Patent Publication Number: US-8531237-B2

Title: Low-pass filter, constant voltage circuit, and semiconductor integrated circuit including same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2009-165466 filed on Jul. 14, 2009 with the Japanese Patent Office. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a low-pass filter, a constant voltage circuit, and a semiconductor integrated circuit including the same, and more particularly, to a low-pass filter and a constant voltage circuit for use in ultra-low noise constant voltage regulation which can be integrally formed on a single semiconductor substrate, and a semiconductor integrated circuit including such a voltage regulator with a low-pass filter incorporated therein. 
     2. Discussion of the Background 
     Electronic low-pass filters are used in various semiconductor circuits which eliminate high frequencies above a given cutoff frequency to provide accurate signals free from high-frequency noise. One typical application is in voltage regulation, where a low-pass filter is connected between a reference voltage generator output terminal and a regulator output terminal to filter out flicker or 1/f noise inherent in the semiconductor device from a reference voltage based on which an output voltage is regulated. 
       FIG. 1  is a circuit diagram schematically illustrating a constant voltage circuit  100  employing a conventional, resistance-capacitance low-pass filter  110  consisting of a resistor R 111  and a capacitor C 111  connected in series. 
     As shown in  FIG. 1 , the constant voltage circuit  100  is a series regulator that regulates an input voltage Vin input to an input terminal IN to output a constant output voltage Vout to an output terminal OUT, including a bipolar, output transistor M 111  connected between the input and output terminals IN and OUT, a resistor R 112  and a Zener diode ZD connected in series between the input terminal IN and ground to form a reference node Nref therebetween, and an error amplifier  111  with a non-inverting input connected to the node Nref through the RC low-pass filter  110 , an inverting input connected to the output terminal OUT, and an output connected to a base terminal of the output transistor M 111 . 
     During operation, the Zener diode ZD generates a reference voltage Vref at the reference node Nref for input to the non-inverting input of the error amplifier  111 , which compares the reference voltage Vref against the output voltage Vout input to its inverting input to output a regulator control signal that controls the base current of the output transistor M 111  so as to maintain the output voltage Vout equal to the reference voltage Vref. 
     Interposed between the reference node Nref and the non-inverting input of the error amplifier  111 , the low-pass filter  110  has the series circuit of the resistor R 111  and the capacitor C 111  connected across the node Nref and ground. The resistor R 111  and the capacitor C 111  are provided with particular resistance and capacitance scaled to yield an appropriate cutoff frequency rated in the range of below one to several hertz (Hz) depending on specific requirements of the voltage regulator. For example, a cutoff frequency of approximately 1 Hz, which is required for proper filtering of 1/f noise, can be obtained in the low-pass filter  110  with the resistor R 111  having a value of 1 megaohms (MΩ) and the capacitor C 111  having a value of 1 microfarad (μF). 
     The conventional low-pass filter  110  is not practical where the cutoff frequency desired is very low. This is because, in practice, all the components of the filtering circuit are constructed on a single semiconductor substrate for integration into a monolithic IC, which imposes limits on the physical sizes and therefore the values of both the resistor and the capacitor in use. 
     For example, consider a case where the capacitor C 111  has its value limited to below 100 picofarads (pF). With such a small capacitance, obtaining a cutoff frequency of 1 Hz requires a resistance of 10 gigaohms (GΩ) or higher of the resistor R 111 , which is technically difficult to form on a single semiconductor substrate on which the capacitor C 111  is disposed. Thus, the conventional low-pass filter  110  is implemented with at least one of the resistor R 111  and the capacitor C 111  built as a discrete component external from the integrated circuit, making the implementation less successful than desired. 
     The problem of the conventional low-pass filter  110  may be overcome by replacing the resistor R 111  with a transistor operated with no gate bias voltage applied thereto. Compared to a simple resistor, a zero-biased transistor provides an extremely high impedance relative to its size, allowing for obtaining a low cutoff frequency with a reasonably small capacitance without requiring large space in the semiconductor circuit. 
       FIG. 2  is a circuit diagram schematically illustrating a constant voltage circuit  200  employing a low-pass filter  210  consisting of a zero-biased transistor M 211  and a capacitor C 211  connected in series. 
     As shown in  FIG. 2 , the constant voltage circuit  200  is a series regulator that regulates an input voltage Vin input to an input terminal IN to output a constant output voltage Vout to an output terminal OUT, including a p-channel metal-oxide semiconductor (PMOS) transistor M 201  connected between the input and output terminals IN and OUT, a reference voltage generator  221 , and a reference voltage amplification circuit formed of an operational amplifier  212  with an inverting input connected to a node between a pair of resistors R 213  and R 214  connected in series, a non-inverting input connected to the reference voltage generator  221 , and an output connected to its non-inverting input through the resistor R 213  to form an amplified reference node Nref, as well as a buffer amplifier  211  with a non-inverting input connected to the node Nref through the low-pass filter  210 , a non-inverting input connected to the output terminal OUT, and an output connected to a gate terminal of the output transistor M 201 . 
