Abstract:
An output circuit has a smaller area and restrains outputs from becoming unstable even if a power supply voltage is lower than an operating voltage. A supply terminal of an inverter circuit is provided with switch circuit, and the switch circuit stops the operation of the inverter circuit when the power supply voltage is lower than the operating voltage of the circuit. Further, the output terminal of the inverter circuit is provided with a current source to fix the output to the power supply voltage when the operation of the inverter circuit is stopped.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2011-013326 filed on Jan. 25, 2011, 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 an output circuit, the output of one end of which has high impedance and, more particularly, to an output circuit which stably operates when a power supply voltage is low. The present invention further relates to a temperature switch IC and a battery pack, each of which is provided with the aforementioned output circuit. 
         [0004]    2. Description of the Related Art 
         [0005]    The following will describe a conventional output circuit.  FIG. 7  is a circuit diagram illustrating a conventional output circuit. 
         [0006]    The conventional output circuit includes an inverter  97  connected to an input terminal, an NMOS transistor  93 , which is an output driver, a diode-connected NMOS transistor  95  and a capacitor  96 , which are provided between a power source and the ground, and an NMOS transistor  94  controlled thereby. 
         [0007]    When the circuit is turned on, a power supply voltage VDD gradually rises. The NMOS transistor  95  remains nonconductive while the power supply voltage VDD is lower than a threshold voltage Vthn  95 . The NMOS transistor  94  turns off, because the gate voltage thereof becomes an earth voltage VSS due to the capacitor  96 . Hence, the output terminal of the output circuit is in a high-impedance state. This ensures that the output terminal of the output circuit is always set in the high-impedance state if the power supply voltage VDD at the time of, for example, turning the circuit on, is lower than a minimum operating voltage of the circuit. 
         [0008]    When the power supply voltage VDD exceeds the threshold voltage Vthn  95  of the NMOS transistor  95 , the NMOS transistor  95  becomes conductive. The capacitor  96  is charged by the current supplied by the NMOS transistor  95 . When the gate voltage thereof gradually rises and exceeds a threshold voltage, the NMOS transistor  94  turns on. When the NMOS transistor  94  turns on, the function of the NMOS transistor  93  is rendered valid, transmitting an output of the inverter  97  to the output terminal. If the voltage of an input terminal of the output circuit is at a low level, then the NMOS transistor  93  turns on, and an output voltage VOUT of the output terminal becomes the earth voltage VSS. If the voltage at the input terminal of the output circuit is at a high level, then the NMOS transistor  93  turns off, causing the output voltage VOUT of the output terminal to be set in the high-impedance state (refer to, for example, patent document 1).
   [Patent Document 1] Japanese Patent Application Laid-Open No. 06-075668   
 
         [0010]    In the conventional output circuit, the NMOS transistor  94  is provided in series with the NMOS transistor  93 . The NMOS transistor  93 , which is an output driver, is required to provide a drive capability. For this reason, a large NMOS transistor is used for the transistor  93 . Thus, the NMOS transistor  94  is required to provide a drive capacity that is equivalent to or higher than that of the NMOS transistor  93 . 
         [0011]    The conventional output circuit has been posing a problem that the large size of the NMOS transistor  94  inconveniently leads to a large area of the output circuit. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention has been made in view of the problem described above, and an object of the invention is to provide an output circuit with a smaller area. 
         [0013]    To this end, the present invention provides an output circuit with open-drain output including: an inverter circuit connected to an input terminal of the output circuit; an output MOS transistor having a gate thereof connected to an output terminal of the inverter circuit; a drain thereof connected to an output terminal of the output circuit, and a source thereof connected to a first supply terminal; a switch circuit provided between the inverter circuit and a second supply terminal; and a current source provided between the gate of the output MOS transistor and the first supply terminal, wherein the switch circuit turns off when a power supply voltage is lower than a minimum operating voltage of the output circuit. 
