Abstract:
A charge/discharge protection circuit prevents a charge control FET from being overheated or ignited so as to improve safety by preventing an oscillating operation of the charge control FET, which is connected to a charge path of a secondary battery in series so as to cut off a charge current. A discharge control FET is connected to the charge path in series so as to cut off a discharge current from the secondary battery. A latch circuit latches an overcharge detection signal output from an overcharge detection circuit and outputs a signal so as to control the charge control FET. A delay circuit delays the discharge over-current detection signal and supplies the delayed discharge over-current detection signal to the discharge control FET. A reset circuit resets the latch circuit so as to turn on the charge control FET when a discharge over-current is detected and an overcharge is not detected.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to a charge/discharge protection circuit for a secondary battery and, more particularly, to a charge/discharge protection circuit protecting a charge/discharge control field effect transistor (FET) from overheating due to repetition of on and off. The present invention also relates to a battery pack incorporating such a charge/discharge control field effect transistor, and an electronic device using such a battery pack.  
           [0003]    2. Description of Related Art  
           [0004]    Many portable electronic devices use a lithium-ion secondary battery. The lithium-ion secondary battery has a problem in that an accident may occur when being overcharged. Moreover, the lithium-ion secondary battery has a problem in that a number of charge/discharge cycles within a service life is reduced if the lithium-ion secondary battery is over-discharged.  
           [0005]    In order to solve such a problem, a protection switch is provided to a charge/discharge path between a lithium secondary battery and a main part of a device. The protection switch is turned off so as to prevent the lithium-ion secondary battery being further overcharged or over-discharged when a state (overcharged state) where the lithium-ion secondary battery is charged to have a voltage more than a predetermined voltage or a state (over-discharged state) where the lithium-ion secondary battery is discharged to have a voltage below a predetermined voltage is detected.  
           [0006]    The charge/discharge protection circuit, which protects a secondary battery by detecting an overcharged state, an over-discharged state or an over-current so as to cut off a charge/discharge path to the secondary battery, is disclosed in, for example, Japanese Laid Open Patent Applications No. 11-103528, No. 10-285810 and No. 9-182283 and Japanese Patent Publication No. 2872365.  
           [0007]    In the above-mentioned conventional charge/discharge protection circuit, an overcharge detection circuit has a hysteresis. That is, if an overcharge of a secondary battery is detected, a charge control field effect transistor (FET) is turned off so as to establish a charge unable state, and, thereafter, the charge control FET is turned on so as to establish a charge enable state when a battery voltage returns to a charge return voltage which is lower than an overcharge detection voltage.  
           [0008]    However, in a semiconductor device of the conventional protection circuit, if a deteriorated battery having a high internal-impedance caused by repeated charge and discharge is charged by a charger having a normal current value but a large voltage value, or if a battery having a normal internal-impedance is charged by a charger having a large voltage value and a large current value, the battery voltage drops when a charge current path is cut off (the charge control FET is turned off) due to an overcharge state being detected by an overcharge detection circuit. Under such circumstances, the battery-voltage drop may exceed the hysteresis of the over-discharge detection circuit, and the battery voltage may reach the level of the over-discharge return voltage.  
           [0009]    In such a case, a charge operation is started again by turning the charge control FET on, and, thereafter, the same operation is repeated. That is, an oscillating operation occurs in that detection of overcharge→cut off of a charge current (turn off the charge control FET)→drop of the battery voltage→start of charge (turn on the charge control FET)→detection of overcharge are repeated. In such a case, there is a risk in that the charge control FET generates heat, which causes an ignition in the worst case.  
         SUMMARY OF THE INVENTION  
         [0010]    It is a general object of the present invention to provide an improved charge/discharge protection circuit in which the above-mentioned problems are eliminated.  
           [0011]    A more specific object of the present invention is to provide a charge/discharge protection circuit which prevents a switching FET from being overheated or ignited so as to improve safety by preventing an oscillating operation of the switching FET due to repeated detection of overcharge and a returning operation in a case where a battery having a high internal impedance is charged by a charger having a normal current value but a large voltage value and also a case where a battery having a normal internal impedance by a charger having both a large voltage value and a large current value.  
           [0012]    Another object of the present invention is to provide a battery pack having the above-mentioned charge/discharge protection circuit and an electronic device, such as a cellular phone, which uses such a battery pack.  
