Patent Publication Number: US-8525580-B2

Title: Semiconductor circuit and constant voltage regulator employing same

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a semiconductor circuit and a constant voltage regulator employing the same, and more particularly, to a semiconductor circuit for use in constant voltage regulation which can prevent variations in output voltage due to abrupt changes in input voltage, and a constant voltage regulator employing such a semiconductor circuit. 
     2. Description of the Background Art 
     Voltage regulators are employed in power supply circuitry which generates a regulated voltage from an input voltage to drive a load circuit that operates with constant power. In electronic applications, a voltage regulator is implemented in a single integrated circuit (IC), typically together with load circuitry, such as a microcontroller or other electronic components, to which electrical power is supplied from an external power source such as battery. 
       FIG. 1  is a circuit diagram schematically illustrating a configuration of a known voltage regulator  101 . 
     As shown in  FIG. 1 , the voltage regulator  101  comprises a series regulator that converts an input voltage V 111  supplied from a power supply terminal  111  to a regulated, constant output voltage V 113  for output to an output terminal  113 , consisting of a driver transistor M 112 , being a p-channel metal-oxide semiconductor (PMOS) device, having a source terminal thereof connected to the power supply terminal  111  and a drain terminal thereof connected to the output terminal  113 ; a pair of voltage divider resistors R 111  and R 112  connected in series between the output terminal  113  and a ground terminal  112  to form a feedback node therebetween; a reference voltage generator  116  connected between the input terminal  114  and the ground terminal  112 ; and a differential amplifier  115  having a non-inverting input thereof connected to the voltage divider node, an inverting input thereof connected to the reference voltage generator  116 , and an output thereof connected to a gate terminal of the driver transistor M 112 , with a pair of power supply inputs thereof connected between the input terminal  114  and the ground terminal  112 . 
     Components of the voltage regulator  101  may be integrated into a single IC, with the input voltage V 111  being input from an external power source connected to the power supply terminal  111 , and the output voltage V 113  output to a load circuit connected to the output terminal  113 . 
     During operation, the driver transistor M 112  conducts an electric current therethrough according to a voltage applied to the gate terminal, so as to output a regulated output voltage V 113  to the output terminal  113 . The voltage divider resistors R 111  and R 112  generate a feedback voltage Vfb proportional to the output voltage V 113  at the feedback node therebetween, whereas the reference voltage generator  116  generates a reference voltage Vref for comparison with the feedback voltage Vfb. The differential amplifier  115 , receiving the feedback voltage Vfb at the non-inverting input and the reference voltage Vref at the inverting input, controls operation of the driver transistor M 112  according to a result of comparison between the differential inputs Vfb and Vref, thereby regulating the output voltage V 113  to a desired constant level. 
       FIGS. 2A and 2B  are graphs showing the voltages V 111  and V 113  in volts (V) plotted against time in microseconds (μs), obtained at the power supply terminal  111  and the output terminal  113 , respectively, during operation of the voltage regulator  101 . 
     As shown in  FIGS. 2A and 2B , the output voltage V 113  of the voltage regulator  101 , which is normally regulated to a constant level of approximately 3.3 V, experiences a sharp, transient change as the power supply voltage V 111  suddenly changes in amplitude. Specifically, the output voltage V 113  “overshoots” (i.e., rises sharply and transiently above the constant level) at time t 0  where the power supply voltage V 111  suddenly increases from 5 V to 25 V, and then “undershoots” (i.e., falls sharply and transiently below the constant level) at time t 1  where the power supply voltage V 111  suddenly decreases from 25 V to 5 V. 
     One problem encountered by the voltage regulator  101  depicted above is that those sharp transient changes of the output voltage V 113 , if significant, can adversely affect proper operation of the load circuit powered through the regulator circuitry. In practice, a large voltage overshoot of e.g., 1.0 V may damage the load circuit where the voltage V 113  exceeds its rated maximum voltage, whereas a large voltage undershoot of e.g., 1.0 V may cause the load circuit to fail or malfunction where the voltage V 113  exceeds its minimum operating voltage. 