         [0014]    The output circuit according to the present invention is configured such that, if a power supply voltage is lower than an operating voltage of the circuit, then the operation of the inverter is interrupted and the gate of an output driver is controlled to turn it off. Hence, a large MOS transistor is no longer required to be provided between the source of the output driver and a power source. This arrangement enables the output circuit to restrain unstable outputs even when the power supply voltage is lower than the operating voltage and to achieve a reduced area. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a circuit diagram illustrating an example of the output circuit according to an embodiment of the present invention; 
           [0016]      FIG. 2  is a circuit diagram illustrating another example of the output circuit according to the embodiment; 
           [0017]      FIG. 3  is a circuit diagram illustrating still another example of the output circuit according to the embodiment; 
           [0018]      FIG. 4  is a block diagram illustrating a battery pack; 
           [0019]      FIG. 5  is a block diagram illustrating a battery protection IC; 
           [0020]      FIG. 6  is a block diagram illustrating a temperature switch IC; and 
           [0021]      FIG. 7  is a circuit diagram illustrating a conventional output circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Embodiments of the present invention will be described below with reference to the accompanying drawings. 
         [0023]      FIG. 1  is a circuit diagram illustrating an output circuit according to an embodiment of the present invention. 
         [0024]    An output circuit  10  has PMOS transistors  11  and  12 , NMOS transistors  21  and  22 , and a current source  31 . 
         [0025]    The PMOS transistor  11  has the gate thereof connected to the input terminal of the output circuit  10 , the source thereof connected to the drain of the PMOS transistor  12 , and the drain thereof connected to the gate of the NMOS transistor  22 . The NMOS transistor  21  has the gate thereof connected to the input terminal of the output circuit, the source thereof connected to an earth terminal (a power supply terminal at an earth voltage side), and the drain thereof connected to the gate of the NMOS transistor  22 . The PMOS transistor  12  has the gate thereof connected to the earth terminal and the source thereof connected to a supply terminal (the power supply terminal at a power supply voltage side). The PMOS transistor  12  is provided on a power supply line of an inverter  36  composed of the PMOS transistor  11  and the NMOS transistor  21 . 
         [0026]    The current source  31  is provided between the drain of the PMOS transistor  11  and the earth terminal. The NMOS transistor  22  has the source thereof connected to the earth terminal and the drain thereof connected to the output terminal of the output circuit  10 . The NMOS transistor  22  is an open-drain output driver. 
         [0027]    The absolute value |Vthp 12 | of the threshold voltage of the PMOS transistor  12  is higher than the absolute value |Vthp 11 | of the threshold voltage of the PMOS transistor  11  and indicates the minimum operating power supply voltage of the output circuit  10 . If a power supply voltage VDD is lower than the minimum operating power supply voltage, then the PMOS transistor  12  turns off so as not to supply the power supply voltage VDD to the inverter  36 . Further, the current source  31  turns the NMOS transistor  22  off. 
         [0028]    The operation of the output circuit  10  will now be described. 
         [0029]    When the power is turned on, the power supply voltage rises. At this time, if the power supply voltage VDD is lower than the absolute value |Vthp 12 | of the threshold voltage of the PMOS transistor  12 , then the PMOS transistor  12  turns off. This prevents the power supply voltage VDD from being supplied to the inverter  36 . Therefore, the output terminal of the inverter  36  is pulled down by the current source  31 , so that the output voltage of the inverter  36  is an earth voltage VSS. The NMOS transistor  22 , which is the output driver, turns off because the gate voltage thereof becomes the earth voltage VSS, thus setting the output terminal of the output circuit  10  in the high impedance state. Hence, the output terminal of the output circuit  10  is pulled up to the power supply voltage of a circuit in a subsequent stage, the aforementioned output terminal being connected to the input terminal of the circuit in the subsequent stage. This arrangement restrains the circuit in the subsequent stage from malfunctioning. 
         [0030]    If the power supply voltage VDD becomes higher than the absolute value |Vthp 12 | of the threshold voltage of the PMOS transistor  12 , then the PMOS transistor  12  turns on. This causes the power supply voltage VDD to be supplied to the inverter  36 . 
