Abstract:
Provided is a switch circuit capable of reliably controlling the transmission or interruption of a voltage of from GND to VDD to an internal circuit even when a positive or negative voltage is input to an input terminal. By adding PMOS transistors to NMOS transistors constituting the switch circuit and controlling gates of the PMOS transistors by a voltage of the input terminal, the transmission or interruption of the voltage of from GND to VDD can be reliably controlled.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of PCT/JP2013/051706 filed on Jan. 28, 2013, which claims priority to Japanese Application No. 2012-044156 filed on Feb. 29, 2012. The entire contents of these applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a switch circuit to be provided to a terminal to which a positive or negative voltage is input. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 4  illustrates a conventional switch circuit. The switch circuit controls whether a positive or negative voltage input to an input terminal IN is transmitted or interrupted to an internal circuit  15  in response to a signal of a switch control terminal EN. 
         [0006]    First consider the case where a positive voltage VIN+ input from the input terminal IN is transmitted to a node B serving as an input terminal of the internal circuit  15 . The signal of the switch control terminal EN is set to a VDD voltage in an active state, to thereby turn ON NMOS transistors  11  and  12 . Therefore, the positive voltage VIN+ input from the input terminal IN is transmitted to the node B serving as the input of the internal circuit  15 . In this case, an NMOS transistor  13  is turned OFF, and hence the NMOS transistor  13  does not affect the voltage of the node B. 
         [0007]    Next consider the case where the positive voltage VIN+ input from the input terminal IN is not transmitted to the node B serving as the input of the internal circuit  15 . The signal of the switch control terminal EN is set to a GND voltage in an inactive state. A drain of the NMOS transistor  11  has the voltage VIN+ and a gate thereof has the GND voltage, and hence the NMOS transistor  11  is turned OFF. The NMOS transistor  13  is turned ON so that a node A has the GND voltage. A drain and a gate of the NMOS transistor  12  have the GND voltage, and hence the NMOS transistor  12  is turned OFF. Therefore, the positive voltage VIN+ input from the input terminal IN is not transmitted to the node B serving as the input of the internal circuit  15 . 
         [0008]    Next consider the case where a negative voltage VIN− input from the input terminal IN is not transmitted to the node B serving as the input of the internal circuit  15 . The signal of the switch control terminal EN is set to the GND voltage in the inactive state. However, the drain of the NMOS transistor  11  is applied with the negative voltage VIN− lower than the GND voltage input from the input terminal IN, and hence the NMOS transistor  11  becomes an ON state in the weak inversion region. In this case, the NMOS transistor  13  is turned ON, and hence the node A has the GND voltage rather than the input negative voltage VIN−. The drain and the gate of the NMOS transistor  12  have the GND voltage, and hence the NMOS transistor  12  is turned OFF. Therefore, the negative voltage VIN− input from the input terminal IN is not transmitted to the node B serving as the input of the internal circuit  15 . 
         [0009]    In this way, even when a negative voltage is input from the input terminal IN, the conventional switch circuit can prevent the negative voltage from being transmitted to the input of the internal circuit  15 , thus preventing an erroneous operation of the internal circuit. 
         [0010]    In the conventional switch circuit, however, when the active state of the signal of the switch control terminal EN is set to the VDD voltage, the positive voltage VIN+ input from the input terminal IN equal to or higher than a voltage (VDD−VGS−VOV) cannot be transmitted to the node B, where VDD represents a power supply voltage, VGS represents a threshold voltage (VGS&gt;0 V) of the NMOS transistors  11  and  12 , and VOV represents an overdrive voltage (VOV&gt;0 V) necessary for reliably turning ON the NMOS transistors  11  and  12 . 
         [0011]    Further, in order to transmit a voltage equal to or higher than the voltage (VDD−VGS−VOV), for example, the power supply voltage VDD, to the node B, the active state signal of the switch control terminal EN needs to be a voltage equal to or higher than a voltage (VDD+VGS+VOV). Therefore, a booster circuit or a level shift circuit is necessary, which increases the circuit scale and the cost of products. 
       SUMMARY OF THE INVENTION 
       [0012]    In order to solve the above-mentioned problem, according to an exemplary embodiment of the present invention, there is provided a switch circuit including: a first NMOS transistor including a drain connected to an input terminal of a semiconductor device, a source connected to a first node, and a gate connected to a control terminal; a second NMOS transistor including a drain connected to the first node, a source connected to a second node, and a gate connected to the control terminal; a first PMOS transistor including a source connected to the input terminal of the semiconductor device, a drain connected to the first node, and a gate connected to the control terminal via an inverter; a second PMOS transistor including a source connected to the first node, a drain connected to the second node, and a gate connected to the control terminal via the inverter; and a third NMOS transistor including a source connected to a ground voltage, a drain connected to the first node, and a gate connected to the control terminal via the inverter, in which the second node is connected to an internal circuit. 
