Abstract:
A disclosed current detection circuit includes a current/voltage converting element for generating a voltage in accordance with an input current, a reference voltage generation circuit for generating a reference voltage, the reference voltage generation circuit having an element configuration for setting a predetermined temperature characteristic to the reference voltage, and a comparator for outputting an output signal in accordance with a magnitude relation between a voltage converted in the current/voltage converting element and the reference voltage generated in the reference voltage generation circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a current detection circuit, and more particularly to a current detection circuit and a protection circuit for converting input current into voltage with a current/voltage converting element, comparing the converted voltage with a reference voltage, and outputting an output signal in accordance with the magnitude relation obtained from the comparison.  
         [0003]     2. Description of the Related Art  
         [0004]     A rechargeable accumulator battery such as a lithium ion battery has its characteristic deteriorated by overcharging and/or over-discharging. Accordingly, a protection circuit is provided to a battery pack using an accumulator battery for protecting the accumulator battery from overcharging and/or over-discharging.  
         [0005]     The protection circuit detects current flowing between the accumulator battery and a load or a charger, and detects an overcharging state or an over-discharging state based on the detection result. The protection circuit turns off a switching element provided between the accumulator battery and the load or the charger. By turning off the switching element with the protection circuit, the accumulator battery can be protected by being separated from the load or the charger. Here, a current detection circuit is provided to the protection circuit for detecting the overcharging state and/or the over-discharging state.  
         [0006]      FIG. 8  is a block diagram showing a configuration of a conventional current detection circuit.  
         [0007]     A current detection circuit  100  includes a sense resistance Rs and a comparator circuit  101 . The sense resistance Rs is connected in series between a terminal Tin 1  and a terminal Tin 2  through which detection current flows. A junction point between one end of the sense resistance Rs and the terminal Tin 1  is connected to an input terminal Tin 11  of the comparator circuit  101 , and a junction point between the other end of the sense resistance Rs and the terminal Tin 2  is connected to an input terminal Tin 12  of the comparator circuit  101 .  
         [0008]     The comparator circuit  101  includes a reference voltage generation circuit  111  and a comparator  112 . The reference voltage generation circuit  111  generates a reference voltage Vref. The reference voltage Vref generated in the reference voltage generation circuit  111  is applied to an inverting input terminal of the comparator  112 .  
         [0009]     A base potential side of the reference voltage generation circuit  111  has the input terminal Tin 12  connected thereto that is applied with the potential of the other end of the sense resistance Rs. A noninverting input terminal of the comparator  112  has the input terminal Tin 11  connected thereto and is thus connected to the one end of the sense resistance Rs.  
         [0010]     The comparator  112  compares a detection voltage Vs generated, in accordance with detection current, in the sense resistance Rs and the reference voltage Vref generated in the reference voltage generation circuit  111 . The comparator  112  provides a high level output when the detection voltage Vs is greater than the reference voltage Vref and provides a low level output when the detection voltage Vs is less than the reference voltage Vref. The output of the comparator  112  is connected to an output terminal Tsout. The output terminal Tsout is supplied to a control circuit of the protection circuit. When the output from the output terminal Tsout of the current detection circuit  100  is a high level output, that is, when an overcharging state is detected, the control circuit switches off a switching element connected in series to the accumulator battery and eliminates the overcharging state (see Japanese Laid-Open Patent Application No. 6-188641).  
         [0011]     However, with the conventional current detection circuit, current detection is executed according to a constant reference voltage Vref without considering the temperature characteristic of the voltage Vs converted by the sense resistance Rs. This leads to a problem where detection current varies depending on temperature.  
       SUMMARY OF THE INVENTION  
       [0012]     It is a general object of the present invention to provide a current detection circuit and a protection circuit that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art.  
         [0013]     Features and advantages of the present invention are set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a current detection circuit and a protection circuit particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.  
         [0014]     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a current detection circuit including: a current/voltage converting element for generating a voltage in accordance with an input current; a reference voltage generation circuit for generating a reference voltage, the reference voltage generation circuit having an element configuration for setting a predetermined temperature characteristic to the reference voltage; and a comparator for outputting an output signal in accordance with a magnitude relation between a voltage converted in the current/voltage converting element and the reference voltage generated in the reference voltage generation circuit.  
         [0015]     In the current detection circuit according to an embodiment of the present invention, the predetermined temperature characteristic set to the reference voltage may be the same as a temperature characteristic of the current/voltage converting element.  