         [0031]    If the voltage at the input terminal of the output circuit  10  is a low level, then the gate voltage of the NMOS transistor  22  becomes a high level due to the inverter  36 , causing the NMOS transistor  22  to turn on and the output voltage VOUT to become the earth voltage VSS. The current source  31  is designed to have a drive capability that is lower than the drive capability of the PMOS transistor  11 . 
         [0032]    If the voltage at the input terminal of the output circuit  10  becomes a high level, then the gate voltage of the NMOS transistor  22  becomes a low level due to the inverter  36 . This causes the NMOS transistor  22  to turn off, placing the output terminal of the output circuit  10  in the high-impedance state. 
         [0033]    The output circuit according to the present embodiment is configured such that, when the power supply voltage is lower than the operating voltage of the circuit, the operation of the inverter is interrupted and the gate of the output driver is turned off by the current source, thus obviating the need for providing a large MOS transistor between an output driver and a power source. This permits a reduced area of the output circuit  10 . 
         [0034]    Further, if the power supply voltage VDD at the time of, for example, turning the power on, is lower than the minimum operating power supply voltage of the output circuit  10 , then the output voltage VOUT always causes high impedance, thus restraining a circuit in a subsequent stage from malfunctioning. 
         [0035]      FIG. 2  is a circuit diagram illustrating another example of the output circuit according to the present embodiment. The output circuit  10  in  FIG. 2  further includes a current source  32  and a PMOS transistor  13 . 
         [0036]    The PMOS transistor  13  and the current source  32  are connected in series between a supply terminal and an earth terminal. The PMOS transistor  13  has the gate and the drain thereof connected to the earth terminal. The connection point of the current source  32  and the source of the PMOS transistor  13  is connected to the gate of the PMOS transistor  12 . 
         [0037]    According to the configuration described above, the minimum operating power supply voltage of the output circuit  10  is set by the current source  32  and the two PMOS transistors  12  and  13 . More specifically, when a power supply voltage VDD becomes higher than the total voltage of the absolute values of the threshold voltages of the two PMOS transistors  12  and  13 , the PMOS transistor  12  is turned on and the power supply voltage VDD is supplied to an inverter  36 . 
         [0038]    At the output end in  FIG. 2 , the diode-connected PMOS transistor  13  is provided between the gate of the PMOS transistor  12  and the earth terminal. Alternatively, however, a diode-connected NMOS transistor may be provided instead of the PMOS transistor  13 . 
         [0039]      FIG. 3  is a circuit diagram illustrating another example of the output circuit according to the present embodiment. As illustrated in  FIG. 3 , the gate of the NMOS transistor  22  may be connected to the drain of the PMOS transistor  11  through the intermediary of a resistor  33 . 
         [0040]    In the aforementioned configuration, a low-pass filter is constituted by the resistor  33  and the capacitor between the gate and the source of the NMOS transistor  22 , thus reducing the malfunctions of the NMOS transistor  22  caused by a surge. The gate of the NMOS transistor  22  may be connected to the connection point of the resistor  33  and the drain of the NMOS transistor  21 . 
         [0041]    In the output circuit shown in  FIG. 1 , the NMOS transistor for controlling the supply of the power supply voltage VDD to the inverter  36  may be provided between the inverter  36  and the earth terminal. Further, in  FIG. 1 , the open-drain NMOS transistor  22  is used, and the output voltage VOUT becomes the high impedance state in the case where the power supply voltage VDD is lower than the minimum operating power supply voltage of the output circuit  10 . However, although not shown, an open-drain PMOS transistor may alternatively be used. At this time, the gate of the NMOS transistor for controlling the supply of the power supply voltage VDD to the inverter  36  is connected to a supply terminal, the source thereof is connected to an earth terminal, and the drain thereof is connected to the source of the NMOS transistor  21 . The gate of the open-drain PMOS transistor is connected to the output terminal of the inverter  36 , the source thereof is connected to the supply terminal, and the drain thereof is connected to the output terminal of the output circuit  10 . The current source  31  is provided between the supply terminal and the output terminal of the inverter  36 . 