     During operation, the reference voltage generator  221  generates a reference voltage Vref for input to the reference amplification circuit, which then generates an amplified reference voltage at the reference node Nref for input to the inverting input of the buffer amplifier  211 . The buffer amplifier  211  compares the amplified reference voltage against the output voltage Vout input to its non-inverting input to generate a regulator control signal that controls the operation of the output transistor M 201  so as to maintain the output voltage Vout equal to the amplified reference voltage. 
     Interposed between the amplified reference node Nref and the input of the buffer amplifier  211 , the low-pass filter  210  has the zero-biased transistor R 211  and the capacitor C 211  connected in series across the node Nref and ground. The transistor M 211  is a PMOS transistor with its gate and source terminals connected together to exhibit an extremely high impedance, higher than that obtained with a simple resistor. Using the zero-biased transistor M 211  as an impedance allows for implementing the low-pass filter  210  on a single integrated circuit, with a sufficiently low cutoff frequency even where the capacitor C 211  is of a small value. 
     Although effective in providing a low cutoff frequency with a relatively small circuit, the low-pass filter  210  depicted above has a drawback. That is, variations in the cutoff frequency can occur due to variations in the impedance of the zero-biased transistor M 211 , which has variations in physical properties from one transistor to the next caused by manufacturing process inconsistencies or environmental changes that are difficult to control and eliminate completely, resulting in reduced accuracy and stability of the low-pass filter  210 . To address this problem, several methods have been proposed to stabilize the impedance of the biased transistor in the low-pass filter  210 . 
       FIG. 3  is a circuit diagram of another conventional low-pass filter  210   a  for use in the constant voltage circuit  200 , shown with an input terminal LPIN for connection with the reference node Nref and an output terminal LPOUT for connection with the error amplifier input. 
     As shown in  FIG. 3 , the low-pass filter  210   a  has the series circuit of the PMOS transistor M 211  and the capacitor C 211  arranged with an additional, PMOS transistor M 212  and a current source I 211  connected in series between the input terminal LPIN and ground. The two PMOS transistors M 211  and M 212  have their source terminals connected together and their gate terminals connected together and to the drain of the transistor M 212  which is connected to the current source I 211 . With the transistors M 211  and M 212  thus forming a current mirror, the transistor M 211  conducts an amount of current proportional to a current i 211  supplied to the transistor M 212  from the current source I 211 . 
     In such a configuration, varying the amount of current i 211  allows adjustment of the impedance of the biased transistor M 211  to a desired value lower than that obtained with no bias voltage applied to the transistor. The ability to adjust the transistor impedance enables the low-pass filter  210   a  to operate with a desired cutoff frequency regardless of manufacturing process inconsistencies and environmental changes. 
       FIG. 4  is a circuit diagram illustrating still another conventional low-pass filter  210   b  for use in the constant voltage circuit  200 . 
     As shown in  FIG. 4 , the low-pass filter  210   b  includes, in addition to the capacitor C 211 , the PMOS transistors M 211  and M 212 , and the current source I 211 , another current mirror formed of a pair of n-channel metal-oxide semiconductor (NMOS) transistors M 213  and M 214  inserted between the current source I 211  and the current mirror of the transistors M 211  and M 212 . The NMOS transistor M 214  is sized twenty-five times larger than the NMOS transistor M 213 , and the PMOS transistor M 212  approximately nine hundred sixty times larger than the PMOS transistor M 211 , so that the amount of current supplied to the transistor M 211  through the two current mirrors is approximately 1/24,000 times smaller than the current i 211  supplied from the current source I 211 . 
     In addition to being capable of adjusting the impedance of the biased transistor M 211 , provision of the dual-current mirror circuit allows the low-pass filter  210   b  to precisely adjust the current through the transistor M 211  relative to the supplied current i 211 , compared to the circuit depicted in  FIG. 3  which requires precise control of an extremely small and consistent current i 211  supplied from the current source I 211  to obtain a sufficiently high impedance of the transistor M 211 . 
     Although obtaining higher accuracy and stability of the transistor impedance compared to those depicted in  FIGS. 2 and 3 , even the improved circuit  210   b  is still susceptible to variations where the current source I 211  itself has variations resulting from manufacturing process inconsistencies or environmental changes. Variations in the supplied current i 211  affect the gate bias voltage of the transistor M 212  that is the gate bias voltage of the transistor M 211 , resulting in significant variations in the impedance of the transistor M 211  and concomitant variations in the cutoff frequency of the low-pass filter  210   b.    