     To counteract the problem, various methods have been proposed to provide a voltage regulation circuitry whose output voltage is stabilized against variations in input power supply voltage. 
     For example, one conventional method provides a voltage regulator formed of a differential amplifier circuit that outputs an output voltage to an output terminal connected with a transistor switch. According to this method, the voltage regulator is equipped with a voltage comparator that monitors the output voltage to control a gate voltage of the transistor switch according to a result of comparison between the output voltage and a reference voltage. Upon detecting a voltage overshoot due to a sudden change in input voltage, the voltage comparator causes the transistor switch to discharge capacitance, thereby stabilizing the output voltage. 
     One drawback of this method is that using the voltage monitor is costly since it includes a comparator adding to cost and power consumption in the voltage regulator. The method also has a drawback in that the feedback control based on the voltage comparator requires a certain period of time until the output voltage is adjusted in response to the feedback signal received, making the system less effective or practical than would be desired for its intended purpose. 
     Another conventional method provides a voltage regulator using an output transistor that regulates an output voltage according to a control signal output from an error amplifier comparing the output voltage against a reference voltage. According to this method, the voltage regulator is equipped with a voltage monitor consisting of a constant current circuit and a capacitor, which monitors a power supply voltage input to the voltage regulator and temporarily increases power supplied to the error amplifier upon detecting a sudden change in the power supply voltage. Increasing power input to the error amplifier enables the error amplifier to operate with a high slew rate, resulting in the control circuit exhibiting good response to the changing power supply voltage. 
     This method has a drawback in that, for proper functioning of the capacitor-based voltage monitor, the voltage regulator involves a capacitor of several picofarads, which is large in size and thus costly to implement on an IC-packaged device. Moreover, the method is not suitable for battery-powered applications, since supplying a large supply voltage to the error amplifier, if temporary, can reduce lifetime of the battery supplying power to the voltage regulator. 
     BRIEF SUMMARY 
     This disclosure describes an improved semiconductor circuit for use in connection with a power supply terminal. 
     In one aspect of the disclosure, the improved semiconductor circuit includes a voltage regulator and a buffer transistor. The voltage regulator converts an input voltage input to an input terminal thereof into an output voltage output to an output terminal thereof. The buffer transistor is an n-channel depletion-mode metal-oxide semiconductor field effect transistor, disposed between the power supply terminal and the voltage regulator with a gate terminal thereof connected to the power supply terminal, a drain terminal thereof connected to the power supply terminal, and a source terminal thereof connected to the input terminal of the voltage regulator. 
     This disclosure also describes an improved voltage regulator for use in connection with a power supply terminal. 