         [0013]    According to the switch circuit of the present invention, even when a negative voltage is input from the input terminal, the negative voltage can be prevented from being transmitted to the input of the internal circuit, and further, a positive voltage up to a VDD voltage can be transmitted to the input of the internal circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the accompanying drawings: 
           [0015]      FIG. 1  is a circuit diagram illustrating a switch circuit according to a first embodiment of the present invention; 
           [0016]      FIG. 2  is a circuit diagram illustrating a switch circuit according to a second embodiment of the present invention; 
           [0017]      FIG. 3  is a circuit diagram illustrating an example of a level shift circuit according to the second embodiment of the present invention; and 
           [0018]      FIG. 4  is a circuit diagram illustrating a conventional switch circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring to the accompanying drawings, a switch circuit according to embodiments of the present invention is described below. The switch circuit controls whether a positive or negative voltage input to an input terminal IN is transmitted or interrupted to an internal circuit  15  in response to a signal of a switch control terminal EN. 
       First Embodiment 
       [0020]      FIG. 1  is a circuit diagram illustrating a switch circuit according to a first embodiment of the present invention. 
         [0021]    The switch circuit in the first embodiment includes NMOS transistors  11 ,  12 , and  13 , PMOS transistors  16  and  17 , and an inverter  14 . 
         [0022]    The NMOS transistor  11  has a drain connected to the input terminal IN, a gate connected to the switch control terminal EN, a source connected to a drain of the NMOS transistor  12 , and a back gate connected to GND. The NMOS transistor  12  has a gate connected to the switch control terminal EN, a source connected to an input terminal (node B) of the internal circuit  15 , and a back gate connected to GND. The PMOS transistor  16  has a source connected to the input terminal IN, a gate connected to an output terminal of the inverter  14 , a drain connected to a source of the PMOS transistor  17 , and a back gate connected to VDD. The PMOS transistor  17  has a gate connected to the output terminal of the inverter  14 , a drain connected to the node B, and a back gate connected to VDD. The inverter  14  has an input terminal connected to the switch control terminal EN. The NMOS transistor  13  has a gate connected to the output terminal of the inverter  14 , a source grounded to GND, a drain connected to the source of the NMOS transistor  11 , to the drain of the NMOS transistor  12 , to the drain of the PMOS transistor  16 , and to the source of the PMOS transistor  17 , and a back gate connected to GND. 
         [0023]    The NMOS transistors  11  and  12  and the PMOS transistors  16  and  17  are controlled to be turned ON/OFF in response to a signal of the switch control terminal EN. The inverter  14  inverts an input signal of a VDD/GND voltage, and outputs the inverted signal. The NMOS transistor  13  is controlled to be turned ON when the NMOS transistors  11  and  12  and the PMOS transistors  16  and  17  are turned OFF. 
         [0024]    Next, the operation of the switch circuit in the first embodiment is described. 
         [0025]    (1) Case where positive voltage VIN+input from input terminal IN is not transmitted to node B 
         [0026]    To the switch control terminal EN, a signal of the GND voltage in an inactive state is input. The drain of the NMOS transistor  11  has the voltage VIN+ and the gate thereof has the GND voltage, and hence the NMOS transistor  11  is turned OFF. The source of the PMOS transistor  16  has the voltage VIN+ and the gate thereof has the VDD voltage, and hence the PMOS transistor  16  is turned OFF. The gate of the NMOS transistor  13  has the VDD voltage, and hence the NMOS transistor  13  is turned ON so that a node A has the GND voltage. The drain and the gate of the NMOS transistor  12  have the GND voltage, and hence the NMOS transistor  12  is turned OFF. The source of the PMOS transistor  17  has the GND voltage and the gate thereof has the VDD voltage, and hence the PMOS transistor  17  is turned OFF. Therefore, the positive voltage VIN+ input from the input terminal IN is not transmitted to the node B. 