     BRIEF SUMMARY 
     This disclosure describes an improved low-pass filter that filters an input signal input to a filter input terminal to output a filtered output signal to a filter output terminal. 
     In one aspect of the disclosure, the improved low-pass filter includes a capacitor, a first field-effect transistor, a first resistor, and a first current source. The capacitor is connected between the filter output terminal and ground. The first field-effect transistor has a gate terminal, a first conduction terminal connected to the filter input terminal, and a second conduction terminal connected to the filter output terminal. The first resistor is connected between the gate and first conduction terminals of the first transistor. The first current source is connected to the first resistor to supply a first current to the first resistor. The first resistor generates a first voltage thereacross based on the supplied first current for electrically biasing the gate terminal of the first transistor. 
     This disclosure also describes an improved constant voltage circuit that converts an input voltage input to a voltage input terminal to generate a constant output voltage output to a voltage output terminal. 
     In one aspect of the disclosure, the constant voltage circuit includes an output transistor, a reference voltage generator, a regulator control circuit, and a low-pass filter. The output transistor is connected between the voltage input and output terminals to control current flow therethrough according to a regulator control signal applied to a control terminal thereof. The reference voltage generator generates a reference voltage. The regulator control circuit is connected to the reference voltage generator and the voltage output terminal to generate the regulator control signal based on a comparison of the output voltage and the reference voltage for application to the control terminal of the output transistor. The low-pass filter has a filter input terminal connected to the reference voltage generator and a filter output terminal connected to the control circuit to filter the reference voltage input to the filter input terminal to output a filtered reference voltage to the filter output terminal. The low-pass filter includes a capacitor, a first field-effect transistor, a first resistor, and a first current source. The capacitor is connected between the filter output terminal and ground. The first field-effect transistor has a gate terminal, a first conduction terminal connected to the filter input terminal, and a second conduction terminal connected to the filter output terminal. The first resistor is connected between the gate and first conduction terminals of the first transistor. The first current source is connected to the first resistor to supply a first current to the first resistor. The first resistor generates a first voltage thereacross based on the supplied first current for electrically biasing the gate terminal of the first transistor. 
    
    
     
       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 constant voltage circuit employing a conventional low-pass filter; 
         FIG. 2  is a circuit diagram schematically illustrating a constant voltage circuit employing another conventional low-pass filter; 
         FIG. 3  is a circuit diagram illustrating an arrangement of the conventional low-pass filter of  FIG. 2 ; 
         FIG. 4  is a circuit diagram illustrating another arrangement of the conventional low-pass filter of  FIG. 2 ; 
         FIG. 5  is a circuit diagram schematically illustrating a low-pass filter according to one embodiment of this patent specification; 
         FIG. 6  is a circuit diagram schematically illustrating in detail one embodiment of a first current source included in the low-pass filter of  FIG. 5 ; 
         FIG. 7  is a circuit diagram schematically illustrating another embodiment of the first current source included in the low-pass filter of  FIG. 5 ; 
         FIG. 8  is a circuit diagram schematically illustrating one embodiment of a constant voltage circuit incorporating the low-pass filter of  FIG. 5 ; 
         FIG. 9  is a circuit diagram schematically illustrating another embodiment of the constant voltage circuit incorporating the low-pass filter of  FIG. 5 ; 
         FIG. 10  is a circuit diagram schematically illustrating a constant voltage regulator with a startup circuit provided to the low-pass filter according to this patent specification; 
         FIG. 11  is a circuit diagram schematically illustrating an example of the startup circuit provided to the low-pass filter according to this patent specification; 
         FIG. 12A  is a plan view schematically illustrating an example of silicon-on-insulator structure for a p-channel metal-oxide semiconductor transistor used in the low-pass filter of  FIG. 5 ; 
         FIG. 12B  is a cross-sectional view of the transistor structure taken along a line B-B of  FIG. 12A ; and 
         FIG. 12C  is a cross-sectional view of the transistor structure taken along a line C-C of  FIG. 12A . 
     
    
    
     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. 5  is a circuit diagram schematically illustrating a low-pass filter  1  according to one embodiment of this patent specification. 
     As shown in  FIG. 5 , the low-pass filter  1  includes a first, p-channel metal-oxide semiconductor (PMOS) transistor M 1 , a capacitor C 1 , a first resistor R 1  having a given resistance r 1 , and a first current source  2 , which together form a filtering circuit that eliminates frequencies higher than a given cutoff frequency from a signal input to an input terminal LPIN to output a filtered signal to an output terminal LPOUT. 
