Abstract:
There is provided a battery voltage detector circuit which uses a multiplexer system and which is capable of reducing the influence of the deviation of a detected voltage attributable to a parasitic capacitance, thus improving the accuracy of voltage detection. The battery voltage detector circuit that monitors the voltages of a plurality of batteries connected in series includes a flying capacitor, a multiplexer switch that sequentially connects the flying capacitor to the plurality of batteries, a voltage detecting unit that detects the voltage of the flying capacitor, a first reference potential detecting unit connected to one terminal of the flying capacitor, a second reference potential connecting unit connected to the other terminal of the flying capacitor, and a control circuit that controls the multiplexer switch, the first reference potential connecting unit and the second reference potential connecting unit.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-028212 filed on Feb. 15, 2013, 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 protection circuit of a secondary battery, such as a lithium battery, and more particularly to a battery voltage detector circuit of a protection circuit that uses a multiplexer. 
         [0004]    2. Background Art 
         [0005]    In general, a secondary battery, such as a lithium battery, uses a protection circuit to protect the battery from overcharge or overdischarge. A protection circuit has a voltage detecting unit for detecting the voltage of a battery. Protecting a plurality of batteries connected in series requires a plurality of voltage detecting units corresponding to the individual batteries, thus leading to a larger scale and a higher withstand voltage of the circuit. 
         [0006]      FIG. 2  is a circuit diagram of a battery voltage detector circuit that uses a conventional multiplexer. The battery voltage detector circuit using the conventional multiplexer includes a battery power supply device  11 , a battery  11   a , switches  21 ,  22 ,  23  and  24 , flying capacitors  28  and  29 , an amplifier  25 , an A/D converter  26 , and a controller  30 . The switch  21  is composed of normally open contacts  21   a  and  21   b,  the switch  23  is composed of normally open contacts  23   a  and  23   b,  and the switch  24  is composed of normally open contacts  24   a  and  24   b.    
         [0007]    To detect the voltage of the battery  11   a,  the switches  21  to  24  are set to an OFF (open) state. In this state, the switch  21  is first set to an ON state and the normally open contacts  21   a  and  21   b  are individually set to a closed state. This causes the voltage in the battery  11   a  to be applied to the flying capacitors  28  and  29 , which are connected in series. Thus, electric charges are accumulated in the flying capacitors  28  and  29 . 
         [0008]    After the switch  21  is held in an ON position for a predetermined period of time, the switch  21  is set to the OFF state and the normally open contacts  21  a and  21   b  are individually set to the open state. This causes the electric charges corresponding to the voltage of the battery  11   a  to be accumulated in the flying capacitors  28  and  29 . 
         [0009]    Thereafter, the switch  22  and the switch  24  are turned on. Turning the switch  22  on causes the connection point of the flying capacitors  28  and  29  to be connected to the ground and fixed to 0V. When the switch  24  is turned on and the normally open contacts  24   a  and  24   b  are individually set to the closed state, an inverting input terminal of the amplifier  25  is fixed to the same potential (0V) of an output of the amplifier  25 , and the voltage of a non-inverting input terminal is fixed to the ground (0V). 
         [0010]    Thereafter, the switch  23  is turned on and the normally open contacts  23   a  and  23   b  are individually set to the closed state. This causes the flying capacitors  28  and  29  to be connected to the inverting input terminal and the non-inverting input terminal, respectively, of the amplifier  25 . However, the switch  24  is on, so that the voltages of the terminals are fixed and the voltages of the flying capacitors  28  and  29  are not applied to the terminals of the amplifier  25 . 
         [0011]    Thereafter, the switch  24  is turned off and the fixation of the voltages at the terminals of the amplifier  25  are cleared, thereby causing the voltages accumulated in the flying capacitors  28  and  29  to be applied to the amplifier  25 . The voltages of the flying capacitors  28  and  29  are applied in the state wherein the voltages of the input terminals of the amplifier  25  are fixed to 0V. This makes it possible to accurately detect the voltages applied from the flying capacitors  28  and  29  without the risk of the voltages of the input terminals exceeding a permissible range or an output of the amplifier  25  being saturated. In addition, the voltages supplied to the amplifier  25  are unlikely to exceed a permissible range, thus protecting the amplifier  25  from deterioration and damage (refer to, for example, Patent Document 1). 