     In one aspect of the disclosure, the improved voltage regulator includes an input terminal, an output terminal, a driver transistor, and a buffer transistor. The input terminal receives an input voltage supplied from the power supply terminal. The output terminal outputs an output voltage to load circuitry. The driver transistor is connected between the input and output terminals to convert the input voltage into the output voltage. The buffer transistor is an n-channel depletion-mode metal-oxide semiconductor field effect transistor, disposed between the power supply terminal and the voltage regulator with a gate terminal thereof connected to the power supply terminal, a drain terminal thereof connected to the power supply terminal, and a source terminal thereof connected to the input terminal of the voltage regulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a circuit diagram schematically illustrating a configuration of a known voltage regulator; 
         FIGS. 2A and 2B  are graphs showing voltages in volts (V) plotted against time in microseconds (μs), obtained at a power supply terminal and an output terminal, respectively, during operation of the voltage regulator of  FIG. 1 ; 
         FIG. 3  is a circuit diagram schematically illustrating a semiconductor circuit according to a first embodiment of this patent specification; 
         FIGS. 4A through 4C  are graphs showing voltages in volts (V) plotted against time in microseconds (μs), obtained at a power supply terminal, an input terminal, and an output terminal, respectively, during operation of the semiconductor circuit of  FIG. 3 ; 
         FIG. 5A  is a circuit diagram showing a buffer transistor with its drain current flowing from the input terminal to the power supply terminal, included in the semiconductor circuit of  FIG. 3 ; 
         FIG. 5B  is a graph showing current-voltage characteristics of the buffer transistor conducting the drain current from the input terminal to the power supply terminal, included in the semiconductor circuit of  FIG. 3 ; 
         FIG. 6  is a circuit diagram schematically illustrating a semiconductor circuit according to a second embodiment of this patent specification; 
         FIG. 7  is a circuit diagram schematically illustrating a semiconductor circuit according to a third embodiment of this patent specification; 
         FIG. 8A  is a circuit diagram showing a buffer transistor with its drain current flowing from the input terminal to the power supply terminal, included in the semiconductor circuit of  FIG. 7 ; 
         FIG. 8B  is a graph showing current-voltage characteristics of the buffer transistor conducting the drain current from the input terminal to the power supply terminal, included in the semiconductor circuit of  FIG. 7 ; 
         FIG. 9  is a circuit diagram schematically illustrating a semiconductor circuit  20  according to a fourth embodiment of this patent specification; 
         FIG. 10  is a circuit diagram schematically illustrating a semiconductor circuit according to a fifth embodiment of this patent specification; 
         FIGS. 11A through 11C  are graphs showing voltages in volts (V) plotted against time in microseconds (μs), obtained at a power supply terminal, an input terminal, and an output terminal, respectively, during operation of the semiconductor circuit of  FIG. 10 ; 
         FIG. 12  is a circuit diagram schematically illustrating a semiconductor circuit according to a sixth embodiment of this patent specification; 
         FIG. 13  is a circuit diagram schematically illustrating a semiconductor circuit according to a seventh embodiment of this patent specification; and 
         FIG. 14  is a circuit diagram schematically illustrating a semiconductor circuit according to an eighth embodiment of this patent specification. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, examples and exemplary embodiments of this disclosure are described. 
       FIG. 3  is a circuit diagram schematically illustrating a semiconductor circuit  20  according to a first embodiment of this patent specification. 
     As shown in  FIG. 3 , the semiconductor circuit  20  includes a constant voltage regulator  1  that converts an input voltage V 11  supplied to an input terminal  14  from a power supply terminal  11  to a regulated, constant output voltage V 13  for output to an output terminal  13 , as well as a buffer transistor M 21 , being a depletion-mode n-channel metal-oxide semiconductor (NMOS) field effect transistor, having a gate terminal thereof connected to the power supply terminal  11 , a drain terminal connected to the power supply terminal  11 , and a source terminal thereof connected to the input terminal  14 . 
     The constant voltage regulator  1  includes a driver transistor M 12 , being a p-channel metal-oxide semiconductor (PMOS) device, having a source terminal thereof connected to the input terminal  14  and a drain terminal thereof connected to the output terminal  13 ; a pair of voltage divider resistors R 11  and R 12  connected in series between the output terminal  13  and a ground terminal  12  to form a feedback node therebetween; a reference voltage generator  16  connected between the input terminal  14  and the ground terminal  12 ; and a differential amplifier  15  having a non-inverting input thereof connected to the voltage divider node, an inverting input thereof connected to the reference voltage generator  16 , and an output thereof connected to a gate terminal of the driver transistor M 12 , with a pair of power supply inputs connected between the input terminal  14  and the ground terminal  12 . 
     Components of the semiconductor circuit  20  depicted above may be integrated into a single integrated circuit (IC), in which case the supply terminal  11  is configured as a power supply terminal of the IC supplied with an external power source, not shown. 
