small, the voltage of the overcurrent detection terminal  14  in the discharge overcurrent state is equal to or higher than the voltage of the reference voltage circuit  28 . Thus, the control circuit  22  sets the overcurrent release voltage to be the voltage of the reference voltage circuit  28  higher than the voltage of the reference voltage circuit  26 . 
     Consequently, even when the load has small impedance and small overcurrent release impedance, the control circuit  22  can release the overcurrent state. After the release, the control circuit  22  turns on the charge/discharge control switch  2  and turns off the pull-down circuit  24 . In this manner, the battery device can return to the normal operating state. 
     As described above, the battery device according to this embodiment can set the overcurrent release voltage depending on the impedance of the load, and hence even when the load has small impedance, the overcurrent can be reliably released. 
     &lt;Second Embodiment&gt; 
       FIG. 2  is a circuit diagram of a battery device according to a second embodiment of the present invention. 
     A battery state monitoring circuit  3  of the battery device according to the second embodiment includes a control circuit  22 , a pull-down circuit  24 , a comparator circuit  25 , a comparator circuit  27 , a reference voltage circuit  29 , a reference voltage circuit  28 , a positive power supply terminal  11 , a negative power supply terminal  12 , a charge/discharge control signal output terminal  13 , and an overcurrent detection terminal  14 . 
     The comparator circuit  27  has an output terminal connected to a control terminal of the reference voltage circuit  29 . The reference voltage circuit  29  switches its output voltage depending on a signal input to the control terminal. Other circuits and connection relationships are the same as those in the battery device according to the first embodiment. 
     Next, an operation of overcurrent detection release of the battery device according to the second embodiment is described. 
     In a discharge overcurrent state, a voltage of the overcurrent detection terminal  14  is increased due to a load. At this time, an overcurrent release voltage is switched depending on the voltage of the overcurrent detection terminal  14 . When the voltage of the overcurrent detection terminal  14  exceeds an output voltage of the reference voltage circuit  28 , the voltage of the reference voltage circuit  29  is switched to a higher voltage based on a detection signal output from the comparator circuit  27 . In this case, the voltage of the reference voltage circuit  29  is switched to a voltage equal to or higher than the voltage of the reference voltage circuit  28 , and this voltage is referred to as “overcurrent release voltage”. 
     When load impedance is small, the voltage of the overcurrent detection terminal  14  in the discharge overcurrent state is equal to or higher than the voltage of the reference voltage circuit  28 . Thus, the voltage of the reference voltage circuit  29  is switched to a higher voltage, and this voltage is referred to as “overcurrent release voltage”. 
     Consequently, even when the load has small impedance and small overcurrent release impedance, the control circuit  22  can release the overcurrent state through the setting of the voltage of the reference voltage circuit  29  to be higher. After the release, the control circuit  22  turns on the charge/discharge control switch  2  and turns off the pull-down circuit  24 . In this manner, the battery device can return to the normal operating state. 
     As described above, the battery device according to this embodiment can set the overcurrent release voltage depending on the impedance of the load, and hence even when the load has small impedance, the overcurrent can be reliably released. 
     &lt;Third Embodiment&gt; 
       FIG. 4  is a circuit diagram of a battery device according to a third embodiment of the present invention. 
     A battery state monitoring circuit  3  of the battery device according to the third embodiment includes a control circuit  22 , a pull-down circuit  24 , a comparator circuit  25 , a comparator circuit  27 , a reference voltage circuit  26 , a reference voltage circuit  28 , a positive power supply terminal  11 , a negative power supply terminal  12 , a resistor  41 , a current source  42 , a charge/discharge control signal output terminal  13 , and an overcurrent detection terminal  14 . 
     The resistor  41  has one end connected to the overcurrent detection terminal  14  and the other end connected to a first input terminal of the comparator circuit  25 . The current source  42  is connected between the resistor  41  and the positive power supply terminal  11 . The comparator circuit  27  has an output terminal connected to a control terminal of the current source  42 . The current source  42  switches its output current depending on a signal input to the control terminal. Other circuits and connection relationships are the same as those in the battery device according to the first embodiment. 
     Next, an operation of the battery device according to the third embodiment is described. 
     When the comparator circuit  27  outputs no detection signal, the current source  42  causes a predetermined current to flow through the resistor  41 . In the resistor  41 , a voltage at the other terminal is higher than the voltage of the overcurrent detection terminal  14  by a voltage generated across the resistor  41  due to the current. The comparator circuit  25  compares the voltage of the reference voltage circuit  26  with the voltage at the other terminal of the resistor  41  to detect a discharge overcurrent. In other words, the current source  42  and the resistor  41  apply an offset voltage to the first input terminal of the comparator circuit  25 . 
     In a discharge overcurrent state, a voltage of the overcurrent detection terminal  14  is increased due to a load. At this time, the offset voltage of the first input terminal of the comparator circuit  25  is switched depending on the voltage of the overcurrent detection terminal  14 . When the voltage of the overcurrent detection terminal  14  exceeds an output voltage of the reference voltage circuit  28 , the current of the current source  42  is switched based on a detection signal output from the comparator circuit  27 . For example, the current of the current source  42  is switched to be off so as to set the offset voltage of the first input terminal of the comparator circuit  25  to be zero. 
     When load impedance is small, the voltage of the overcurrent detection terminal  14  in the discharge overcurrent state is equal to or higher than the voltage of the reference voltage circuit  28 . Thus, the voltage of the reference voltage circuit  26  is switched to an apparently higher voltage, and this voltage is referred to as “overcurrent release voltage”. 
     Consequently, even when the load has small impedance and small overcurrent release impedance, the control circuit  22  can release the overcurrent state. After the release, the control circuit  22  turns on the charge/discharge control switch  2  and turns off the pull-down circuit  24 . In this manner, the battery device can return to the normal operating state. 
     As described above, the battery device according to this embodiment can set the overcurrent release voltage depending on the impedance of the load, and hence even when the load has small impedance, the overcurrent can be reliably released.