         [0012]    [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-201548 
         [0013]    However, the conventional art has been posing a problem in that, at the instant the switch  22  is turned on, electric charges move to a parasitic capacitance produced between the switch  22  and the flying capacitors  28  and  29  and the holding voltages of the flying capacitors  28  and  29  change, leading to deteriorated voltage detection accuracy. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention has been made with a view toward solving the problem described above and an object of the invention is to achieve improved voltage detection accuracy by reducing the influence of a detected voltage deviation attributable to a parasitic capacitance in a battery voltage detector circuit that uses a multiplexer system. 
         [0015]    To solve the problem with the prior art, a battery voltage detector circuit in accordance with the present invention is configured as described below. 
         [0016]    The battery voltage detector circuit is adapted to monitor the voltages of a plurality of batteries connected in series, and includes: a flying capacitor; a multiplexer switch that sequentially connects the flying capacitor to a plurality of batteries; a voltage detecting unit that detects the voltage of the flying capacitor; a first reference potential connecting unit connected to one terminal of the flying capacitor; a second reference potential connecting unit connected to the other terminal of the flying capacitor; and a control circuit that controls the multiplexer switch, the first reference potential connecting unit, and the second reference potential connecting unit. 
         [0017]    The battery voltage detector circuit in accordance with the present invention is capable of reducing the influence of a parasitic capacitance on the detection of a battery voltage thereby to improve the accuracy of voltage detection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a circuit diagram illustrating a battery voltage detector circuit according to a first embodiment; 
           [0019]      FIG. 2  is a circuit diagram illustrating a battery voltage detector circuit according to a second embodiment; and 
           [0020]      FIG. 3  is a circuit diagram illustrating a conventional battery voltage detector circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The following will describe embodiments of the present invention with reference to the accompanying drawings. 
       First Embodiment  
       [0022]      FIG. 1  is a circuit diagram of a battery voltage detector circuit according to a first embodiment. The battery voltage detector circuit according to the present embodiment is constituted of a row of batteries  100  and a battery voltage detector circuit  200 . The row of batteries  100  is formed of batteries  100 _ 1 ,  100 _ 2 ,  100 _ 3 , and  100 _ 4 . The battery voltage detector circuit  200  is constituted of a multiplexer switch  210 , a flying capacitor  240 , switches  280  and  281 , constant current circuits  290  and  291 , an amplifier  250 , a comparator  260 , a reference voltage circuit  270 , a control circuit  220 , a resistor  271 , a VDD terminal, and a VSS terminal. The multiplexer switch  210  is constituted of switches  210 _ 1 ,  210 _ 2 ,  210 _ 3 ,  210 _ 4 ,  210 _ 5 ,  210 _ 6 ,  210 _ 7 , and  210 _ 8 . One end of the flying capacitor  240  is denoted as a node A, while the other end thereof is denoted as a node B. 
         [0023]    The positive electrode of the battery  100 _ 1  is connected to the VDD terminal and one terminal of the switch  210 _ 1 , while the negative electrode thereof is connected to the positive electrode of the battery  100 _ 2  and one terminal of each of the switch  210 _ 2  and the switch  210 _ 3 . The other terminal of each of the switch  210 _ 1  and the switch  210 _ 2  is connected to the node A, while the other terminal of the switch  210 _ 3  is connected to the node B. The positive electrode of the battery  100 _ 3  is connected to the negative electrode of the battery  100 _ 2  and one terminal of each of the switch  210 _ 4  and the switch  210 _ 5 , while the negative electrode thereof is connected to the positive electrode of the battery  100 _ 4  and one terminal of each of the switch  210 _ 6  and the switch  210 _ 7 . The other terminal of each of the switch  210 _ 4  and the switch  210 _ 6  is connected to the node A, and the other terminal of each of the switch  210 _ 5  and the switch  210 _ 7  is connected to the node B. 
         [0024]    One terminal of the switch  210 _ 8  is connected to the VSS terminal and the negative electrode of the battery  100 _ 4 , while the other terminal is connected to the node B. One terminal of the switch  280  is connected to the constant current circuit  290 , while the other terminal thereof is connected to the node A. The other terminal of the constant current circuit  290  is connected to the VDD terminal. One terminal of the switch  281  is connected to the constant current circuit  291 , while the other terminal thereof is connected to the node B. The other terminal of the constant current circuit  291  is connected to the VDD terminal. An inverting input terminal of the amplifier  250  is connected to one terminal of the resistor  271 , while the non-inverting input terminal thereof is connected to the node B, while the output thereof is connected to the other terminal of the resistor  271  and the inverting input terminal of the comparator  260 . The non-inverting input terminal of the comparator  260  is connected to the negative electrode of the reference voltage circuit  270 , while the positive electrode of the reference voltage circuit  270  is connected to the VDD terminal. The amplifier  250 , the comparator  260 , the reference voltage circuit  270 , and the resistor  271  constitute a voltage detecting unit. The switches  280  and  281  and the multiplexer switch  210  are controlled to be turned on/off by the control circuit  220 . A parasitic capacitance  230  exists on the node B. 