         [0016]     In the current detection circuit according to an embodiment of the present invention, the predetermined temperature characteristic set to the reference voltage may be different from a temperature characteristic of the current/voltage converting element.  
         [0017]     In the current detection circuit according to an embodiment of the present invention, the. reference voltage generation circuit may further include a control part or controlling the reference voltage to a voltage smaller than a predetermined voltage when the reference voltage reaches the predetermined voltage.  
         [0018]     Furthermore, the present invention provides a protection circuit including: a current/voltage converting element for generating a voltage in accordance with an input current; a reference voltage generation circuit for generating a reference voltage, the reference voltage generation circuit having an element configuration for setting a temperature characteristic to the reference voltage; a comparator for outputting an output signal in accordance with a magnitude relation between a voltage converted in the current/voltage converting element and the reference voltage generated in the reference voltage generation circuit; and a control part for controlling the input current in accordance with the output from the comparator.  
         [0019]     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a block diagram showing a configuration of an embodiment of the present invention;  
         [0021]      FIG. 2  is a circuit diagram showing a configuration of a current detection circuit  21 ;  
         [0022]      FIG. 3  is a circuit diagram showing a configuration of a reference voltage generation circuit  41 ;  
         [0023]      FIG. 4  is a plan view of a transistor Q 1  on a semiconductor chip;  
         [0024]      FIG. 5  is a graph showing the characteristic in the change of temperature coefficient with respect to (A/B);  
         [0025]      FIG. 6  is a circuit diagram showing a configuration of an exemplary variation of the reference voltage generation circuit  41 ;  
         [0026]      FIG. 7  is a graph for explaining an operation of a reference voltage generation circuit  61 ; and  
         [0027]      FIG. 8  is a circuit diagram showing a configuration of a current detection circuit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]      FIG. 1  is a block diagram showing a configuration of an embodiment of the present invention.  
         [0029]     A battery pack  1  according to this embodiment includes an accumulator battery  11 , a protection IC  12 , a switching transistor M 1 , and a sense resistance Rs.  
         [0030]     The accumulator battery  11  has a positive electrode connected to an output terminal Tout 0  via the switching transistor Ml and the sense resistance Rs, and a negative electrode connected to a ground terminal Tgnd.  
         [0031]     The protection IC  12  includes a current detection circuit  21  and a control circuit  22 . The current detection circuit  21  detects the current flowing in the accumulator battery  11  from the voltages of both ends of the sense resistance Rs. The current detection circuit  21  outputs a high level output when the voltage generated by the detected current exceeds a reference voltage Vref. The control circuit  22  switches off the switching transistor M 1  when the high level output is output from the current detection circuit  21 . By switching off the switching transistor Ml, the accumulator battery  11  and the output terminal Tout 0  are disconnected and the current flowing in the accumulator battery  11  is cut off. Thereby, the accumulator battery  11  is protected.  
         [0032]     Next, the current detection circuit  21  is described in detail.  FIG. 2  is a circuit diagram showing an exemplary configuration of the current detection circuit  21 .  
         [0033]     The current detection circuit  21  in this embodiment includes a reference voltage generation circuit  41  and a comparator  42 .  
         [0034]     The sense resistance Rs is connected in series between a terminal Tin 1  and a terminal Tin 2  through which a detection current Is flows. A detection voltage Vs (Vs=Rs×Is) is generated across the sense resistance Rs in accordance with the detection current Is. One end of the sense resistance Rs is connected to an input terminal Tin 11  of the current detection circuit  21 , and the other end of the sense resistance Rs is connected to an input terminal Tin 12  of the current detection circuit  21 .  
         [0035]      FIG. 3  is a circuit diagram showing an exemplary configuration of the reference voltage generation circuit  41 .  
         [0036]     The reference voltage generation circuit  41  includes MOS field effect transistors Q 1 -Q 3  and a voltage divider circuit  51 . The transistor Q 1 , being formed of an N-channel depletion type MOS field effect transistor, has a drain applied with a drive voltage Vcc, a source connected to a drain of the transistor Q 3 , and a gate connected to a source of the transistor Q 1  and a gate of the transistor Q 2 .  
         [0037]     The transistor Q 2 , being formed of an N-channel depletion type MOS field effect transistor, has a drain applied with the drive voltage Vcc, a source connected to the voltage divider circuit  51  connecting a resistance R 11  and a resistance R 12  in series, and a gate connected to the gate and the source of the transistor Q 1 .  