         [0027]    (2) Case where negative voltage VIN− input from input terminal IN is not transmitted to node B 
         [0028]    To the switch control terminal EN, the signal of the GND voltage in the inactive state is input. The drain of the NMOS transistor  11  has the negative voltage VIN− and the gate thereof has the GND voltage, and hence the NMOS transistor  11  becomes an ON state in the weak inversion region. However, the gate of the NMOS transistor  13  has the VDD voltage, and hence the NMOS transistor  13  is turned ON so that the node A has the GND voltage. The drain of the NMOS transistor  12  has the GND voltage and the gate thereof also has the GND voltage, and hence the NMOS transistor  12  is turned OFF. The gates of the PMOS transistors  16  and  17  have the VDD voltage, and hence the PMOS transistors  16  and  17  are turned OFF. Therefore, the negative voltage VIN− input from the input terminal IN is not transmitted to the node B. 
         [0029]    (3) Case where positive voltage (VDD voltage) input from input terminal IN is transmitted to node B 
         [0030]    To the switch control terminal EN, a signal of the VDD voltage in an active state is input. The drain and the gate of the NMOS transistor  11  have the VDD voltage, and hence a voltage (VDD−VGS−VOV) is transmitted to the source (node A), where VDD represents a power supply voltage, VGS represents a threshold voltage (VGS&gt;0 V) of the NMOS transistors  11  and  12 , and VOV represents an overdrive voltage (VOV&gt;0 V) necessary for reliably turning ON the NMOS transistors  11  and  12 . 
         [0031]    On the other hand, the source of the PMOS transistor  16  has the VDD voltage and the gate thereof has the GND voltage, and hence the PMOS transistor  16  is turned ON so that the VDD voltage is transmitted to the drain. In this case, the PMOS transistor  16  is fully turned ON, and hence the drain voltage of the PMOS transistor  16  is dominant in the voltage of the node A. Therefore, the voltage of the node A becomes the VDD voltage. 
         [0032]    The same holds true for the relation between the NMOS transistor  12  and the PMOS transistor  17 , and hence the VDD voltage can be transmitted to the node B. 
         [0033]    As described above, the switch circuit in this embodiment can transmit an input voltage in the range of from the GND voltage to the VDD voltage. 
       Second Embodiment 
       [0034]      FIG. 2  is a circuit diagram illustrating a switch circuit according to a second embodiment of the present invention. 
         [0035]    The switch circuit in the second embodiment includes a level shift circuit  18  in addition to the circuit of  FIG. 1 . The level shift circuit  18  has a power supply terminal V connected to the input terminal IN, an output terminal O connected to the gate of the PMOS transistor  16 , and an input terminal I connected to the switch control terminal EN. The PMOS transistor  16  has a back gate connected to the input terminal IN. 
         [0036]    The level shift circuit  18  performs voltage conversion on a signal of the input terminal I based on a voltage of the power supply terminal V, and outputs the converted signal from the output terminal O. In this embodiment, the level shift circuit  18  is configured to invert the logic between input and output. When a signal of the input terminal I is the VDD voltage, a signal of the GND voltage is output from the output terminal O. When the signal of the input terminal I is the GND voltage, a signal of the voltage of the power supply terminal V is output from the output terminal O. 
         [0037]    (1) Case where voltage higher than VDD voltage input from input terminal IN is not transmitted to node B. 
         [0038]    If the back gate of the PMOS transistor  16  is connected to VDD, the back gate voltage becomes lower than a source voltage. Therefore, an unnecessary current flows from the input terminal IN to VDD via the PMOS transistor  16  (from source to back gate). 
         [0039]    According to the switch circuit in the second embodiment, the back gate and the source of the PMOS transistor  16  are connected to the input terminal IN, and hence there is no potential difference between the back gate and the source, and no current path to VDD exists. The gate of the PMOS transistor  16  has the voltage of the input terminal IN which is output from the output terminal O of the level shift circuit  18 . Therefore, the gate of the PMOS transistor  16  is applied with a voltage necessary for turning OFF the PMOS transistor  16 . 
         [0040]    Other combinations of the voltage of the input terminal IN and the control of the switch circuit have the same circuit operations as those described in the first embodiment. 
         [0041]      FIG. 3  is an example of the level shift circuit configuration used in the switch circuit of the second embodiment. The level shift circuit only needs to have a simple and general configuration including PMOS transistors  21  and  22  and NMOS transistors  23  and  24 . 
         [0042]    As described above, according to the switch circuit in the second embodiment, even when a voltage higher than the VDD voltage is applied from the input terminal IN when the switch is OFF, it is possible to prevent an unnecessary current from flowing to the input terminal IN, thus preventing the PMOS transistor  16  from being broken down.