         [0025]    The operation of the battery voltage detector circuit will be described. The flying capacitor  240  is controlled by a signal of the control circuit  220  such that the flying capacitor  240  becomes parallel to one of the row of batteries  100 . The signal of the control circuit  220  causes the switch  210 _ 1  and the switch  210 _ 3  to turn on and the remaining switches to turn off, thus connecting the battery  100 _ 1  and the flying capacitor  240 . The flying capacitor  240  is charged to the same voltage (V0) as that of the battery  100 _ 1 . Thereafter, the signal of the control circuit  220  causes the switch  210 _ 1  and the switch  210 _ 3  to turn off, thus disconnecting the battery  100 _ 1  and the flying capacitor  240 . 
         [0026]    Then, when the switch  280  is turned on by the signal of the control circuit  220 , the node A is pulled up by the constant current circuit  290  to the voltage of the VDD terminal (VDD). The voltage of the node B becomes VDD−V0, and the voltage of VDD−V0 is applied to the non-inverting input terminal of the amplifier  250 . The voltage of VDD−V0 is supplied to the output of the amplifier  250 , so that the voltage of the reference voltage circuit  270  (comparison voltage) and the voltage of VDD−V0 are compared by the comparator  260 , thus making it possible to detect whether the voltage of the flying capacitor  240  is higher or lower than the comparison voltage. In other words, it is possible to detect whether the voltage of the battery  100 _ 1  is higher or lower than the comparison voltage by comparing the voltage of the battery  100 _ 1  with the comparison voltage. The voltages of all the batteries can be monitored by carrying out the foregoing series of operations also on the remaining batteries. 
         [0027]    The case where the parasitic capacitance  230  exists on the node B will be discussed. All the batteries of the row of batteries  100  are assumed to have V0, and the flying capacitor  240  is connected to the battery  100 _ 4  at the lowermost end. At this time, the voltage of the node B is VSS, and the voltage of the parasitic capacitance  230  becomes 0V. All the switches of the multiplexer switch  210  are turned off to open the flying capacitor  240 , and then the switch  280  is turned on. This causes the node A to be pulled up by the constant current circuit  290 . The parasitic capacitance  230  is charged by the constant current circuit  290 . The charging current flows via the flying capacitor  240 , so that the holding voltage of the flying capacitor  240  becomes higher than V0. The voltage of the flying capacitor  240  when the voltage of the node A eventually becomes equivalent to VDD will be represented by the following expression. 
         [0000]        V 0+ Cx /( C+Cx )×3 V 0
 
         [0000]    where C denotes the capacitance value of the flying capacitor  240 , and Cx denotes the capacitance value of the parasitic capacitance  230 . The deviation width of the voltage of the flying capacitor  240  is denoted by Cx/(C+Cx)×3V0 of the second term. In this case, Cx/(C+Cx) denotes the capacitance ratio between the flying capacitor  240  and the parasitic capacitance  230 , and 3V0 denotes the voltage deviation width observed from a state, in which the node A is connected to the battery  100  at the lowermost end, to the moment when the node A is pulled up to VDD. 
         [0028]    Thus, it is understood that the larger the capacitance value of the parasitic capacitance  230  as compared with the capacitance value of the flying capacitor  240 , the larger the deviation amount becomes. The deviation amount also increases as the voltage of a battery to be monitored is farther from a reference potential. 
         [0029]    A case where a battery voltage monitoring circuit is integrated on an IC is assumed. It is also assumed that the capacitance value of the flying capacitor 240 is 10 pF and the capacitance value of the parasitic capacitance  230  is 100 fF. It is further assumed that the battery voltages are all 4.0 V and that the batteries are connected in 4 series, as illustrated in  FIG. 1 . The amount of deviation that occurs when detecting the voltage of the battery at the lowermost end will be as follows: 
         [0000]      100 fF/(100 pF+100 fF)×3×4.0=12 mV
 
         [0030]    The overcharge detection voltage of a general lithium battery is required to have an accuracy of approximately ±20 mV. This means that the foregoing amount of deviation will have a significant adverse effect on the accurate performance of the battery voltage detector circuit. 
