Abstract:
There is provided a voltage regulator equipped with a reverse-current prevention function capable of ensuring safe performance without causing a large overshoot at an output terminal against a sharp fluctuation in source voltage. The voltage regulator provides a source voltage fluctuation detecting circuit for detecting a fluctuation in source voltage in a comparison circuit for comparing the source voltage with output voltage so that when the source voltage rises sharply, the current through constant current circuits for limiting the consumption current of the comparison circuit will be increased to improve the response characteristics.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2012-043224 filed on Feb. 29, 2012, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage regulator, and more particularly to a voltage regulator equipped with a reverse-current prevention function to prevent reverse current from an external power supply such as a backup battery connected to an output terminal. 
     2. Background Art 
       FIG. 3  is a circuit diagram of a voltage regulator equipped with a reverse-current prevention function. 
     The voltage regulator equipped with a reverse-current prevention function includes a reference voltage circuit  401 , an error amplifier  402 , an Nch transistor  400 , Pch transistors  403 ,  404 ,  405 , and  406 , voltage dividing resistors  407  and  408 , and a comparison circuit  430 . 
     The source voltage (VBAT 1) is applied between a VDD terminal and a VSS terminal. A backup battery  412  and a load  413  (e.g., a semiconductor memory device) are connected to an output terminal OUT. 
     First, the operation of the voltage regulator when the source voltage is being supplied between the VDD terminal and the VSS terminal will be described. The relation between the source voltage and the voltage (VBAT 2) of the backup battery  412  is generally as follows: VBAT 1&gt;VBAT 2. 
     The error amplifier  402  amplifies a difference voltage between feedback voltage VFB obtained by dividing output voltage VOUT of the output terminal OUT through the resistor  407  and the resistor  408  and reference voltage Vref output from the reference voltage circuit  401  to control the gate of the Pch transistor  403 . The output voltage VOUT at the output terminal OUT is kept constant. The comparison circuit  430  compares the source voltage input to an input terminal  121  with the output voltage VOUT input to an input terminal  122  to output a signal to a CONTX terminal  110  and a CONT terminal  111 . 
       FIG. 4  shows a conventional comparison circuit  430 . The comparison circuit  430  is composed of a constant current circuit  103 , a constant current circuit  104 , a Pch transistor  101 , a Pch transistor  102 , an inverter  105 , an inverter  106 , an inverter  108 , and a level shifter  107 . 
     Since the source voltage is higher than the output voltage VOUT, the gate-source voltage of the Pch transistor  101  is higher than the gate-source voltage of the Pch transistor  102 . Therefore, the voltage at the drain of the Pch transistor  102  becomes an “L” level (the voltage at the VSS terminal). The inverters  105  and  106  for waveform shaping cause the voltage at the CONT terminal  111 , to which the output of the inverter  106  is connected, to become the “L” level. The voltage at the CONTX terminal  110  becomes an “H” level (source voltage) because of going through the level shifter  107  and the inverter  108 . Therefore, since the Pch transistor  405  is turned ON and the Pch transistor  406  is turned OFF, the substrate voltage of the Pch transistor  403  becomes the source voltage. 
     Next, the operation of the voltage regulator when the supply of the source voltage is reduced will be described. The relation between the source voltage and the voltage of the backup battery  412  is as follows: VBAT 1&lt;VBAT 2. 
     When the source voltage drops below the output voltage VOUT, the gate-source voltage of the Pch transistor  101  becomes lower than the gate-source voltage of the Pch transistor  102 . Therefore, the potential of the drain of the Pch transistor  102  becomes an “H” level (output voltage VOUT). The inverters  105  and  106  for waveform shaping cause the voltage at CONT terminal  111  as the output of the inverter  106  to become the “H” level (output voltage VOUT). The voltage at the CONTX terminal  110  becomes an “L” level because of going through the level shifter  107  and the inverter  108 . Therefore, since the Pch transistor  405  is turned OFF and the Pch transistor  406  is turned ON, the substrate voltage of the Pch transistor  403  becomes the output voltage VOUT. 