           [0013]    In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a charge/discharge protection circuit comprising: an overcharge detection circuit which detects an overcharge of a secondary battery connected to the charge/discharge protection circuit and outputs an overcharge detection signal; a discharge over-current detection circuit which detects a discharge over-current of the secondary battery and outputs a discharge over-current detection signal; a charge control field effect transistor (FET) connected to a charge path of the secondary battery in series so as to cut off a charge current supplied to the secondary battery; a discharge control field effect transistor (FET) connected to the charge path of the secondary battery in series so as to cut off a discharge current supplied from the secondary battery; a latch circuit which latches the overcharge detection signal output from the overcharge detection circuit and outputs a signal so as to control the charge control FET; a first delay circuit which delays the discharge over-current detection signal output from the discharge over-current detection circuit and supplies the delayed discharge over-current detection signal to the discharge control FET; and a reset circuit which resets the latch circuit so as to turn on the charge control FET when the discharge over-current is detected by the discharge over-current detection circuit and when the overcharge is not detected by the overcharge detection circuit.  
           [0014]    In the above-mentioned invention, the reset circuit may cause the latch circuit to continuously output the signal to turn on the charge control FET when the over-current detected by the discharge over-current detection circuit disappears during a delay time of the delay circuit. Additionally, the reset circuit may includes: a second delay circuit which delays the charge over-current detection signal for a predetermined time; and a logic circuit which logically operates the overcharge detection signal after being delayed by the second delay circuit and the discharge over-current detection signal output by the discharge over-current detection circuit. Further, the logic circuit may includes: an inverter inverting the overcharge detection signal after being delayed by the second delay circuit; and an AND circuit performing an AND operation on the inverted overcharge detection signal and the discharge over-current detection signal output by the discharge over-current detection circuit.  
           [0015]    Additionally, the charge/discharge protection circuit according to the present invention may further comprise a logic circuit between the first delay circuit and the discharge control FET so as to maintain the discharge control FET being turned on when the latch circuit is reset during a delay time of the first delay circuit. The logic circuit may include: a first inverter inverting the discharge over-current detection signal output from the discharge over-current detection circuit; a second inverter inverting the discharge over-current detection signal inverting the discharge over-current detection signal after being delayed by the first delay circuit; and an OR circuit which performs a summing operation on outputs of the first and second inverters and supplies a result of the summing operation to discharge control FET.  
           [0016]    Additionally, according to other aspects of the present invention, there are provided a battery pack using the charge/discharge protection circuit and an electronic device having such a battery pack.  
           [0017]    Other objects, features and advantages of the present invention will become more apparent from the following detailed descriptions when read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram of a battery protection circuit including a charge/discharge protection circuit; and  
         [0019]    [0019]FIG. 2 is a block diagram of a part of a charge/discharge protection circuit according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    First, a description will be given of the charge/discharge protection circuit proposed in Japanese Patent Application No. 2000-296457 (prior application) by the present inventor. In the charge/discharge protection circuit in the prior application, a charge over-current is taken into consideration.  
         [0021]    [0021]FIG. 1 is a block diagram of a battery protection circuit including the charge/discharge protection circuit disclosed in the prior application. The battery protection circuit is incorporated into a battery pack A to which a charger B is connected to charge a battery cell provided in the battery pack A.  
         [0022]    In the prior application, the charge/discharge protection circuit  1 , which constitutes a principal part of a battery pack A, comprises an overcharge detection circuit  11 , an over-discharge detection circuit  12 , a discharge over-current detection circuit  13 , a short-circuit detection circuit  14 , an abnormal charger detection circuit  15 , an oscillation circuit  16 , a counter circuit  17 , logic circuits  18  and  20 , a level shift circuit  19  and a charge over-current detection circuit  21 .  
         [0023]    If an overcharge, over-discharge, discharge over-current, charge over-current or short-circuit is detected by the respective overcharge detection circuit  11 , over-discharge detection circuit  12 , discharge over-current detection circuit  13 , charge over-current detection circuit  21  or short-circuit detection circuit  14 , the oscillation circuit  16  starts an operation and the counter circuit  17  starts a counting operation. When a delay time, which is previously set up at the time of each detection, is counted by the counter circuit  17  and if an overcharge or a charge over-current is detected, an output Cout is set to a low level through the logic circuits (latch etc.)  18  and the level shift circuit  19 , and a charge control field effect transistor (FET) Q 1  is turned off. On the other hand, when an over-discharge or a discharge over-current or hsort circuit is detected, an output Dout is set to a low level. As a result, a discharge control field effect transistor (FET) Q 2  is turned off.  