     During operation, the constant voltage regulator  1  performs voltage regulation with the driver transistor M 12  conducting an electric current therethrough according to a voltage applied to the gate terminal, so as to output an output voltage V 13  to the output terminal  113 . The voltage divider resistors R 11  and R 12  generate a feedback voltage Vfb proportional to the output voltage V 13  at the feedback node therebetween, whereas the reference voltage generator  16  generates a reference voltage Vref for comparison with the feedback voltage Vfb. The differential amplifier  15 , receiving the feedback voltage Vfb at the non-inverting input and the reference voltage Vref at the inverting input, controls operation of the driver transistor M 12  according to a result of comparison between the differential inputs Vfb and Vref, thereby regulating the output voltage V 13  to a desired constant level. 
     The depletion-mode buffer transistor M 21  conducts current as long as the voltage V 11  at the power supply terminal  11  remains positive, so that the voltage V 14  at the input terminal  14  remains substantially equal to or slightly lower than the power supply voltage V 11 . In this state, the voltage regulator  1  can properly regulate the output voltage V 13  at a constant level, which in the present example is approximately 3.3 V. 
       FIGS. 4A through 4C  are graphs showing the voltages V 11 , V 14 , and V 13  in volts (V) plotted against time in microseconds (μs), obtained at the power supply terminal  11 , the input terminal  14 , and the output terminal  13 , respectively, during operation of the semiconductor circuit  20 . 
     As shown in  FIGS. 4A through 4C , as the power supply voltage V 11  suddenly decreases from 25 V to 5 V at time t 1 , the input voltage V 14  of the voltage regulator  1  in turn decreases from 24.5 V to 4.5 V, causing the output voltage V 13  to transiently decrease from 3.3 V to 3.0 V. 
     Note that the input voltage V 14 , whose amplitude is generally consistent with that of the power supply voltage V 11 , does not experience an abrupt, steep transition as that experienced by the power supply voltage V 11  at time t 1 . Instead, the input voltage V 14  gradually decreases over a period of time (for example, approximately 10 μs in the present embodiment) between time t 1  and time t 2 . The transition of the input voltage, thus buffered or slowed down, results in an reduced amount of “undershoot” exhibited by the output voltage V 13  falling below the constant level of 3.3 V, which is significantly smaller than that would otherwise be obtained. 
     Such undershoot suppression capability of the semiconductor circuit  20  upon a sudden decrease in the power supply voltage V 11  is derived from provision of the depletion-mode MOSFET M 21  between the power supply terminal  11  and the input terminal  14 , which serves as a constant current circuit conducting a drain current id from the input terminal  14  to the power supply terminal  11  where the input voltage V 14  becomes higher than the power supply voltage V 11 . 
     Specifically, with additional reference to  FIG. 5A , the buffer transistor M 21  is shown with its drain current id flowing from the input terminal  14  to the power supply terminal  11  where the input voltage V 14  exceeds the power supply voltage V 11 , causing a potential difference V 14 -V 11  applied between the drain and source terminals of the transistor M 21 . 
       FIG. 5B  is a graph showing current-voltage characteristics of the transistor M 21  conducting the drain current id from the input terminal V 14  to the power supply terminal V 11 . As shown in  FIG. 5B , the drain current id remains substantially constant at approximately 1 microampere (μA) where the drain-source voltage V 14 -V 11  is sufficiently large, that is, above approximately 0.5 V in the present embodiment. 
     Thus, as the power supply voltage V 11  suddenly falls below the input voltage V 14 , the buffer transistor M 21  serves as a constant current circuit through which any electric charges present at the input terminal  14 , such as those stored in the parasitic capacitance, are discharged to the power supply terminal  11  from the input terminal  14 . Discharging capacitance through the transistor M 21  effectively prevents an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . Further buffering or slowing down of the input voltage V 14  may be accomplished by providing a capacitor between the input terminal  14  and the ground terminal  12 . 
       FIG. 6  is a circuit diagram schematically illustrating a semiconductor circuit  20 A according to a second embodiment of this patent specification. 