         [0038]     The transistor Q 3 , being formed of an N-channel enhancement type MOS field effect transistor, has a drain connected to the source and drain of the transistor Q 1 , a source applied with a base voltage V 0  of the reference voltage generation circuit  21 , and a gate connected to a junction point between the transistor Q 2  and the voltage divider circuit  51 .  
         [0039]     The voltage divider circuit  51  includes the resistance R 11  and the resistance R 12  connected in series. The voltage divider circuit  51  is connected between a junction point between the source of the transistor Q 2  and the gate of the transistor Q 3 , and the base voltage V 0 . The voltage divider circuit  51  divides a constant voltage, generated at the junction point between the source of the transistor Q 2  and the gate of the transistor Q 3 , with the resistance R 11  and the resistance R 12 , and outputs the divided voltage as reference voltage Vref from a junction point between the resistance R 11  and the resistance R 12 .  
         [0040]     The junction point between the resistance R 11  and the resistance R 12  is connected to a reference voltage output terminal Tvref, and the reference voltage Vref is output from the reference voltage output terminal Tvref. The reference voltage output terminal Tvref is connected to an inverting input terminal of the comparator  42 .  
         [0041]     The comparator  42  has a noninverting input terminal connected to the input terminal Tin 11 . It is. to be noted that, in the reference voltage generation circuit  41 , the voltage from the input terminal Tin 12  to the other end of the sense resistance Rs is supplied as the base voltage V 0 .  
         [0042]     The comparator  42  compares the detection voltage Vs generated in the sense resistance Rs and the reference voltage Vref generated in the reference voltage generation circuit  41 . The comparator  42  outputs a high level output when the detection voltage Vs is greater than the reference voltage Vref and outputs a low level output when the detection voltage Vs is less than the reference voltage Vref.  
         [0043]     Here, the temperature characteristic of the reference voltage Vref generated in the reference voltage generation circuit  41  is set for reducing the influence of the temperature characteristic of the sense resistance Rs. For example, in a case where the temperature characteristic of the sense resistance Rs has a positive temperature characteristic, the reference voltage Vref is set to have a positive temperature characteristic.  
         [0044]     Next, a method of setting the temperature characteristic of the reference voltage Vref is described.  
         [0045]     The reference voltage Vref is determined in accordance with a ratio between a channel area of the transistor Q 1  (Q 2 ) and a channel area of the transistor Q 3  in the reference voltage generation circuit  41 .  
         [0046]      FIG. 4  is a plan view showing the transistor Q 1  on a semiconductor chip.  
         [0047]     In  FIG. 4 , letter AG indicates a configuration space of the transistor Q 1 , and a space A 0  is formed having a width of W 1 . Furthermore, letter S indicates a source, and letter D indicates a drain. On the configuration space A 0 , a gate electrode GL is wired on the configuration space A 0  via an insulating layer. The gate electrode GL is formed having a width of L 1 . Here, the ratio of the sides of the channel forming the transistor Q 1  (Q 2 ) is represented as W 1 /L 1 . It is to be noted that the same applies to the transistor Q 3 , in which the ratio of the sides of the channel forming the transistor Q 3  is represented as W 3 /L 3 , wherein the configuration space A 0  is formed having a width of W 3 , and the gate electrode GL is formed having a width of L 3 .  
         [0048]     In a case where the letter A is a ratio of the sides of the channel forming the transistor Q 1  (W 1 /L 1 ) and where the letter B is a ratio of the sides of the channel forming the transistor Q 3 , respective characteristics of the reference voltage Vref are, in general, determined according to these ratios.  
         [0049]      FIG. 5  is a drawing showing the characteristic in the change of temperature coefficient with respect to a ratio (A/B) of the transistor Q 1  and the transistor Q 3 .  
         [0050]     As shown in  FIG. 5 , the temperature coefficient has a characteristic of becoming larger as the ratio (A/B) increases. Furthermore, at a predetermined ratio (A 0 /B 0 ), the temperature coefficient becomes  1  and the reference voltage Vref becomes unaffected by temperature.  
         [0051]     Therefore, by setting the ratio (A/B) of the transistor Q 1  and the transistor Q 3 , the temperature characteristic of the reference voltage Vref can be set to a desired characteristic. Accordingly, by setting the ratio (A/B) of the transistor Q 1  and the transistor Q 3  to a ratio enabling the temperature characteristic of the reference voltage to offset the temperature characteristic of the sense resistance Rs, the output of the comparator  42  can be unaffected by temperature.  
