Abstract:
Provided is a battery state monitoring circuit including: a charge/discharge control circuit for detecting and controlling a state of a secondary battery; a temperature sensor circuit connected to an overcurrent detection terminal; an amplifier for comparing a reference voltage and a divided voltage of the over-current detection terminal; and a transistor that receives an output signal of the amplifier, for controlling a charge control output terminal. When a charger is connected in a state in which the secondary battery has an ultra-low voltage, a charge control FET is controlled so that a constant voltage may appear between output terminals, to thereby ensure an operating voltage of the temperature sensor circuit to enable an overheat protection function.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2010-073334 filed on Mar. 26, 2010, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a battery state monitoring circuit for detecting a voltage and an abnormality of a secondary battery and a battery device including the battery state monitoring circuit, and more particularly, to a battery state monitoring circuit incorporating a temperature sensor in a charge/discharge control circuit and a battery device including the battery state monitoring circuit. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 3  illustrates a circuit diagram of a conventional battery device incorporating a temperature sensor. The conventional battery device incorporating the temperature sensor includes a secondary battery  101 , an overcharge detection circuit  301 , an overdischarge detection circuit  305 , a control circuit  302 , an over-heat detection circuit  304 , a temperature sensor  307 , an overcurrent detection circuit  303 , a discharge field-effect transistor (FET)  306 , a charge FET  308 , an external terminal  106 , and an external terminal  107 . 
         [0006]    Connection is made as follows. The positive terminal side of the secondary battery  101  is connected to the overcharge detection circuit  301 , the overdischarge detection circuit  305 , and the external terminal  106 , and the negative terminal side thereof is connected to a source of the discharge FET  306 . The control circuit  302  is connected to an output of the overcharge detection circuit  301 , an output of the overdischarge detection circuit  305 , an output of the overcurrent detection circuit  303 , and an output of the overheat detection circuit  304 . The discharge FET  306  has a gate connected to an output of the control circuit  302  and a drain connected to a drain of the charge FET  308 . The charge FET  308  has a gate connected to another output of the control circuit  302  and a source connected to an input of the overcurrent detection circuit  303  and the external terminal  107 . 
         [0007]    When the charge FET  308  enters an abnormal overheated state due to overcurrent during charge, the overheat detection circuit  304  notifies the control circuit  302  of the abnormal state so that the control circuit  302  operates to change the gate of the charge FET  308  to L to disconnect a charge current. Therefore, the charge FET  308  for charge control can be protected from abnormal overheat (see, for example, Japanese Patent Application Laid-open No. 2007-124775). 
         [0008]    However, the conventional technology has a problem that the overheat detection circuit cannot operate when the secondary battery is charged from an ultra-low voltage (around 0 V) after long-term standing. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made to solve the problem described above, and provides a battery state monitoring circuit in which an overheat protec-tion circuit operates for overheat protection when a charger is connected even if a secondary battery voltage is an ultra-low voltage, and also provides a battery device including the battery state monitoring circuit. 
         [0010]    In order to solve the conventional problem, a battery device including a battery state monitoring circuit according to the present invention has the following configuration. 
         [0011]    A battery state monitoring circuit includes: a charge/discharge control circuit for detecting and controlling a state of a secondary battery; a temperature sensor circuit connected to an overcurrent detection terminal; an amplifier for comparing a reference voltage and a divided voltage of the overcurrent detection terminal; and a transistor that receives an output signal of the amplifier, for controlling a charge control output terminal. 
         [0012]    According to the battery device of the present invention, immediately after the charger is connected, the amplifier compares the output of the reference voltage circuit and the divided voltage of the overcurrent detection terminal, and outputs a signal to control the transistor, with the result that the voltage of the overcurrent detection terminal can be maintained to be a constant voltage higher than a minimum operating voltage of the temperature sensor circuit so that the temperature sensor circuit can operate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    In the accompanying drawings: 
           [0014]      FIG. 1  is a circuit diagram of a battery device including a battery state monitoring circuit according to a first embodiment of the present invention; 
           [0015]      FIG. 2  is a circuit diagram of a battery device including a battery state monitoring circuit according to a second embodiment of the present invention; and 
           [0016]      FIG. 3  is a circuit diagram of a conventional battery device including a battery state monitoring circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring to the accompanying drawings, modes for embodying the present invention are described. 