     As shown in  FIG. 6 , the overall configuration of the second embodiment is similar to that depicted in  FIG. 3 , except that the input terminal  14 , that is, the source terminal of the buffer transistor M 21  is connected solely to the driver transistor M 12 , instead of being connected in common with the driver transistor M 12 , the reference voltage generator  16 , and the differential amplifier  15 . 
     In such a configuration, the semiconductor circuit  20 A operates in a manner similar to that depicted primarily with reference to  FIG. 3 , wherein the depletion-mode transistor M 21  provided between the power supply terminal  11  and the input terminal  14  serves as a constant current circuit conducting a drain current from the input terminal  14  to the power supply terminal  11  to discharge capacitance at the node  14  where the power supply voltage V 11  suddenly falls below the input voltage V 14 , so as to prevent an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . 
     In the second embodiment, the buffer transistor M 12  exerts a buffering effect solely on the drain voltage of the driver transistor M 12 , compared to the first embodiment which can buffer or slow down the transition not only in the input voltage of the driver transistor M 12  but also in the reference voltage generator  16  and the differential amplifier  15 . Such arrangement saves power consumed in the voltage regulator  1 , which is particularly suitable for applications where the semiconductor circuit is operated at relatively low input voltages. 
       FIG. 7  is a circuit diagram schematically illustrating a semiconductor circuit  20 B according to a third embodiment of this patent specification. 
     As shown in  FIG. 7 , the overall configuration of the third embodiment is similar to that depicted in  FIG. 3 , except that the circuit  20 B further includes a resistor R 21  disposed between the power supply terminal  11  and the drain terminal of the buffer transistor M 21 . 
     In such a configuration, the semiconductor circuit  20 A operates in a manner similar to that depicted primarily with reference to  FIG. 3 , wherein the depletion-mode transistor M 21  provided between the power supply terminal  11  and the input terminal  14  serves as a constant current circuit conducting a drain current from the input terminal  14  to the power supply terminal  11  to discharge capacitance at the node  14  where the power supply voltage V 11  suddenly falls below the input voltage V 14 , so as to prevent an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . 
     Specifically, with additional reference to  FIG. 8A , the buffer transistor M 21  is shown with its drain current id flowing from the input terminal  14  to the power supply terminal  11  where the input voltage V 14  exceeds the power supply voltage V 11 , causing a potential difference V 14 -V 11  applied between the drain and source terminals of the transistor M 21 . 
       FIG. 8B  is a graph showing current-voltage characteristics of the transistor M 21  conducting the drain current id from the input terminal V 14  to the power supply terminal V 11 . As shown in  FIG. 8B , the drain current id remains substantially constant at approximately 1 μA where the drain-source voltage V 14 -V 11  is sufficiently large, that is, above approximately 0.45 V in the present embodiment. 
     Thus, as the power supply voltage V 11  suddenly falls below the input voltage V 14 , the buffer transistor M 21  serves as a constant current circuit through which any electric charges present at the input terminal  14 , such as those stored in the parasitic capacitance, are discharged to the power supply terminal  11  from the input terminal  14 . Discharging capacitance through the transistor M 21  effectively prevents an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot of the output voltage V 13 . 
     Further, in the third embodiment, addition of the resistor R 21  between the power supply terminal  11  and the drain terminal of the buffer transistor M 21  establishes a negative feedback in the buffer circuitry, wherein the current flow id induces a corresponding voltage across the resistor R 21 , which in turn increases a threshold voltage of the transistor M 21 , resulting in a limited amount of current id through the transistor M 21 . Such arrangement allows the semiconductor circuit  20 B to more effectively prevent an abrupt transition in the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , compared to the first embodiment depicted in  FIG. 3 . 
       FIG. 9  is a circuit diagram schematically illustrating a semiconductor circuit  20 C according to a fourth embodiment of this patent specification. 