     In other words, the potential of the substrate (NWELL) of the Pch transistor  403  is switched to a higher side of the source voltage and the output voltage to prevent electric current from flowing from the output terminal OUT through an inter-substrate parasitic diode of the Pch transistor  403  even when the source voltage drops below the voltage at the input terminal  122  (for example, see Patent Document 1). 
     [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-65634 
     SUMMARY OF THE INVENTION 
     However, in the conventional comparison circuit  430 , the reverse current flowing from the input terminal  122  is minimized and hence the circuit response speed is slow. This arises a problem that the signal for switching the substrate voltage of the Pch transistor  403  against a sharp voltage fluctuation is delayed. For example, when the source voltage rises sharply, electric current flows from the VDD terminal to the output terminal OUT through the inter-substrate parasitic diode of the Pch transistor  403  during the delay in the switching signal, resulting in causing an overshoot at the output terminal OUT. 
     Therefore, it is an object of the present invention to solve the above problem and provide a voltage regulator equipped with a reverse-current prevention function capable of ensuring safe performance without causing a large overshoot at an output terminal OUT against a sharp fluctuation in source voltage. 
     The voltage regulator equipped with a reverse-current prevention function of the present invention is configured such that a source voltage fluctuation detecting circuit for detecting a rise of source voltage is provided in a comparison circuit for comparing source voltage with output voltage, and when the source voltage rises sharply, current through a constant current circuit for limiting the consumption current of the comparison circuit is increased to improve the response characteristics. 
     According to the voltage regulator equipped with a reverse-current prevention function of the present invention, since the comparison circuit for comparing the source voltage with the output voltage includes a circuit for detecting a rise of the source voltage, the circuit controls the constant current circuit for limiting the consumption current, there is an advantage of being able to switch the substrate potential of a Pch transistor with a response speed enough for a fluctuation in source voltage without steadily increasing reverse current flowing into an output terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a comparison circuit in a voltage regulator of the present invention. 
         FIG. 2  is a circuit diagram showing an example of a source voltage fluctuation detecting circuit in the comparison circuit of the voltage regulator of the present invention. 
         FIG. 3  is a circuit diagram of a voltage regulator of the present invention. 
         FIG. 4  is a circuit diagram of a conventional comparison circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment for carrying out the present invention will be described with reference to the accompanying drawings. 
     As shown in  FIG. 3 , a voltage regulator equipped with a reverse-current prevention function according to the present invention includes a reference voltage circuit  401 , an error amplifier  402 , an Nch transistor  400 , Pch transistors  403 ,  404 ,  405 , and  406 , voltage dividing resistors  407  and  408 , and a comparison circuit  430 . 
     The Pch transistor  403  as an output transistor is connected between a VDD terminal and an output terminal OUT. The voltage dividing resistors  407  and  408 , and the Nch transistor  400  are connected in series between the output terminal OUT and a VSS terminal. The error amplifier  402  is configured such that the output terminal of the reference voltage circuit  401  is connected to an inverting input terminal thereof, a connection point between the voltage dividing resistors  407  and  408  is connected to a non-inverting input terminal thereof, and an output terminal thereof is connected to the gate of the Pch transistor  403 . The comparison circuit  430  is configured such that the VDD terminal is connected to an input terminal  121  thereof, the output terminal OUT is connected to an input terminal  122  thereof, the VSS terminal is connected to an input terminal  123  thereof, an output terminal  110  thereof is connected to the gates of the Nch transistor  400  and the Pch transistors  404  and  406 , and an output terminal  111  thereof is connected to the gate of the Pch transistor  405 . The source and drain of the Pch transistor  405  are connected to the VDD terminal and the substrate of the Pch transistor  403 . The source and drain of the Pch transistor  406  are connected to the output terminal OUT and the substrate of the Pch transistor  403 . The source and drain of the Pch transistor  404  are connected to the output terminal OUT and the gate of the Pch transistor  403 . 
