Patent Publication Number: US-2005127921-A1

Title: Voltage detecting circuit

Description:
BACKGROUND OF THE INVENTTON  
      1. Field of the Invention  
      The present invention relates to a voltage detecting circuit for detecting a value of a voltage developed across detection terminals to change an output.  
      2. Description of the Related Art  
       FIG. 3  is a circuit block diagram of a voltage detecting circuit (refer to JP 2002-296306 A). Terminals which detect a voltage developed across them are connected to terminals  11  and  10 , respectively. In case of the voltage detecting circuit shown in  FIG. 3 , terminals of a battery  1  are connected to the terminals  11  and  10 , respectively. Voltage division resistors  13  and  14  are connected between the terminals  11  and  10 . A node between the voltage division resistors  13  and  14 , and a reference voltage source  15  are connected to inputs of a comparator  17 . An output buffer circuit  18  is connected to an output of the comparator  17 , and an output of the output buffer circuit  18  is connected to an output terminal  12 . Illustration of power supply terminals of the comparator  17  is omitted in the drawing. In addition, it is supposed that the terminal  10  is given the GND electric potential.  
      The comparator  17  compares a voltage Va appearing at the node between the voltage division resistors  13  and  14  with a reference voltage Vb of the reference voltage source  15 , thereby detecting a voltage of the battery  1 . That is to say, a voltage at which an output of the comparator  17  is inverted is Va =Vb. In this example, the voltage Va changes due to a resistance ratio between the voltage division resistors  13  and  14 , or due to the voltage of the battery  1 . When a resistance value of the voltage division resistor  13  is assigned R 1 , a resistance value of the voltage division resistor  14  is assigned R 2 , and the voltage of the battery  1  is assigned V 1 , the detected voltage of the battery  1  is expressed by Equation (1): 
 
Detected voltage=( R 1+ R 2)/ R 2× Vb   (1) 
 
      When the voltage of the battery  1  is higher than the voltage value expressed by Equation (1) (hereinafter, this state is referred to as “a release state”), the output of the comparator  17  goes to a high level, while when the voltage of the battery  1  is lower than the voltage value expressed by Equation (1) (hereinafter, this state is referred to as “a detection state”), the output of the comparator  17  goes to a low level. In other words, whether the voltage detecting circuit is in the release state or in the detection state can be known in correspondence to the output of the comparator  17  which is adapted to be at the high level or at the low level.  
      In general, since the voltage detecting circuit usually operates in order to detect an arbitrary voltage, a quantity of current consumed in the operation is desirably as small as possible. That is to say, a quantity of current consumed in the release state is desirably made as small as possible.  
      As shown in  FIG. 4 , the comparator  17  generally includes a current mirror circuit having P-channel MOS transistors  26  and  27 , an input differential pair having N-channel MOS transistors  28  and  29 , and a constant current circuit  30  for supplying a constant current I 1 .  
      In addition, as shown in  FIG. 4 , the output buffer circuit  18  includes an inverter  42 , an output N-channel MOS transistor  43 , and a pull-up resistor  40 . In this example, the pull-up of the pull-up resistor  40  is carried out for a positive electrode of the battery  1 . However, the pull-up of the pull-up resistor  40  may be carried out for a positive electrode of a second battery. In such cases, the voltage at the high level is determined based on the voltage of the second battery.  
      In order to reduce the consumed current in the release state in the conventional voltage detecting circuit, it is necessary to suppress a leakage current of the output N-channel MOS transistor, in other words, it is necessary to reduce a size of the output N-channel MOS transistor. However, if the size of the output N-channel MOS transistor is reduced, then there is encountered a problem that a SINK current driving ability in the detection state is reduced.  
     SUMMARY OF THE INVENTION  
      In light of the foregoing, the present invention has been made in order to solve the above-mentioned problem associated with the prior art, and it is, therefore, an object of the present invention to provide a voltage detecting circuit which is capable of suppressing a leakage current to reduce a consumed current in a release state without reducing a SINK current driving ability in a detection state of an output N-channel MOS transistor.  
      A voltage detecting circuit of the present invention is configured so as to control a voltage of a back gate of an output transistor in accordance with whether the voltage detecting circuit is in a detection state and in a release state.  
      According to the voltage detecting circuit of the present invention, a leakage current of the output transistor can be suppressed to reduce a consumed current in the release state without reducing the SINK current driving ability of the output transistor. 
    