     As shown in  FIG. 9 , the overall configuration of the fourth embodiment is similar to that depicted in  FIG. 6 , except that the circuit  20 C further includes a resistor R 21  disposed between the power supply terminal  11  and the drain terminal of the buffer transistor M 21 . 
     In such a configuration, the semiconductor circuit  20 C operates in a manner similar to that depicted primarily with reference to  FIG. 6 , wherein the depletion-mode transistor M 21  provided between the power supply terminal  11  and the input terminal  14  serves as a constant current circuit conducting a drain current from the input terminal  14  to the power supply terminal  11  to discharge capacitance at the node  14  where the power supply voltage V 11  suddenly falls below the input voltage V 14 , so as to prevent an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . 
     As is the case with the third embodiment, in the fourth embodiment, addition of the resistor R 21  between the power supply terminal  11  and the drain terminal of the buffer transistor M 21  establishes a negative feedback in the buffer circuitry, wherein the current flow id induces a corresponding voltage across the resistor R 21 , which in turn increases a threshold voltage of the transistor M 21 , resulting in a limited amount of current id through the transistor M 21 . Such arrangement allows the semiconductor circuit  20 C to more effectively prevent an abrupt transition in the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , compared to the second embodiment depicted in  FIG. 6 . 
       FIG. 10  is a circuit diagram schematically illustrating a semiconductor circuit  20 D according to a fifth embodiment of this patent specification. 
     As shown in  FIG. 9 , the overall configuration of the fifth embodiment is similar to that depicted in  FIG. 3 , except that the circuit  20 D further includes a resistor R 22  disposed between the power supply terminal  11  and the gate terminal of the buffer transistor M 21 , and a capacitor C 21  disposed between the ground and the gate terminal of the buffer transistor M 21 . 
     In such a configuration, the semiconductor circuit  20 D operates in a manner similar to that depicted primarily with reference to  FIG. 3 , wherein the depletion-mode transistor M 21  provided between the power supply terminal  11  and the input terminal  14  serves as a constant current circuit conducting a drain current from the input terminal  14  to the power supply terminal  11  to discharge capacitance at the node  14  where the power supply voltage V 11  suddenly falls below the input voltage V 14 , so as to prevent an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . 
       FIGS. 11A through 11C  are graphs showing the voltages V 11 , V 14 , and V 13  in volts (V) plotted against time in microseconds (μs), obtained at the power supply terminal  11 , the input terminal  14 , and the output terminal  13 , respectively, during operation of the semiconductor circuit  20 D. 
     As shown in  FIGS. 11A through 11C , as the power supply voltage V 11  suddenly increases from 5 V to 25 V at time to, the input voltage V 14  of the voltage regulator  1  in turn increases from 4.5 V to 24.5 V, causing the output voltage V 13  to transiently increase from 3.3 V to 3.6 V. 
     Note that the input voltage V 14 , whose amplitude is generally consistent with that of the power supply voltage V 11 , does not experience an abrupt, steep transition as that experienced by the power supply voltage V 11  at time t 0 . Instead, the input voltage V 14  gradually increases over a period of time after time t 0 . The transition of the input voltage, thus buffered or slowed down, results in an reduced amount of “overshoot” exhibited by the output voltage V 13  rising above the constant level of 3.3 V, which is significantly smaller than that would otherwise be obtained. 
     Such overshoot suppression capability of the semiconductor circuit  20  upon a sudden increase in the power supply voltage V 11  is derived from provision of the additional resistor R 21  and capacitor C 21 , which forms a series RC circuit whose time constant limits the rate at which the gate voltage of the buffer transistor M 21  increases, so as to effectively prevent an abrupt transition of the input voltage V 14  due to a sudden increase in the power supply voltage V 11 , resulting in a small amount of overshoot exhibited by the output voltage V 13 . 
       FIG. 12  is a circuit diagram schematically illustrating a semiconductor circuit  20 E according to a sixth embodiment of this patent specification. 