     The source voltage (VBAT 1) is applied between the VDD terminal and the VSS terminal. A backup battery  412  and a load  413  (e.g., semiconductor memory device) are connected to the output terminal OUT. 
       FIG. 1  is a circuit diagram of the comparison circuit in the voltage regulator according to the present invention. The comparison circuit  430  includes a Pch transistor  101 , a Pch transistor  102 , a constant current circuit  103 , a constant current circuit  104 , an inverter  105 , an inverter  106 , an inverter  108 , a level shifter  107 , and a source voltage fluctuation detecting circuit  109 . 
     The Pch transistor  101  is configured such that the gate is connected to the drain thereof, the gate of the Pch transistor  102 , and the constant current circuit  103 , and the source is connected to the VDD terminal. The Pch transistor  102  is configured such that the drain is connected to the inverter  105  and the constant current circuit  104 , and the source and back gate are connected to the input terminal  122 . The source voltage fluctuation detecting circuit  109  is connected between the VDD terminal and the VSS terminal  123 , and an output terminal thereof is connected to the constant current circuit  103  and the constant current circuit  104 . The inverter  105  and the inverter  106  are connected in series so that power will be supplied from the input terminal  122 . The output of the inverter  106  is connected to the level shifter  107  and a CONT terminal  111 . The output of the level shifter  107  is connected to a CONTX terminal  110  through the inverter  108 . Power is supplied to the level shifter  107  and the inverter  108  from the VDD terminal. 
     Next, the operation of the voltage regulator equipped with a reverse-current prevention function will be described. 
     First, the operation of the voltage regulator when the source voltage is being supplied between the VDD terminal and the VSS terminal will be described. The relation between the source voltage and the voltage (VBAT 2) of the backup battery  412  is as follows: VBAT 1&gt;VBAT 2. 
     The error amplifier  402  amplifies a difference voltage between feedback voltage VFB, obtained by dividing output voltage VOUT of the output terminal OUT through the resistor  407  and the resistor  408 , and reference voltage Vref output from the reference voltage circuit  401  to control the gate of the Pch transistor  403 . The output voltage VOUT of the output terminal OUT is kept constant. The comparison circuit  430  compares the source voltage input to the input terminal  121  with the output voltage VOUT input to the input terminal  122  to output a signal to the CONTX terminal  110  and the CONT terminal  111 . 
     Since the source voltage is higher than the output voltage VOUT, the gate-source voltage of the Pch transistor  101  is higher than the gate-source voltage of the Pch transistor  102 . Therefore, the voltage of the drain of the Pch transistor  102  becomes an “L” level (the voltage at the VSS terminal). The inverters  105  and  106  for waveform shaping cause the voltage at the CONT terminal  111 , to which the output of the inverter  106  is connected, to become the “L” level. The voltage at the CONTX terminal  110  becomes an “H” level (source voltage) because of going through the level shifter  107  and the inverter  108 . Therefore, the Nch transistor  400  is turned ON and the Pch transistor  404  is turned OFF. In other words, the voltage regulator operates normally. 
     Further, since the Pch transistor  405  is turned ON and the Pch transistor  406  is turned OFF, the substrate voltage of the Pch transistor  403  becomes the source voltage. 
     Next, the operation of the voltage regulator when the supply of the source voltage is reduced will be described. The relation between the source voltage and the voltage of the backup battery  412  is as follows: VBAT 1&lt;VBAT 2. 
     When the source voltage drops below the output voltage VOUT, the gate-source voltage of the Pch transistor  101  becomes lower than the gate-source voltage of the Pch transistor  102 . Therefore, the potential of the drain of the Pch transistor  102  becomes an “H” level (output voltage VOUT). The inverters  105  and  106  for waveform shaping cause the voltage at the CONT terminal  111  as the output of the inverter  106  to become the “H” level (output voltage VOUT). The voltage at the CONTX terminal  110  becomes an “L” level because of going through the level shifter  107  and the inverter  108 . Therefore, the Nch transistor  400  is turned OFF and the Pch transistor  404  is turned ON. Even when the source voltage drops to make the output of the error amplifier  402  unstable, since the “H” level of voltage is applied to the gate of the Pch transistor  403  by means of the Pch transistor  404 , the Pch transistor  403  can be maintained in the OFF state. 