     In the low-pass filter  1 , the first resistor R 1  and the first current source  2  are connected in series between the input terminal LPIN and ground, forming a first node N 1  therebetween. The first transistor M 1  has its source terminal connected to the input terminal LPIN, its drain terminal connected to the output terminal LPOUT, and its gate terminal connected to the node N 1 . The capacitor C 1  is connected between the output terminal LOUT and ground. 
     During operation, the first current source  2  supplies a given first current i 1  to the first resistor R 1 , which in turn generates a first voltage or potential drop Vb 1  thereacross proportional to its resistance r 1  and the supplied current i 1 . The first transistor M 1  thus biased with the voltage Vb 1  applied between its gate and source terminals exhibits an impedance corresponding to the gate bias voltage Vb 1 , which, together with a capacitance of the capacitor C 1 , determines the cutoff frequency with which the low-pass filter  1  performs filtering on an input signal. 
     The low-pass filter  1  is configured with sufficiently small values of the resistor R 1  and the current source  2  so that the transistor bias voltage Vb 1  determined by the product of r 1  and i 1  is smaller than a threshold voltage of the first transistor M 1 . That is, the first transistor M 1  operates in a subthreshold region where it conducts an extremely small, subthreshold current substantially exponentially proportional to the applied bias voltage Vb 1 , which means an extremely high impedance across the first transistor M 1 . 
     In such a configuration, the low-pass filter  1  can operate with extremely low cutoff frequencies even where the capacitor C 1  is of a relatively small value. For example, to obtain a cutoff frequency of 1 hertz (Hz) with the capacitor C 1  having a capacitance of 100 picofarads (pF), the first transistor M 1  is required to have an impedance of approximately 10 gigaohms (GΩ). Such a high impedance is obtained with a small bias voltage Vb 1  applied to the transistor M 1 , established with reasonably small values of the resistor R 1  and the current source  2 , which allows accommodation of these electronic components in a single semiconductor substrate so that the entire low-pass filter circuit  1  may be integrated into a single integrated circuit. 
     Thus, the low-pass filter  1  according to this patent specification provides a simple, reliable filtering circuit, wherein the biased first transistor M 1  exhibits a stable, high impedance to determine the cutoff frequency of the low-pass filter  1 . Biasing the first transistor M 1  with the gate bias voltage Vb 1  generated by the first resistor R 1  supplied with the first current source  2  enables precise setting of a desired cutoff frequency even with a small value of the capacitor C 1 , while allowing for simple and compact structure of the low-pass filter  1  which can be integrated into a semiconductor integrated circuit. 
       FIG. 6  is a circuit diagram schematically illustrating in detail one embodiment of the first current source  2   a  included in the low-pass filter  1  according to this patent specification. 
     As shown in  FIG. 6 , the first current source  2   a  includes a second, constant current source  3 , an operational amplifier  4 , a second, PMOS transistor M 2 , a third, n-channel metal-oxide semiconductor (NMOS) transistor M 3 , and a second resistor R 2  having a given resistance r 2 . 
     The second transistor M 2  has an electrical conductivity and other physical properties substantially identical to those of the first transistor M 1 , and the second resistor R 2  has physical properties substantially identical to those of the first resistor R 1 . As used herein, the term “physical properties” denotes characteristics and behaviors determined, for example, by the material and manufacturing process used to obtain the electronic component. Components identical in the physical properties operate in a substantially identical manner and can exhibit similar variations due to changes in environmental conditions, such as temperature, under which the low-pass filter  1  is operated. 
     In the first current source  2   a , the constant current source  3  and the second transistor M 2  are connected in series between a power supply input Vdd and ground, forming a second node N 2  therebetween. The second transistor M 2  has its source terminal connected to the current source  3 , and its drain and gate terminals grounded. The third transistor M 3  and the second resistor R 2  are connected in series between the first node N 1  and ground, forming a third node N 3  therebetween. The operational amplifier  4  has a non-inverting input connected to the second node N 2 , an inverting input connected to the third node N 3 , and an output connected to a gate terminal of the third transistor M 3 . 
     During operation, the constant current source  3  supplies a second, constant current i 2  to the source of the second transistor M 2 , which generates a second voltage Vb 2  corresponding to the supplied current i 2  at its source or node N 2  for input to the non-inverting input of the operational amplifier  4 . The second voltage Vb 2  thus determined by the amount of the second current i 2  acts as a gate bias voltage of the second transistor M 2 . 
     The third transistor M 3  conducts a first current i 1  for flowing through the first resistor R 1  as well as the second resistor R 2 , the amount of which is regulated according to a control signal applied to the gate terminal of the transistor M 3 . The second resistor R 2 , thus supplied with the first current i 1 , generates a third voltage Vb 3  proportional to its resistance r 2  and the current i 1  at the node N 3  for input to the inverting input of the operational amplifier  4 . 