       First Embodiment 
       [0018]      FIG. 1  is a circuit diagram of a battery device including a battery state monitoring circuit according to a first embodiment of the present invention. 
         [0019]    The battery device according to the first embodiment includes a secondary battery  101 , a discharge control P-channel field-effect transistor (FET)  102 , a charge control P-channel FET  103 , external terminals  106  and  107  to which a charger  105  is to be connected, a charge/discharge control circuit  111 , a temperature sensor circuit  112 , a 0 V charge control circuit  113 , a reference voltage circuit  114 , resistors  115  and  116 , an amplifier  117 , an NMOS transistor  118 , and a PMOS transistor  119 . 
         [0020]    The charge/discharge control circuit  111  uses the secondary battery  101  as power supply. The temperature sensor circuit  112 , the 0 V charge control circuit  113 , the reference voltage circuit  114 , and the amplifier  117  are each con-nected to an overcurrent detection terminal  123  for positive power supply and connected to a negative terminal of the secondary battery  101  for negative power supply. 
         [0021]    The temperature sensor circuit  112  has an output connected to the 0 V charge control circuit  113 . The charge/discharge control circuit  111  has an input connected to the overcurrent detection terminal  123 , one output connected to a discharge control output terminal  121 , and another output connected to the 0 V charge control circuit  113 . The NMOS transistor  118  has a gate connected to an output of the 0 V charge control circuit  113 , a source connected to the negative terminal of the secondary battery  101 , and a drain connected to a drain of the PMOS transistor  119 . The resistor  115  has one end connected to the overcurrent detection terminal  123  and the other end connected to the resistor  116 . The resistor  116  has one end connected to the negative terminal of the secondary battery  101  and the other end connected to the resistor  115 . The amplifier  117  has a non-inverting input terminal connected to an output of the reference voltage circuit  114 , an inverting input terminal connected to a node  131  between the resistors  115  and  116 , and an output connected to a gate of the PMOS transistor  119 . The PMOS transistor  119  has a source connected to a charge control output terminal  122 . The discharge control P-channel FET  102  has a gate connected to the discharge control output terminal  121 , a source connected to the positive terminal of the secondary battery  101 , and a drain connected to a drain of the charge control P-channel FET  103 . The charge control P-channel FET  103  has a gate connected to the charge control output terminal  122  and a source connected to the external terminal  106 . A resistor  108  has one end connected to the charge control output terminal  122  and another end connected to the external terminal  106 . The overcurrent detection terminal  123  is connected to the external terminal  106 , and the external terminal  107  is connected to the negative terminal of the secondary battery  101 . 
         [0022]    Next, an operation of the battery device according to the first embodiment is described. 
         [0023]    In a state in which the secondary battery  101  has an ultra-low voltage (around 0 V) after long-term standing, a charger  105  is connected between the external terminals  106  and  107 . Immediately after the charger  105  is connected, because the overcurrent detection terminal  123  has the ultra-low voltage, the 0 V charge control circuit  113  outputs H to turn ON the NMOS transistor  118 . At this time, the PMOS transistor  119  is turned OFF and accordingly a voltage of the charge control terminal  122  becomes equal to that of the external terminal  106  to turn OFF the charge control P-channel FET  103 . When the voltage of the over-current detection terminal  123  increases from the ultra-low voltage (around 0 V) and the node  131  has a voltage exceeding an output voltage of the reference voltage circuit  114 , the amplifier  117  outputs L to turn ON the PMOS transistor  119 . Then, the voltage of the charge control terminal  122  is changed to L to turn ON the charge control P-channel FET  103 , with the result that the overcurrent detection terminal  123  is connected to the positive terminal of the secondary battery  101  to be reduced in voltage. 
         [0024]    When the voltage of the node  131  falls below the output voltage of the reference voltage circuit  114 , the output voltage of the amplifier  117  increases to reduce a current flowing through the PMOS transistor  119 . Then, the voltage of the charge control terminal  122  becomes close to the voltage of the external terminal  106 , and the charge control P-channel FET  103  performs analog opera-tion to reduce the current to flow. Then, the voltage of the overcurrent detection terminal  123  increases again. The action of feedback described above allows the voltage of the overcurrent detection terminal  123  to be a constant value deter-mined by multiplying the output voltage of the reference voltage circuit  114  by a “resistance of the resistor  116 /(a resistance of the resistor  115 +a resistance of the resistor  116 )”. With this voltage set to be equal to or higher than a minimum operating voltage of the temperature sensor circuit  112 , the temperature sensor circuit  112  can operate immediately after the charger  105  is connected. Therefore, even in the case of charging the secondary battery  101  having the ultra-low voltage, the overheat protection function can work immediately after the charger  105  is connected. 