     As shown in  FIG. 12 , the overall configuration of the sixth embodiment is similar to that depicted in  FIG. 6 , except that the circuit  20 E further includes a resistor R 22  disposed between the power supply terminal  11  and the gate terminal of the buffer transistor M 21 , and a capacitor C 21  disposed between the ground and the gate terminal of the buffer transistor M 21 . 
     In such a configuration, the semiconductor circuit  20 E operates in a manner similar to that depicted primarily with reference to  FIG. 6 , wherein the depletion-mode transistor M 21  provided between the power supply terminal  11  and the input terminal  14  serves as a constant current circuit conducting a drain current from the input terminal  14  to the power supply terminal  11  to discharge capacitance at the node  14  where the power supply voltage V 11  suddenly falls below the input voltage V 14 , so as to prevent an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . 
     Further, in the sixth embodiment, provision of the additional resistor R 21  and capacitor C 21 , which forms a series RC circuit whose time constant limits the rate at which the gate voltage of the buffer transistor M 21  increases, effectively prevents an abrupt transition of the input voltage V 14  due to a sudden increase in the power supply voltage V 11 , resulting in a small amount of overshoot exhibited by the output voltage V 13 . 
       FIG. 13  is a circuit diagram schematically illustrating a semiconductor circuit  20 F according to a seventh embodiment of this patent specification. 
     As shown in  FIG. 13 , the overall configuration of the seventh embodiment is similar to that depicted in  FIG. 3 , except that the circuit  20 F employs an NMOS transistor, instead of a PMOS transistor, as a driver transistor M 12  of the voltage regulator  1 . 
     In such a configuration, the semiconductor circuit  20 F operates in a manner similar to that depicted primarily with reference to  FIG. 3 , wherein the depletion-mode transistor M 21  provided between the power supply terminal  11  and the input terminal  14  serves as a constant current circuit conducting a drain current from the input terminal  14  to the power supply terminal  11  to discharge capacitance at the node  14  where the power supply voltage V 11  suddenly falls below the input voltage V 14 , so as to prevent an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . 
     In the seventh embodiment  20 F, configuring the driver transistor M 13  as an NMOS device allows for implementing the semiconductor circuit  20 F in an IC that contains one or more circuit components integrated into a single integrated unit, which are in most cases designed to operate with a voltage regulated through a voltage regulator employing an NMOS driver transistor. 
     Thus, the seventh embodiment  20 F is applicable to IC implementation not only where the output of the voltage regulator  1  is supplied to a load circuit outside of the IC, but also where the output of the voltage regulator  1  is supplied to a load circuit inside of the IC. The semiconductor circuit  20 F is particularly effective as a voltage regulator to drive internal circuitry of an IC, where providing a capacitor inside the same IC for preventing variations in the output voltage is difficult due to space limitations or other design constraints. 
       FIG. 14  is a circuit diagram schematically illustrating a semiconductor circuit  20 G according to an eighth embodiment of this patent specification. 
     As shown in  FIG. 13 , the overall configuration of the eighth embodiment is similar to that depicted in  FIG. 6 , except that the circuit  20 G employs an NMOS transistor, instead of a PMOS transistor, as a driver transistor M 12  of the voltage regulator  1 . 
     In such a configuration, the semiconductor circuit  20 G operates in a manner similar to that depicted primarily with reference to  FIG. 6 , wherein the depletion-mode transistor M 21  provided between the power supply terminal  11  and the input terminal  14  serves as a constant current circuit conducting a drain current from the input terminal  14  to the power supply terminal  11  to discharge capacitance at the node  14  where the power supply voltage V 11  suddenly falls below the input voltage V 14 , so as to prevent an abrupt transition of the input voltage V 14  due to a sudden decrease in the power supply voltage V 11 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . 
     As is the case with the seventh embodiment, in the seventh embodiment  20 G, configuring the driver transistor M 13  as an NMOS device allows for implementing the semiconductor circuit  20 F in an IC that contains one or more circuit components integrated into a single integrated unit, which are in most cases designed to operate with a voltage regulated through a voltage regulator employing an NMOS driver transistor. 