     Comparing the inverting input voltage Vb 3  against the non-inverting input voltage Vb 2 , the operational amplifier  4  outputs the control signal to control the operation of the transistor M 3  so that the voltage Vb 3  at the third node N 3  is substantially equal to the voltage Vb 2  at the second node N 2 . This results in the first current i 1  flowing through the resistor R 2  substantially proportional to the gate bias voltage Vb 2  of the second transistor M 2 , as represented by the following Equation 1:
 
 i 1 =Vb 2 /r 2  Eq. 1
 
     The first current i 1  thus output by the first current source  2  flows through the first resistor R 1  in the low-pass filter  1  to generate the first voltage Vb 1 , determined by the product of the resistance r 1  and the current i 1  across the first resistor R 1 . Substituting Eq. 1 into Vb 1 =r 1 *i 1 , the gate bias voltage Vb 1  applied to the first transistor M 1  is given by the following Equation 2:
 
 Vb 1 =Vb 2 *r 1 /r 2  Eq. 2
 
     As mentioned, the second transistor M 2  has an electrical conductivity and other properties substantially identical to those of the first transistor M 1 . This means that variations in the gate bias voltage Vb 2  of the second transistor M 2  occurring, e.g., due to changes in temperature, are cancelled out by variations in the gate bias voltage Vb 1  of the first transistor M 1 . The result is that the impedance of the first transistor M 1  is substantially insensitive to process or environmental variations, leading to high stability of the cutoff frequency of the low-pass filter  1  supplied with the current source  2   a.    
     Also as mentioned, the first and second resistors R 1  and R 2  have substantially identical physical properties. This means that the ratio of the first and second resistances r 1  and r 2 , to which the gate bias voltage Vb 1  of the first transistor M 1  is proportional (see Eq. 2), remains substantially constant and does not affect the first voltage Vb 1  regardless of process and environmental variations. Moreover, should there be variations in the constant current i 2  due to process or environmental variations to affect the second voltage Vb 2 , the first voltage Vb 1  may remain unaffected by variations in the second voltage Vb 2  where the ratio of the first and second resistances r 1  and r 2  is smaller than one. 
     Thus, the low-pass filter  1  according to this patent specification can operate with a stable cutoff frequency, wherein the current source  2   a , formed of the second transistor M 2  substantially identical in properties to the first transistor M 1 , and the second resistor R 2  substantially identical in properties to the first resistor R 1 , supplies the low-pass filter  1  without causing variations in the impedance of the first transistor M 1  even where there are variations in the electronic components resulting from variations in process or environmental conditions. 
       FIG. 7  is a circuit diagram schematically illustrating another embodiment of the first current source  2   b  included in the low-pass filter  1  according to this patent specification. 
     As shown in  FIG. 7 , the present embodiment is similar to that depicted in  FIG. 6 , except that the first current source  2   b  includes a pair of fourth and fifth, PMOS transistors M 4  and M 5  forming a first current mirror, and a pair of sixth and seventh, NMOS transistors M 6  and M 7  forming a second current mirror, in addition to the second current source  3 , the operational amplifier  4 , the second transistor M 2 , the third transistor M 3 , and the second resistor R 2 . 
     In the first current source  2   b , the components included in the current source  2   a  are connected in a manner similar to that depicted with reference to  FIG. 6 , except that the third transistor M 3  has its drain terminal connected to the drain terminal of the fourth transistor M 4  instead of the first node N 1 . The fourth and fifth transistors M 4  and M 5  have their source terminals connected together to the power supply input Vdd, and their gate terminals connected together to the drain terminal of the fourth transistor M 4 . The sixth and seventh transistors M 6  and M 7  have their source terminals connected together to ground, and their gate terminals connected together to the drain terminal of the sixth transistor M 6 . The drain terminal of the fifth transistor M 5  is connected to the drain terminal of the sixth transistor M 6 . The drain terminal of the seventh transistor M 7  is connected to the first node N 1 . 
     During operation, a current flowing through the third transistor M 3  is replicated through the first current mirror and then through the second current mirror to generate a first current i 1  flowing through the seventh transistor M 7 , which is supplied to the first resistor R 1  to generate the gate bias voltage Vb 1  applied to the first transistor M 1  in the low-pass filter  1 . 
     As is the case with the embodiment of  FIG. 6 , the first current source  2   b , formed of the second transistor M 2  substantially identical in properties to the first transistor M 1 , and the second resistor R 2  substantially identical in properties to the first resistor R 1 , supplies the low-pass filter  1  without causing variations in the impedance of the first transistor M 1 . 