       Second Embodiment 
       [0025]      FIG. 2  is a circuit diagram of a battery device including a battery state monitoring circuit according to a second embodiment of the present invention.  FIG. 2  is different from  FIG. 1  in that the discharge control P-channel FET  102 , the charge control P-channel FET  103 , the NMOS transistor  118 , and the PMOS transistor  119  are changed to a discharge control N-channel FET  201 , a charge control N-channel FET  202 , a PMOS transistor  221 , and an NMOS transistor  222 , respectively. 
         [0026]    Connection is made as follows. The charge/discharge control circuit  111  uses the secondary battery  101  as power supply. The temperature sensor circuit  112 , the 0 V charge control circuit  113 , the reference voltage circuit  114 , and the amplifier  117  are each connected to the positive terminal of the secondary battery  101  for positive power supply and connected to the overcurrent detection terminal  123  for negative power supply. 
         [0027]    The discharge control N-channel FET  201  has a gate connected to the discharge control output terminal  121 , a source connected to the negative terminal of the secondary battery  101 , and a drain connected to a drain of the charge control N-channel FET  202 . The charge control N-channel FET  202  has a gate connected to the charge control output terminal  122  and a source connected to the external terminal  107 . The amplifier  117  has an inverting input terminal connected to the output of the reference voltage circuit  114 , a non-inverting input terminal con-nected to the node  131  between the resistors  115  and  116 , and an output connected to a gate of the NMOS transistor  222 . The NMOS transistor  222  has a source connected to the charge control terminal  122 . The PMOS transistor  221  has a gate connected to the output of the 0 V charge control circuit  113 , a source connected to the positive terminal of the secondary battery  101 , and a drain connected to a drain of the NMOS transistor  222 . 
         [0028]    Next, an operation of the battery device according to the second embodiment is described. 
         [0029]    In a state in which the secondary battery  101  has an ultra-low voltage (around 0 V) after long-term standing, the charger  105  is connected between the external terminals  106  and  107 . Immediately after the charger  105  is connected, because the overcurrent detection terminal  123  has the ultra-low voltage, the 0 V charge control circuit  113  outputs L to turn ON the PMOS transistor  221 . At this time, the NMOS transistor  222  is turned OFF and accordingly a voltage of the charge control terminal  122  becomes equal to that of the external terminal  107  to turn OFF the charge control N-channel FET  202 . When the voltage of the over-current detection terminal  123  decreases from the ultra-low voltage (around 0 V) and the node  131  has a voltage exceeding an output voltage of the reference voltage circuit  114 , the amplifier  117  outputs H to turn ON the NMOS transistor  222 . Then, the voltage of the charge control terminal  122  is changed to H to turn ON the charge control N-channel FET  202 , with the result that the overcurrent detection terminal  123  is connected to the negative terminal of the secondary battery  101  to be increased in voltage. 
         [0030]    When the voltage of the node  131  falls below the output voltage of the reference voltage circuit  114 , the output voltage of the amplifier  117  decreases to reduce a current flowing through the NMOS transistor  222 . Then, the voltage of the charge control terminal  122  becomes close to the voltage of the external terminal  107 , and the charge control N-channel FET  202  performs analog operation to reduce the current to flow. Then, the voltage of the overcurrent detection terminal  123  decreases again. The action of feedback described above allows the voltage between the external terminal  106  and the overcurrent detection terminal  123  to be a constant value determined by multiplying the output voltage of the reference voltage circuit  114  by a “resistance of the resistor  115 /(a resistance of the resistor  115 +a resistance of the resistor  116 )”. With this voltage set to be equal to or higher than a minimum operating voltage of the temperature sensor circuit  112 , the temperature sensor circuit  112  can operate immediately after the charger  105  is connected. Therefore, even in the case of charging the secondary battery  101  having the ultra-low voltage, the overheat protection function can work.