         [0031]    In order to reduce the influence of the parasitic capacitance  230 , all the switches of the multiplexer switch  210  are turned off to open the flying capacitor  240  and then the switches  280  and  281  are simultaneously turned on. This enables not only the constant current circuit  290  but also the constant current circuit  291  to contribute to the charging of the parasitic capacitance  230 . Control is carried out such that, when the potential of the node A reaches VDD, the switch  281  is turned off by the control circuit  220 . This prevents the constant current circuit  291  from charging the flying capacitor  240 , so that a voltage deviation of the flying capacitor  240  will not occur. 
         [0032]    The deviation taking place in the flying capacitor  240  decreases as the parasitic capacitance  230  is charged more by the constant current circuit  291  than by the constant current circuit  290  by the time when the node A reaches VDD. Therefore, in order to increase the voltage detection accuracy, the amount of current of the constant current circuit  291  is desirably larger than that of the constant current circuit  290 . If the current values of the constant current circuits  290  and  291  are the same, then the deviation width of the voltage of the flying capacitor  240  will be Cx/(2C+Cx)×3V0, making it possible to obtain equivalently the same effect that would be obtained by increasing the capacitance value of the flying capacitor  240 . 
         [0033]    In the above description, the amplifier  250  has been used to detect the voltage of the flying capacitor  240 . However, the amplifier may not necessarily be used as long as the configuration is capable of detecting the voltage of the flying capacitor  240 . 
         [0034]    Further, in the above description, the constant current circuits  290  and  291  have been used to pull up the flying capacitor  240  and to charge the parasitic capacitance  230 . However, the constant current circuits may not necessarily be used, and direct connection to the resistor or VDD or a different configuration may be used as long as the configuration is capable of pulling up the flying capacitor  240  and charging the parasitic capacitance  230 . 
         [0035]    Thus, the battery voltage detector circuit according to the first embodiment is capable of improving the voltage detection accuracy by charging the parasitic capacitance  230  by using the constant current circuit  291 . Further, the improved accuracy can be achieved without increasing the size of the flying capacitor  240 , so that the layout area can be reduced accordingly. 
       Second Embodiment 
       [0036]      FIG. 2  is a circuit diagram of a battery voltage detector circuit according to a second embodiment, which differs from the one illustrated in  FIG. 1  in that a switch  300  has been added. Regarding the connection, one terminal of the switch  300  is connected to the connection point of a switch  281  and a flying capacitor  240 , while the other terminal thereof is connected to a non-inverting input terminal of the amplifier  250 . The switch  300  is controlled to be turned on/off by a control circuit  220 . 
         [0037]    The operation the battery voltage detector circuit will be described. The flying capacitor  240  is controlled by a signal of the control circuit  220  such that the flying capacitor  240  becomes parallel to one of a row of batteries  100 . The signal of the control circuit  220  causes a switch  210 _ 1  and a switch  210 _ 3  to turn on and the remaining switches to turn off, thus connecting the battery  100 _ 1  and the flying capacitor  240 . The flying capacitor  240  is charged to the same voltage (V0) as that of the battery  100 _ 1 . Thereafter, the signal of the control circuit  220  causes the switch  210 _ 1  and the switch  210 _ 3  to turn off, thus disconnecting the battery  100 _ 1  and the flying capacitor  240 . 
         [0038]    Then, when a switch  280  is turned on by the signal of the control circuit  220 , a node A is pulled up by a constant current circuit  290  to the voltage of a VDD terminal (VDD). The voltage of a node B becomes VDD−V0, and the voltage of VDD−V0 is applied to the non-inverting input terminal of the amplifier  250  when the switch  300  is turned on by a signal of the control circuit  220 . The voltage of VDD−V0 is supplied to the output of the amplifier  250 , so that the voltage of a reference voltage circuit  270  (comparison voltage) and the voltage of VDD−V0 are compared by a comparator  260 , thus making it possible to detect whether the voltage of the flying capacitor  240  is higher or lower than the comparison voltage. In other words, the voltage of the battery  100 _ 1  can be compared with the comparison voltage to detect whether the voltage of the battery  100 _ 1  is higher or lower than the comparison voltage. The voltages of all the batteries can be monitored by carrying out the series of operations described above also on the remaining batteries. 
         [0039]    The case where a parasitic capacitance  230  exists on a node B will be discussed. All the batteries of the row of batteries  100  are assumed to have V0, and the flying capacitor  240  is connected to the battery  100 _ 4  at the lowermost end. At this time, the voltage of the node B is VSS, and the voltage of a parasitic capacitance  230  becomes 0V. All the switches of a multiplexer switch  210  are turned off to open the flying capacitor  240 , and then the switch  280  is turned on. This causes the node A to be pulled up by a constant current circuit  290 . The parasitic capacitance  230  is charged by the constant current circuit  290 . The charging current flows via the flying capacitor  240 , so that the holding voltage of the flying capacitor  240  becomes higher than V0. The voltage of the flying capacitor  240  when the voltage of the node A eventually becomes equivalent to VDD is represented by the following expression. 