     Moreover, provision of the first and second current mirrors inserted between the third transistor M 3  and the output N 1  of the first current source  2   b  results in the low-pass filter  1  having only one NMOS transistor M 7  interposed between the resistor R 1  and ground. Compared to the configuration of  FIG. 6 , where there is one NMOS transistor M 3  and one resistor R 2  between the resistor R 1  and ground, this arrangement enables the low-pass filter  1  to operate with an extremely low input voltage input to the input terminal LPIN, allowing low-voltage application of the low-pass filter  1  using the first current source  2   b.    
       FIG. 8  is a circuit diagram schematically illustrating one embodiment of a constant voltage circuit  10  incorporating the low-pass filter  1  according to this patent specification. 
     As shown in  FIG. 8 , the constant voltage circuit  10  is configured as a series regulator that converts an input voltage Vin input to an input terminal IN to generate a given constant voltage Vout for output to an output terminal OUT, including, in addition to the low-pass filter  1 , an output, PMOS transistor M 11 , a reference voltage generator  11 , and an error amplification circuit EA formed of a pair of voltage divider resistors R 11  and R 12  having given resistances r 11  and r 12 , respectively, and an error amplifier  12 . All the components of the voltage regulator  10 , or in certain applications, all except for the output transistor M 11 , may be integrally formed on a single semiconductor substrate for integration into a semiconductor integrated circuit. 
     In the constant voltage regulator  10 , the output transistor M 11  is connected between the input and output terminals IN and OUT. The voltage divider resistors R 11  and R 12  are connected in series between the output terminal OUT and ground, forming a feedback node Nfb 1  therebetween. The error amplifier  12  has an inverting input connected to the reference voltage generator  11  through the low-pass filter  1 , a non-inverting input connected to the node Nfb 1 , and an output connected to a gate terminal of the output transistor M 11 . 
     The low-pass filter  1 , thus inserted between the reference voltage generator  11  and the error amplifier  12 , has its input terminal LPIN connected to the output of the reference voltage generator  11  and its output terminal LOUT connected to the inverting input of the error amplifier  12 . 
     During operation, the voltage divider resistors R 11  and R 12  generate a feedback voltage Vfb 1  at the feedback node Nfb 1  by dividing the output voltage Vout. The reference voltage generator  11  generates a given reference voltage Vref 1  for input to the low-pass filter  1 , which filters out high-frequency noise on the incoming signal Vref 1  for output to the error amplifier  12 . 
     Upon receiving the filtered reference voltage Vref 1  at the inverting input and the feedback voltage Vfb 1  at the non-inverting input, the error amplifier  12  amplifies a difference between the input voltages Vref 1  and Vfb 1  to generate a control signal for application to the gate of the output transistor M 11 , which controls operation of the transistor M 11  so that the feedback voltage Vfb 1  is substantially equal to the reference voltage Vref 1 . This results in the transistor M 11  regulating current flow from the input terminal IN to the output terminal OUT to maintain the output voltage Vout at a given constant level. 
     Given the feedback voltage Vfb 1  is maintained substantially equal to the reference voltage Vref 1 , the output voltage Vout is represented by the following Equation 3:
 
 V out= V ref1*( r 11 +r 12)/ r 12  Eq. 3
 
     In such a configuration, any noise contained in the reference voltage Vref 1  at the input to the error amplifier  12  is multiplied by a factor of (r 11 +r 12 )/r 12  for superimposition on the resulting output signal Vout, as indicated by Equation 3. Providing the low-pass filter  1  between the reference voltage generator output Vref 1  and the error amplifier  12  input can effectively reduce noise in the output voltage Vout of the constant voltage regulator  10 , wherein filtering is performed on the reference voltage Vref 1  input to the input terminal LPIN prior to amplification through the error amplifier  12 . 
       FIG. 9  is a circuit diagram schematically illustrating another embodiment of a constant voltage circuit  20  incorporating the low-pass filter  1  according to this patent specification. 
     As shown in  FIG. 9 , the constant voltage circuit  20  is a series regulator that converts an input voltage Vin input to an input terminal IN to generate a given constant voltage Vout for output to an output terminal OUT, including, in addition to the low-pass filter  1 , an output, PMOS transistor M 21 , a reference voltage generator  21 , a controller or buffer amplifier  23 , and a reference voltage amplification circuit RA formed of a pair of resistors R 21  and R 22 , and an operational amplifier  22 . All the components of the voltage regulator  20 , or in certain applications, all except for the output transistor M 21 , may be integrally formed on a single semiconductor substrate for integration into a semiconductor integrated circuit. 