         [0042]    When the power supply voltage VDD becomes higher than the threshold voltage Vthn of the NMOS transistor, the NMOS transistor turns on and the power supply voltage VDD is supplied to the inverter  36 . 
         [0043]    If the power supply voltage VDD is lower than the absolute value |Vthp 12 | of the threshold voltage of the PMOS transistor  12 , then the PMOS transistor  12  turns off. Thus, the power supply voltage VDD is not supplied to the inverter  36 . Hence, the output terminal of the inverter  36  is pulled up by the current source  31 , so that the output voltage of the inverter  36  becomes the power supply voltage VDD. The PMOS transistor turns off, and the output voltage VOUT becomes the high impedance state. 
         [0044]    An application example of the output circuit  10  will now be described. First, the construction of a temperature switch IC provided with the output circuit  10  and the construction of a battery pack provided with a battery protection IC will be described. The temperature switch IC detects an abnormal temperature. The battery protection IC protects a battery from overcharge/overdischarge.  FIG. 4  is a block diagram illustrating the battery pack.  FIG. 5  is a block diagram illustrating the battery protection IC.  FIG. 6  is a block diagram illustrating the temperature switch IC. 
         [0045]    As illustrated in  FIG. 4 , a battery pack  50  includes a battery protection IC  51 , a temperature switch IC  52 , p-type FETs  53  to  55 , a resistor  57 , and a battery  58 . Further, the battery pack  50  has an external terminal EB+ and an external terminal EB−. 
         [0046]    As illustrated in  FIG. 5 , the battery protection IC  51  includes reference voltage generating circuits  61  and  62 , an overcharge detection comparator  64 , and an overdischarge detection comparator  63 . Further, the battery protection IC  51  has a supply terminal, an earth terminal, a charge control terminal CO, and a discharge control terminal DO. 
         [0047]    As illustrated in  FIG. 6 , the temperature switch IC  52  has a temperature voltage generating circuit  75 , reference voltage generating circuits  71  and  72 , a high temperature detection comparator  73 , a low temperature detection comparator  74 , a NOR circuit  76 , and an output circuit  10 . The temperature voltage generating circuit  75  is constituted of a PNP bipolar transistor and the like, although not shown. Further, the temperature switch IC  52  has a supply terminal, an earth terminal, and an output terminal DET. 
         [0048]    The supply terminal of the battery protection IC  51  is connected to the positive terminal of the battery  58 , the earth terminal thereof is connected to the negative terminal of the battery  58 , the discharge control terminal DO is connected to the gate of the p-type FET  53 , and the charge control terminal CO is connected to the gate of the p-type FET  54  and the drain of the p-type FET  55 . The supply terminal of the temperature switch IC  52  is connected to the positive terminal of the battery  58 , the earth terminal thereof is connected to the negative terminal of the battery  58 , and the output terminal DET thereof is connected to the gate of the p-type FET  55 . 
         [0049]    The resistor  57  is provided between the external terminal EB+ and the connection point of the output terminal DET and the gate of the p-type FET  55 . The source and the back gate of the p-type FET  53  are connected to the positive terminal of the battery  58 , and the drain thereof is connected to the drain of the p-type FET  54 . The source and the back gate of the p-type FET  54  are connected to the external terminal EB+. The source and the back gate of the p-type FET  55  are connected to the external terminal EB+. The external terminal EB− is connected to the negative terminal of the battery  58 . In other words, the p-type FETs  53  and  54  are provided in series in the charge/discharge path of the battery  58 . 
         [0050]    The reference voltage generating circuits  61  and  62 , the overcharge detection comparator  64 , and the overdischarge detection comparator  63  are provided between the supply terminal and the earth terminal. The inverting input terminal of the overcharge detection comparator  64  is connected to the output terminal of the reference voltage generating circuit  62 , the non-inverting input terminal thereof is connected to the supply terminal, and the output terminal thereof is connected to the charge control terminal CO. The inverting input terminal of the overdischarge detection comparator  63  is connected to the supply terminal, the non-inverting input terminal thereof is connected to the output terminal of the reference voltage generating circuit  61 , and the output terminal thereof is connected to the discharge control terminal DO. 