         [0024]    When the charger B is connected to the battery pack A and a charge current flows in the battery pack A, a source voltage of the charge control FET Q 1  becomes lower than a source voltage of the discharge control FET Q 2 . The source voltage of the discharge control FET Q 2  is equal to a Vss terminal voltage of the semiconductor device. Although a resistor is connected to a V-terminal of the semiconductor device, the source voltage of the charge control FET Q 1  becomes almost equal to the V-terminal voltage of the semiconductor device since the V-terminal has a high impedance. Therefore, when a charge current flows, the V-terminal voltage becomes lower than the Vss terminal voltage. When the V-terminal voltage becomes lower than the Vss terminal voltage by a predetermined voltage (charge over-current detection voltage), a charge over-current is detected, and, thereby the output Cout is set to a low level and the charge control FET Q 1  is turned off.  
         [0025]    A description will now be given, with reference to FIG. 2, of a charge/discharge protection circuit according to an embodiment of the present invention. FIG. 2 is a block diagram of a part of the charge/discharge protection circuit according to the present invention. The charge/discharge protection circuit is provided in the form of a semiconductor device.  
         [0026]    The overcharge detection circuit  101 , the delay circuit  102 , the latch circuit  103 , the charge control FET  104 , the discharge over-current detection circuit  105 , the delay circuit  108  and the discharge control FET  109  shown in FIG. 2 correspond to the over-discharge detection circuit  11 , the oscillation circuit  16  and the counter circuit  17 , the logic circuit  18 , the charge control FET Q 1 , the discharge over-current detection circuit  13 , the oscillation circuit  16 , the counter circuit  17  and the discharge control FET Q 2  shown in FIG. 1, respectively. In FIG. 2, other parts shown in FIG. 1 are omitted for the sake of simplification. In addition, unlike the over-discharge detection circuit  11  shown in FIG. 11, the over-discharge detection circuit  101  according to the present embodiment is constituted by a comparator having no hysteresis. An AND circuit  107  and an inverter  110  shown in FIG. 2 are newly added parts.  
         [0027]    In the circuit structure shown in FIG. 2, if the overcharge detection circuit  101  detects an overcharge, the output of the overcharge detection circuit  101  is supplied to the latch circuit  103 , after being delayed for a predetermined time by the delay circuit  102 . The output of the latch circuit  103  is supplied through the inverter  112  to a gate of the charge control FET  104  as a low-level signal “L”. Thereby, the charge control FET  104  is turned off, and a charge current is interrupted (an overcharged state).  
         [0028]    A description will now be given of a structure which does not return from the overcharged state to a normal state when the charger is connected to the battery pack.  
         [0029]    In order to return from an overcharged state, the latch circuit  103  must be reset. In order to reset the latch circuit  103 , the output of the overcharge detection circuit  101  must be the low level “L” and a high-level signal “H” must be supplied to one of input terminals of the AND circuit  107  through the delay circuit  102  and the inverter  110 . Moreover, at the same time, the output of the electric discharge over-current detection circuit  105  must be the high level “H”, and the high-level signal “H” must be supplied to the other of the terminals of the AND circuit  107 .  
         [0030]    Here, the fact that the output of the overcharge detection circuit  101  is the low level “L” means that the battery voltage is below an overcharge detection voltage (Vref 1 ). Moreover, the fact that the output of the electric discharge over-current detection circuit  105  is high level “H” means that a V-potential is higher than a discharge over-current detection voltage (Vref 2 ). Therefore, the conditions to reset the latch circuit  103  are that the battery voltage is lower than the overcharge detection voltage (Vref 1 ) and the V-potential is higher than the discharge over-current detection voltage (Vref 2 ).  
         [0031]    However, after the overcharge detection circuit  101  detects an overcharge and turns off the charge control FET  104  so as to interrupt a charge current (overcharged state), the V-potential does not exceed the discharge over-current detection voltage (Vref 2 ) in a state where the charger is continuously connected. Accordingly, in the state where the charger is continuously connected, the charge control FET  104  will never be turned on no matter how the battery voltage drops. Therefore, the above-mentioned oscillation of the charge control FET  104  will never occur. Consequently, the problem of heat generation or an ignition of the charge control FET  104  is eliminated.  
         [0032]    It should be noted that the inverter  110  and the AND circuit  107  together forms a logic circuit, which may be referred to as a reset circuit for resetting the latch circuit  103 .  
         [0033]    A description will now be given of a return operation from the overcharged state at the time a load is connected to the battery pack.  
         [0034]    If the charger is disconnected and a load is connected in a state where the battery voltage is less than the overcharge detection voltage (Vref 1 ), a load current flows though a parasitic diode  106 , which exists within the charge control FET  104  although the charge control FET  104  is turned off. The load current generates a normal direction voltage Vf in the diode  106 . Therefore, the V-potential is raised higher than Vss by the normal direction voltage Vf. The normal direction voltage Vf is about 0.5 V to 0.7 V. Since the discharge over-current detection voltage (Vref 2 ) is generally set to about 0.1 V to 0.2 V, the output of the discharge over-current detection circuit  105  is inverted and becomes the high level “H”.  