     Further, since the Pch transistor  405  is turned OFF and the Pch transistor  406  is turned ON, the substrate voltage of the Pch transistor  403  becomes the output voltage VOUT. In other words, the potential of the substrate (NWELL) of the Pch transistor  403  is switched to a higher side of the source voltage and the output voltage to prevent electric current from flowing from the output terminal OUT through an inter-substrate parasitic diode of the Pch transistor  403  even when the source voltage drops below the output voltage VOUT. 
     Next, the operation of the voltage regulator when the source voltage rises sharply in this condition will be described. Although the potential of the drain of the Pch transistor  102  becomes the “L” level (the potential of the VSS terminal), the time required for the switching is limited by the constant current circuit  104 . The source voltage fluctuation detecting circuit  109  detects a source voltage fluctuation to control current flowing into the constant current circuit  103  and the constant current circuit  104  according to the fluctuation. In other words, when the voltage at the VDD terminal rises sharply, the current flowing into the constant current circuit  103  and the constant current circuit  104  is temporarily increased to reduce the time for switching the potential of the drain of the Pch transistor  102  to the “L” level. 
     As described above, according to the voltage regulator of the present invention, the source voltage fluctuation detecting circuit  109  detects a sharp fluctuation in source voltage and temporarily increases the current flowing into the constant current circuit  103  and the constant current circuit  104  to reduce the switching time of the signal to the CONT terminal  111  and the CONTX terminal  110 , enabling the reverse-current prevention function to work promptly. This can prevent the occurrence of overshoot at the VOUT terminal  122  without affecting the operating time of the backup battery  412 . 
       FIG. 2  is a circuit diagram showing an example of the source voltage fluctuation detecting circuit in the comparison circuit of the voltage regulator of the present invention. 
     The source voltage fluctuation detecting circuit  109  is composed of a capacitance  201  and a depression-type Nch transistor  301  as a resistance element, which are connected in series between the VDD terminal and the VSS terminal, and Nch transistors  203  and  204 . The constant current circuit  103  and the constant current circuit  104  are composed of depression-type Nch transistors  302 ,  303  and depression-type Nch transistors  304 ,  305 , respectively. 
     The capacitance  201  and the depression-type Nch transistor  301  function as a differentiating circuit to control the gates of the Nch transistors  203  and  204  according to the fluctuation at the VDD terminal. In other words, when the source voltage rises sharply, since the voltage of the drain of the depression-type Nch transistor  301  rises to raise the voltage of the gates of the Nch transistors  203  and  204  and turn them on, the current through the constant current circuit  103  and the constant current circuit  104  increases. This can reduce the switching time of the signal to the CONT terminal  111  and the CONTX terminal  110 , enabling the reverse-current prevention function to work promptly. 
     Note that the circuitry including the inverter  105  and subsequent elements is not limited to this circuit diagram as long as a signal after being subjected to waveform shaping and level conversion can be output. 
     Further, since the depression-type Nch transistor  301  functioning as a resistance element of the differentiating circuit is of the same depression-type Nch as the depression-type Nch transistors  302  to  305  that make up the constant current circuits, they are correlated with each other in terms of variability in the process of manufacture. For example, when the threshold voltage of the depression-type Nch transistor drops, the response time of the comparison circuit  430  is slowed down steadily but quickened against the source voltage fluctuation. This allows the responsiveness of the comparison circuit  430  to be relatively less correlated with the variability in the process of manufacture. Therefore, the transistors that make up the resistance element in the differentiating circuit and the constant current circuits are not limited to those mentioned above as long as they are correlated with each other in terms of the variability in the process of manufacture.