     In the constant voltage regulator  20 , the output transistor M 21  is connected between the input and output terminals IN and OUT. The resistors R 21  and R 22  are connected in series between an output terminal of the operational amplifier  22  and ground, forming a feedback node Nfb 2  therebetween. The operational amplifier  22  has an inverting input connected to the node Nfb 2 , and a non-inverting input connected to the reference voltage generator  21 . The output of the operational amplifier  22  is connected to the buffer amplifier  23  through the low-pass filter  1 . The buffer amplifier  23  has an inverting input connected to the output of the operational amplifier  22  through the low-pass filter  1 , a non-inverting input connected to the output terminal OUT, and an output connected to a gate terminal of the output transistor M 21 . 
     The low-pass filter  1 , thus inserted between the reference amplification circuit RA and the buffer amplifier  23 , has its input terminal LPIN connected to the output of the operational amplifier  22  and its output terminal LOUT connected to the inverting input of the buffer amplifier  23 . 
     During operation, the resistors R 21  and R 22  generate a feedback voltage Vfb 2  at the feedback node Nfb 2  for input to the operational amplifier  22  by dividing the voltage at the output of the operational amplifier  22 . The reference voltage generator  21  generates a given reference voltage Vref 1  for input to the operational amplifier  22 . Upon receiving the feedback voltage Vfb 2  at the inverting input and the reference voltage Vref 1  at the non-inverting input, the operational amplifier  22  amplifies a difference between the input voltages Vfb 2  and Vref 1  to generate an amplified reference voltage Vref 2 . The amplified reference voltage Vref 2  is input to the low-pass filter  1 , which filters out high-frequency noise on the incoming signal for output to the buffer amplifier  23 . 
     Upon receiving the filtered reference voltage Vref 2  at the inverting input and the output voltage Vout at the non-inverting input, the buffer amplifier  23  amplifies a difference between the input voltages to generate a control signal for application to the gate of the output transistor M 21 , which controls operation of the transistor M 21  so that the output voltage Vout is substantially equal to the amplified reference voltage. This results in the transistor M 21  regulating current flow from the input terminal IN to the output terminal OUT to maintain the output voltage Vout at a given constant level. 
     Given the output voltage Vout is maintained substantially equal to the amplified reference voltage Vref 2  output by the reference voltage amplifier RA, the output voltage Vout is represented by the following Equation 4:
 
 V out= V ref1*( r 21+ r 22)/ r 22  Eq. 4
 
     In such a configuration, providing the low-pass filter  1  between the reference voltage amplifier RA output and the buffer amplifier  23  input can effectively reduce noise in the output voltage Vout of the constant voltage regulator  20 , where filtering is performed on the relatively large voltage input to the input terminal LPIN subsequent to amplification through the reference amplification circuit RA. 
     Thus, the constant voltage circuit according to this patent specification can provide reliable voltage regulation with extremely low noise contained in the output signal owing to the low-pass filter  1  effectively filtering out high-frequency noise from the reference voltage based on which the output voltage is regulated. As described in the embodiments above, the constant voltage circuit may be configured with the low-pass filter  1  filtering the reference voltage either downstream or upstream of voltage amplification, and either configuration can be selectively used according to specific applications of the constant voltage regulator. 
     Preferably, the constant voltage circuit according to this patent specification has a startup circuit provided to the low-pass filter  1  to temporarily reduce the impedance of the first transistor M 1  to enable the capacitor C 1  to swiftly charge up during startup. Such a fast startup capability can reduce the overall time required for the constant voltage circuit to initiate voltage regulation, compared to the embodiments depicted above with reference to  FIGS. 8 and 9 , wherein the low-pass filter  1  takes time to charge up the capacitor C 1  after power on (e.g., approximately 1 second for a cutoff frequency of 1 Hz), which translates into a corresponding delay for the output voltage Vout to reach the constant level. 
       FIG. 10  is a circuit diagram schematically illustrating a constant voltage regulator  30  with a startup circuit  15  provided to the low-pass filter  1  according to this patent specification. 
     As shown in  FIG. 10 , the constant voltage regulator  30  is similar to that depicted in  FIG. 8 , including the low-pass filter  1 , the output transistor M 11 , and the error amplification circuit EA formed of the reference voltage generator  11 , the error amplifier  12 , and the voltage divider resistors R 11  and R 12 , except for the startup circuit  15  connected to the low-pass filter  1 . 
     During operation, the startup circuit  15  supplies current to the first resistor R 1  upon application of power to the input terminal IN, and stops the supply of current when a predetermined period of time has elapsed after power on. This results in the additional current temporarily flowing through the resistor R 1  in addition to the first current i 1  to increase the bias voltage Vb 1  applied to the first transistor M 1 , so that the biased transistor M 1  exhibits a reduced impedance to immediately charge up the capacitor C 1 , leading to a reduced startup time of the voltage regulator  30  employing the low-pass filter  1 . 