     Thus, the eighth embodiment  20 G is applicable to IC implementation not only where the output of the voltage regulator  1  is supplied to a load circuit outside of the IC, but also where the output of the voltage regulator  1  is supplied to a load circuit inside of the IC. The semiconductor circuit  20 G is particularly effective as a voltage regulator to drive internal circuitry of an IC, where providing a capacitor inside the same IC for preventing variations in the output voltage is difficult due to space limitations or other design constraints. 
     To recapitulate, the semiconductor circuit  20  according to this patent specification includes a voltage regulator  1  to convert an input voltage V 14  input to an input terminal  14  thereof from a power supply terminal  11  into an output voltage V 13  output to an output terminal  13  thereof; and a buffer transistor M 21 , being an n-channel depletion-mode metal-oxide semiconductor field effect transistor, disposed between the power supply terminal  11  and the voltage regulator  1 , with a gate terminal thereof connected to the power supply terminal  11 , a drain terminal thereof connected to the power supply terminal  11 , and a source terminal thereof connected to the input terminal  14  of the voltage regulator  1 . 
     The semiconductor circuit  20  is protected against a significant undershoot of the output voltage V 13  due to a sudden decrease in the power supply voltage V 11 , owing to the buffer transistor M 21  serving as a constant current circuit conducting current from its source, input terminal  14  to its drain, power supply terminal  11  where the power supply voltage V 11  falls below the input voltage V 14 , which can buffer or slow down the transition of the input voltage V 14 , resulting in a small amount of undershoot exhibited by the output voltage V 13 . 
     Providing the undershoot suppression capability through the single depletion-mode transistor M 21  connected to the voltage regulator  1  does not require a large amount of power consumed by the buffering circuitry, while allowing for a fast response time to a change in the power supply input, compared to those provided by a known feedback circuit. 
     In further embodiment, the source terminal of the buffer transistor M 21  may be connected solely to a conductive terminal of a driver transistor M 12  connected between the input and output terminals of the voltage regulator  1 . Such arrangement saves power consumed in the voltage regulator  1 , which is particularly suitable for applications where the semiconductor circuit is operated at relatively low input voltages. 
     In still further embodiment, the semiconductor circuit  20  may include a resistor R 21  disposed between the power supply terminal  11  and the drain terminal of the buffer transistor M 21 . Such arrangement allows the semiconductor circuit  20  to more effectively prevent an abrupt transition in the input voltage V 14  due to a sudden decrease in the power supply voltage V 11  without requiring additional power consumption. 
     In yet still further embodiment, the semiconductor circuit  20  may include a resistor R 22  disposed between the power supply terminal  11  and the gate terminal of the buffer transistor M 21 , and a capacitor C 21  disposed between a ground and the gate terminal of the buffer transistor M 21 . Such arrangement provides the semiconductor circuit  20  with an overshoot suppression capability, in addition to the undershoot suppression capability, without requiring additional power consumption, in which the additional resistor and capacitor R 22  and C 21  form a series RC circuit whose time constant limits the rate at which the gate voltage of the buffer transistor M 21  increases, so as to effectively prevent an abrupt transition of the input voltage V 14  due to a sudden increase in the power supply voltage V 11 , resulting in a small amount of overshoot exhibited by the output voltage V 13 . 
     Hence, the semiconductor circuit according to this patent specification is provided with undershoot/overshoot suppression capabilities that can operate with relatively low operating current, which protects the voltage regulator against significant undershoot/overshoot of the output voltage where the power supply voltage suddenly changes. Such semiconductor circuit may find application in high-voltage regulator or any suitable electronic device incorporating voltage regulation circuitry. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
     This patent specification is based on Japanese patent application No. 2010-160572 filed on Jul. 15, 2010 in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference herein.