         [0035]    At this time, both two inputs of the AND circuit  107  become high level “H”, and the output of the AND circuit  107 , i.e., the input to the reset terminal R of the latch circuit  103 , becomes high level “H”. Thereby, the latch circuit  103  changes from a latch state to a reset state, and outputs a low-level signal “L”. The low-level signal “L” is supplied to the gate of the charge control FET  104  after being inverted to a high-level signal “H” by the inverter  112 . Therefore, the charge control FET  104  is turned on, thereby returning from the overcharged state.  
         [0036]    On the other hand, in a case in which only the inverter  111  is provided between the output terminal of the discharge over-current detection circuit  105  and the gate of the discharge control FET  109 , if the output of the discharge over-current detection circuit  105  is changed to the high level “H” when the charger is disconnected and the load is connected, the high-level signal “H” acts to supply a low-level signal “L” to the gate of the discharge control FET  109  via the inverter  111 . When the gate voltage of the discharge control FET  109  is changed to the low level “L”, the discharge control FET  109  is turned off, and the load current is interrupted. Therefore, there is a problem in that it becomes impossible to pass a load current even if a load is connected after detecting an overcharge.  
         [0037]    Then, in the present invention, as shown in the circuit structure of FIG. 2, a delay circuit  108 , an inverter  111  and an inverter  113  are provided between the output terminal of the discharge over-current detection circuit  105  and the gate of the discharge control FET  109  so that the delay circuit  108  and the inverter  111  are series-connected, and the inverter  113  is connected parallel to the series-connected delay circuit  108  and the inverter  111 . Signals from the inverter  111  and the inverter  113  are supplied to an OR (logical sum) circuit  114 , and an output of the OR circuit  114  is supplied to the gate of the discharge control FET  109 .  
         [0038]    Even when the output of the discharge over-current detection circuit  105  changes to the high level “H”, the discharge control FET  109  is not immediately turned off. That is, the discharge control FET  109  is turned off after the delay of a predetermined time (for example, 10 msec). However, if the charge control FET  104  is turned on by resetting the latch circuit  103  from a latch state during the predetermined delay time, the above-mentioned normal direction voltage Vf of the parasitic diode  106  disappears since the delay time by the AND circuit  107 , the latch circuit  103  and the inverter  112 , etc. is less than 1 msec.  
         [0039]    If the normal direction voltage Vf of the parasitic diode  106  does not exist, the V-potential becomes lower than the discharge over-current detection voltage (Vref 2 ), and the output of the discharge over-current detection circuit  105  is changed to the low level “L”. Thereby, a high-level signal “H” is supplied to the gate of the discharge control FET  109  through the inverter  113 . Consequently, the gate of the electric discharge does not change to the low level “L” while maintaining the high level “H”.  
         [0040]    Therefore, it becomes possible to pass a load current, and the above-mentioned problem is eliminated. It should be noted that since the output of the discharge over-current detection circuit  105  is supplied to the OR circuit  114  through the inverter  113 , if the output of the electric discharge over-current detection circuit  105  becomes a low level “L”, the discharge control FET  109  is not turned off so as to maintain the on state after the predetermined delay time (for example, 10 msec) according to the delay circuit  108  has elapsed. Thus, it becomes possible to continuously supply a load current when a load is connected after detecting an overcharge.  
         [0041]    It should be noted that the circuit structure shown in FIG. 2, which includes the delay circuit  108 , the inverter  111 , the inverter  113  and the OR circuit  114 , is merely an example of the present invention. Namely, any circuit structure may be used if the circuit structure turns off the discharge control FET  109  with a predetermined time delay when the discharge over-current detection circuit  105  detects an over-current, while maintaining the discharge control FET being turned on without being turned off when the discharge over-current detection circuit  105  does not detect an over-current during the delay time.  
         [0042]    In the above-mentioned structure, the inverters  111  and  113  and the OR circuit  114  together form a logic circuit to maintain the discharge control FET  109  being turned on when the latch circuit  103  is reset during the delay time of the delay circuit  108 .  
         [0043]    It should be noted that the structure according to the present composition invention is suitable for charge/discharge protection circuits, such as a lithium-ion secondary battery. Additionally, the charge/discharge protection circuit according to the present embodiment is also applicable to a battery pack incorporating a charge/discharge circuit or an electronic device using a battery pack, such as, for example, a cellular phone, a digital camera, a portable audio equipment, etc.  
         [0044]    The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.  
         [0045]    The present application is based on Japanese priority application No. 2000-394035 filed on Dec. 26, 2000, the entire contents of which are hereby incorporated by reference.