         [0052]     In a case where the detection voltage Vs of the sense resistance Rs shows a tendency of increasing in correspondence with temperature, the output of the comparator  42  can be prevented from being affected by temperature by setting the reference voltage Vref so that it too may increase in correspondence with temperature. This allows the output of the comparator to be prevented from being reversed by temperature.  
         [0053]     It is to be noted that, although this embodiment is applied to a case of overcurrent, the embodiment may also be applied to a case of detecting reduction of current, that is, detection of overcharging.  
         [0054]     Furthermore, although this embodiment sets the temperature characteristic in accordance with the shapes of the channels of the transistors Q 1  (Q 2 ) and Q 3 , the temperature difference may, for example, be set in accordance with the widths, lengths and/or thickness of the resistances R 11  and R 12 .  
         [0055]     In this embodiment, the shapes of the channels of the transistor Q 1  (Q 2 ) and transistor Q 3  in the reference voltage generation circuit  41  are set only to offset the temperature characteristic of the sense resistance Rs. However, since the temperature characteristic can be set to a desired temperature characteristic by setting the shapes of the channels of the transistors Q 1 , Q 2  and Q 3 , in addition to offsetting the temperature characteristic of the sense resistance Rs, the temperature characteristic may alternatively be controlled to have a desired temperature characteristic when necessary.  
         [0056]     Furthermore, in addition to controlling the temperature characteristic of the reference voltage to offset the temperature characteristic of the sense resistance Rs, the reference voltage Vref may be controlled so that it makes a steep drop at a predetermined temperature.  
         [0057]      FIG. 6  is a circuit diagram showing a configuration of an exemplary variation of the reference voltage generation circuit  41 . In  FIG. 6 , like components are denoted with like numerals as of  FIG. 3  and will not be further described.  
         [0058]     In this variation, a reference voltage generation circuit  61  is added with a reference voltage source  71 , a comparator  72 , and a switching element  73 . The reference voltage source  71  generates a reference voltage Vref 10  having no temperature characteristic and supplies the reference voltage Vref 10  to an inverting input terminal of the comparator  72 . A reference voltage Vref is supplied from the voltage divider circuit  51  to a noninverting input terminal of the comparator  72 . The comparator  72  compares the reference voltage Vref and the reference voltage Vref 10 . The comparator  72  outputs a high level output when the reference voltage Vref is greater than the reference voltage Vref 10  and outputs a low level output when the reference voltage Vref is less than the reference voltage Vref 10 .  
         [0059]     The output of the comparator  72  is supplied to the switching element  73 . The switching element  73 , being supplied with the reference voltage Vref and a base voltage Vref 0  from the voltage divider circuit  51 , supplies the reference voltage Vref to the inverting input terminal of the comparator  72  when the output of the comparator  72  is a low level output and supplies the base voltage Vref 0  when the output of the comparator  72  is a high level output.  
         [0060]      FIG. 7  is a diagram showing the change of the reference voltage Vref with respect to the temperature in the reference voltage generating circuit  61 .  
         [0061]     As shown in  FIG. 7 , when the reference voltage Vref reaches a predetermined temperature T 0  and thus when the reference voltage Vref reaches a constant voltage Vref 10 , the switching element  73  switches to supply the base voltage V 0  to the inverting input terminal of the comparator  42 .  
         [0062]     In this variation, when the reference voltage Vref reaches a predetermined voltage, that is, when the reference voltage Vref reaches a predetermined temperature, the base voltage V 0  is applied as the voltage to the inverting input terminal of the comparator  72 . Accordingly, the high level output can be surely output from the comparator  22 , the switching transistor M 1  can be switched off, and the current flowing in the accumulator battery  11  can be disconnected. Thereby the accumulator battery  11  can be protected from, for example, overheating.  
         [0063]     It is to be noted that although this variation applies the base voltage V 0  as the reference voltage to the inverting input terminal of the comparator  72  when the reference voltage Vref becomes a predetermined voltage, that is, when the temperature reaches the predetermined temperature T 0 , alternatively, the reference voltage Vref may be maintained at a predetermined voltage by employing, for example, a capacitor, and the predetermined voltage may be applied to the inverting input terminal of the comparator  22 .  
         [0064]     Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention.  
         [0065]     The present application is based on Japanese Priority Application No. 2004-052182 filed on Feb. 26, 2004, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.