         [0051]    The reference voltage generating circuits  71  and  72 , the high temperature detection comparator  73 , the low temperature detection comparator  74 , the temperature voltage generating circuit  75 , the NOR circuit  76 , and the output circuit  10  are connected between the supply terminal and the earth terminal. The non-inverting input terminal of the high temperature detection comparator  73  is connected to the output terminal of the reference voltage generating circuit  71 , while the inverting input terminal thereof is connected to the output terminal of the temperature voltage generating circuit  75 . The non-inverting input terminal of the low temperature detection comparator  74  is connected to the output terminal of the temperature voltage generating circuit  75 , while the inverting input terminal thereof is connected to the output terminal of the reference voltage generating circuit  72 . The first input terminal of the NOR circuit  76  is connected to the output terminal of the high temperature detection comparator  73 , the second input terminal thereof is connected to the output terminal of the low temperature detection comparator  74 , and the output terminal thereof is connected to the input terminal of the output circuit  10 . The output terminal of the output circuit  10  is connected to the output terminal DET. 
         [0052]    Upon detection of an abnormal temperature, the temperature switch IC  52  emits an output current. The resistor  57  generates a voltage on the basis of the output current. The voltage generated in the resistor  57  turns the p-type FET  55  on. This causes the p-type FET  54  for charge control to turn off, thus controlling charge. If the battery  58  is overcharged, then the battery protection IC  51  operates to turn the p-type FET  54  off. If the battery  58  is overdischarged, then the battery protection IC  51  operates to turn the p-type FET  53  for discharge control off. 
         [0053]    The operation of the battery pack  50  will now be described. 
         [0000]    [Operation Performed when the Battery  58  is Overcharged] 
         [0054]    A charger (not shown) is connected to the battery pack  50 . The reference voltage generating circuit  62  generates a reference voltage VREF 2  based on an overcharge voltage indicating that the battery  58  is in an overcharged state. The overcharge detection comparator  64  compares a divided voltage of the voltage of the battery  58  with the reference voltage VREF 2 , and reverses an output voltage according to the comparison result. More specifically, if the divided voltage of the voltage of the battery  58  exceeds the reference voltage VREF 2 , then the output voltage of the overcharge detection comparator  64  is reversed to a high level. This turns the p-type FET  54  off, stopping the charging of the battery  58 . 
         [0000]    [Operation Performed when the Battery  58  is Overdischarged] 
         [0055]    A load (not shown) is connected to the battery pack  50 . The reference voltage generating circuit  61  generates a reference voltage VREF  1  based on an overdischarge voltage indicating that the battery  58  is in an overdischarged state. The overdischarge detection comparator  63  compares a divided voltage of the voltage of the battery  58  with the reference voltage VREF 1 , and reverses an output voltage according to the comparison result. More specifically, if the divided voltage of the voltage of the battery  58  drops to the reference voltage VREF 1  or lower, then the output voltage of the overdischarge detection comparator  63  is reversed to a high level. This turns the p-type FET  53  off, stopping the discharging of the battery  58 . 
       [Operation in Case of Abnormally High Temperature] 
       [0056]    The temperature voltage generating circuit  75  generates a temperature voltage VTEMP based on a temperature. The temperature voltage generating circuit  75  is characteristic in that the temperature voltage VTEMP drops as the temperature rises. The reference voltage generating circuit  71  generates a reference voltage VREF 3  based on an abnormally high temperature to be detected. The high temperature detection comparator  73  compares the temperature voltage VTEMP with the reference voltage VREF 3  and reverses the output voltage according to the comparison result. More specifically, as the temperature rises, the temperature voltage VTEMP drops, and the output voltage of the high temperature detection comparator  73  switches to a high level when the temperature voltage VTEMP drops to the reference voltage VREF3 or less. In other words, if the temperature reaches an abnormally high temperature level or more, then the output voltage of the high temperature detection comparator  73  is switched to the high level. As a result, the output voltage of the NOR circuit  76  is set to a low level, the output circuit  10  is turned on to supply current to the resistor  57 , a voltage is generated at the resistor  57 , and the voltage of the output terminal DET is set to the low level. This causes the p-type FET  55  to turn on and the p-type FET  54  to turn off, thus stopping the charging of the battery  58 . 