         [0000]        V 0 +Cx/ ( C+Cx )×3 V 0
 
         [0000]    where C denotes the capacitance value of the flying capacitor  240 , and Cx denotes the capacitance value of the parasitic capacitance  230 . The deviation width of the voltage of the flying capacitor  240  is expressed by Cx/(C+Cx)×3V0 of the second term. In this case, Cx/(C+Cx) denotes the capacitance ratio between the flying capacitor  240  and the parasitic capacitance  230 , and 3V0 denotes the width of voltage shift that occurs from a state, in which the node A is connected to the battery  100  at the lowermost end, to the moment when the node A is pulled up to VDD. 
         [0040]    Thus, it is understood that the deviation amount increases as the capacitance value of the parasitic capacitance  230  is larger than the capacitance value of the flying capacitor  240 . The deviation amount also increases as the voltage of a battery to be monitored is farther from a reference potential. 
         [0041]    It is assumed that a battery voltage monitoring circuit is integrated on an IC, and the capacitance value of the flying capacitor  240  is 10 pF and the capacitance value of the parasitic capacitance  230  is 100 fF. It is further assumed that the battery voltages are all 4.0 V and that the batteries are connected in 4 series, as illustrated in  FIG. 1 . The amount of deviation that occurs when detecting the voltage of the battery at the lowermost end will be as follows: 
         [0000]      100 fF/(100 pF+100 fF)×3×4.0=12 mV
 
         [0042]    The overcharge detection voltage of a general lithium battery is required to have an accuracy of approximately ±20 mV. This means that the foregoing amount of deviation will have a significant adverse effect on the accurate performance of the battery voltage detector circuit. 
         [0043]    In order to reduce the influence of the parasitic capacitance  230 , all the switches of the multiplexer switch  210  and the switch  300  are turned off to open the flying capacitor  240  and then the switches  280  and  281  are simultaneously turned on. This enables not only the constant current circuit  290  but also the constant current circuit  291  to charge the parasitic capacitance  230 . The constant current circuit  291  will not charge the flying capacitor  240 , so that a voltage deviation of the flying capacitor  240  will not occur. The switch  281  is turned off by the control circuit  220  when or before the node A reaches VDD. 
         [0044]    The deviation taking place in the flying capacitor  240  decreases as the parasitic capacitance  230  is charged more by the constant current circuit  291  than by the constant current circuit  290  by the time the node A reaches VDD. Therefore, in order to increase the voltage detection accuracy, the amount of current of the constant current circuit  291  is desirably larger than that of the constant current circuit  290 . If the current values of the constant current circuits  290  and  291  are the same, then the deviation width of the voltage of the flying capacitor  240  will be Cx/(2C+Cx)×3V0, making it possible to obtain equivalently the same effect that would be obtained by increasing the capacitance value of the flying capacitor  240 . 
         [0045]    Turning the switch  300  off while the flying capacitor  240  is being pulled up makes it possible to eliminate the influences of the parasitic capacitance existing from the switch  281  to the input terminal of the amplifier  250  and the gate capacitance of an input transistor of the amplifier  250 . Thus, using the switch  300  permits a reduced influence of the parasitic capacitance  230 , resulting in improved voltage detection accuracy. 
         [0046]    In the above description, the amplifier  250  has been used to detect the voltage of the flying capacitor  240 . However, the amplifier may not necessarily be used and a different configuration may be used as long as the configuration is capable of detecting the voltage of the flying capacitor  240 . 
         [0047]    Further, in the above description, the constant current circuits  290  and  291  have been used to pull up the flying capacitor  240  and to charge the parasitic capacitance  230 . However, the constant current circuits may not necessarily be used, and direct connection to or connection through the resistor to VDD may be implemented, or a different configuration may be used as long as the configuration is capable of pulling up the flying capacitor  240  and charging the parasitic capacitance  230 . 
         [0048]    Thus, the battery voltage detector circuit according to the present embodiment is capable of improving the voltage detection accuracy by charging the parasitic capacitance  230  by using the constant current circuit  291 . The improved accuracy can be achieved without increasing the size of the flying capacitor  240 , so that the layout area can be reduced accordingly. Further, the influences of the parasitic capacitances of the amplifier and the like and the gate capacitance can be removed, thus permitting further improved voltage detection accuracy.