    
     BRIEF DESCRIPTTON OF THE DRAWINGS  
      In the accompanying drawings:  
       FIG. 1  is a circuit block diagram of a voltage detecting circuit according to an embodiment of the present invention;  
       FIG. 2  is a circuit diagram of the voltage detecting circuit according to an embodiment of the present invention;  
       FIG. 3  is a circuit block diagram of a conventional voltage detecting circuit;  
       FIG. 4  is a circuit diagram of the conventional voltage detecting circuit; and  
       FIG. 5  is a circuit diagram of a voltage detecting circuit according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  is a circuit block diagram of a voltage detecting circuit according to a first embodiment of the present invention. An input of a voltage selecting circuit  50  is connected to an output of a comparator  17 , and an output thereof is connected to a back gate of an output N-channel MOS transistor  41 . The voltage selecting circuit  50  serves to reduce a back gate bias electric potential of the output N-channel MOS transistor  41  when the voltage detecting circuit is in a release state, and to increase the back gate bias electric potential of the output N-channel MOS transistor  41  when the voltage detecting circuit is in a detection state in accordance with an output from the comparator  17 .  
       FIG. 2  shows a circuit diagram of the voltage selecting circuit  50 . The voltage selecting circuit  50  includes an inverter  51 , an N-channel MOS transistor  52 , an N-channel MOS transistor  53 , and a battery  54 .  
      When the voltage detecting circuit is in the release state, in the voltage selecting circuit  50 , since an input of the inverter  51  is at a high level, the N-channel MOS transistor  52  is turned OFF and the N-channel MOS transistor  53  is turned ON.  
      Now, when it is supposed that a voltage of the battery  54  is assigned V 54 , the back gate bias electric potential of the output N-channel MOS transistor  41  is given as a negative value, i.e., −V 54 .  
      On the other hand, when the voltage detecting circuit is in the detection state, in the voltage selecting circuit  50 , since the input of the inverter  51  is at a low level, the N-channel MOS transistor  52  is turned ON and the N-channel MOS transistor  53  is turned OFF. Consequently, the back gate bias electric potential of the output N-channel MOS transistor  41  is given as the GND electric potential.  
      As described above, the voltage selecting circuit  50  controls the back gate bias electric potential of the output N-channel MOS transistor  41  so that the back gate bias electric potential when the voltage detecting circuit is in the release state becomes lower than that when the voltage detecting circuit is in the detection state. Hence, a threshold voltage of the output N-channel MOS transistor  41  in the case of the release state becomes higher than that in the case of the detection state. Consequently, a leakage current in the release state can be suppressed to a low level, i.e., a consumed current can be reduced without reducing a SINK current driving ability of the output N-channel MOS transistor  41  in the detection state.  
      In the above description, the voltage selecting circuit  50  has been explained as having the circuit configuration shown in  FIG. 2 . However, in case as well of any other circuit configuration having such a function that an output voltage value changes in accordance with a change in input signal, the same effects can be obtained.  
      In addition, while the output buffer circuit  16  has been described as having the circuit configuration shown in  FIG. 2 , for example, in case as well of a circuit configuration as shown in  FIG. 5 , the same effects can be obtained. In an output buffer circuit  19  of a voltage detecting circuit shown in  FIG. 5 , a constant current circuit  31  is provided instead of a pull-up resistor  40  in the output buffer circuit  16  of the voltage detecting circuit shown in  FIG. 2 .