       [Operation in Case of Abnormally Low Temperature] 
       [0057]    The reference voltage generating circuit  72  generates a reference voltage VREF 4  based on an abnormally low temperature to be detected. The low temperature detection comparator  74  compares the temperature voltage VTEMP with the reference voltage VREF 4  and reverses the output voltage according to the comparison result. More specifically, as the temperature decreases, the temperature voltage VTEMP rises, and the output voltage of the low temperature detection comparator  74  switches to a high level when the temperature voltage VTEMP reaches the reference voltage VREF 4  or more. In other words, if the temperature decreases to an abnormally low temperature level or less, then the output voltage of the low temperature detection comparator  74  is switched to the high level. This causes the charging of the battery  58  to be stopped as described above. 
         [0058]    Thus, the operation of the aforementioned output circuit  10  ensures that the output circuit  10  of the temperature switch IC  52  always turns off if the power supply voltage VDD is lower than the minimum operating power supply voltage of the output circuit  10 . As a result, the voltage at the output terminal of the output circuit  10 , i.e., the voltage at the output terminal DET of the temperature switch IC  52  is invariably pulled up to the voltage at the external terminal EB+ by the resistor  57 . Hence, if the power supply voltage VDD is lower than the minimum operating power supply voltage of the output circuit  10 , the p-type FET  55  invariably turns off, thereby invariably disabling the temperature switch IC  52  to control the p-type FET  54  through the intermediary of the p-type FET  55 . Accordingly, when, for example, the charging of the battery  58  is started from a state wherein the voltage thereof is in the vicinity of zero volt, it is possible to prevent the temperature switch IC  52  from malfunctioning to turn the p-type FET  54  off due to the low voltage (power supply voltage VDD) of the battery  58  and erroneously stopping the charging despite that the voltage of the battery  58  is low. 
         [0059]    As illustrated in  FIG. 6 , the overcharge detection comparator  64  and the overdischarge detection comparator  63  are required as the protective functions for the battery pack  50 . However, in the case where the specifications of the battery pack  50  do not require the overdischarge detection function as the protective function, the overdischarge detection comparator  63  may be deleted, although not shown. In this case, the p-type FET  53  would be also deleted. 
         [0060]    Further, illustrated in  FIG. 6 , the high temperature detection comparator  73  and the low temperature detection comparator  74  are required as the protective functions for the battery pack  50 . However, in the case where the specifications of the battery pack  50  do not require the low temperature detection function or the high temperature detection function as the protective function, the low temperature detection comparator  74  or the high temperature detection comparator  73  may be omitted. 
         [0061]    Further, the resistor  57 , the p-type FET  55  or the like may be incorporated in the temperature switch IC  52 . 
         [0062]    In  FIG. 4 , the p-type FETs  53  and  54  for controlling charge/discharge are provided between the external terminal EB+ and the positive terminal of the battery  58 . Alternatively, however, two n-type FETs may be provided between the external terminal EB− and the negative terminal of the battery  58 , although now shown. In this case, the p-type FET  55 , the resistor  57 , the internal circuit of the battery protection IC  51 , and the internal circuit of the temperature switch IC  52  would be changed, as necessary. 
         [0063]    In  FIG. 4 , the temperature switch IC  52  controls only the p-type FET  54  for charge control. Alternatively, however, the temperature switch IC  52  may control only the p-type FET  53  for discharge control, although not shown, or may control both the p-type FETs  53  and  54 .