     Although the embodiment above depicts the startup circuit  15  provided to the voltage regulator  10  of  FIG. 8 , a similar arrangement may be provided for the voltage regulator  20  of  FIG. 9 , of which a detailed description is omitted for brevity. 
       FIG. 11  is a circuit diagram schematically illustrating an example of the startup circuit  15  provided to the low-pass filter  1  in the constant voltage circuit according to this patent specification. 
     As shown in  FIG. 11 , the startup circuit  15  includes a PMOS transistor M 31 , a diode D 31 , a resistor R 31 , and a capacitor C 31 . 
     In the startup circuit  15 , the resistor R 31  and the capacitor C 31  are connected in series between the input terminal IN and ground, forming a node Nc therebetween. The transistor M 31  has its source terminal connected to the input terminal IN, its drain terminal connected to the second node N 2  between the second current source  3  and the second transistor M 2 , and its gate terminal connected to the node Nc. The diode D 31  has its cathode connected to the input terminal IN and its anode connected to the node Nc. 
     During operation, the capacitor C 31  charges through the resistor R 31  as the input voltage Vin is supplied to the input terminal IN, resulting in a voltage Vc at the node Nc gradually increasing from a ground voltage for application to the gate of the PMOS transistor M 31 . The transistor M 31  remains conductive during a given period of time after power on where the gate voltage Vc gradually increases from ground to a threshold voltage of the transistor M 31 . The capacitor C 31  discharges through the diode D 31  when there is no voltage input to the input terminal IN. 
     Specifically, immediately after power on where the gate voltage Vc remains below the threshold voltage of the transistor M 31 , the transistor M 31  conducts current flowing from the input terminal IN to the source of the second transistor M 2 , resulting in a high value of the second voltage Vb 2  at the non-inverting of the operational amplifier  4 . Since the gate bias voltage Vb 1  of the first transistor M 1  is proportional to the second voltage Vb 2  (see, for example, Eq. 2), this causes the first transistor M 1  to exhibit a relatively low impedance, enabling the capacitor C 1  to swiftly charge up during startup of the low-pass filter  1 . 
     Then, as a given period of time elapses after power on, the voltage Vc at the node Nc exceeds the threshold voltage of the transistor M 31 . This turns off the transistor M 31  so as to stop the supply of current from the startup circuit  15  to the second transistor M 2 . With the second transistor M 2  thus supplied only with the second current source  3 , the low-pass filter  1  enters a normal state so that the first transistor M 1  exhibits a sufficiently high impedance to obtain a desired cutoff frequency of the low-pass filter  1 . 
     Thus, the startup circuit  15  included in the constant voltage circuit according to this patent specification can temporarily reduce the impedance of the first transistor M 1  by increasing the amount of current flowing through the first resistor R 1  for a given period of time after power on, so as to enable the capacitor C 1  to immediately charge up during startup. Increasing the current flow across the first resistor R 1  may be accomplished by providing an additional current, or by supplying a startup signal to cause the first current source  2  to temporarily increase the first current i 1 . In either case, by using the startup circuit  15  in conjunction with the low-pass filter  1 , the constant voltage circuit according to this patent specification can swiftly enter operation without requiring excessive time for starting up the low-pass filter  1 . 
     More preferably, the low-pass filter  1  according to this patent specification has at least the first transistor M 1  formed in a silicon-on-insulator (SOI) structure, which enables the transistor M 1  to operate with extremely high ON resistance without causing junction leak between the source and drain terminals. 
       FIG. 12A  is a plan view schematically illustrating an example of SOI structure for the PMOS transistor M 1 , and  FIGS. 12B and 12C  are cross-sectional views of the transistor structure taken along lines B-B and C-C, respectively, of  FIG. 12A . 
     As shown in  FIGS. 12A through 12C , the transistor structure includes a gate electrode  51  formed above an n-type body  52  provided with a body contact  53  and electrode  54 , a p-type drain region  55  with a drain contact  59  and electrode  56 , and a p-type source region  57  with a source contact  60  and electrode  58 , which together form a p-channel transistor built on a buried oxide or insulator layer  63  overlying a bulk substrate, not shown, and insulated with silicon dioxide  61  formed by local oxidation of silicon (LOCOS) on which lies an intermediate layer  62  separating one layer from another of the multilayered structure. 
     In the SOI structure, the drain region  55  and the source region  57  are formed on the insulator of buried oxide  63  so that there is no p-n junction or interface between each p-type region and the bulk substrate. This means there is substantially no risk of current leaking across the semiconductor junctions, allowing the PMOS transistor M 1  to have an extremely high ON resistance ranging from several to several tens of gigaohms without junction leakage, as is required for operation in the low-pass filter  1  according to this patent specification. 
     The semiconductor structure depicted above may be fabricated using a known SOI technique, of which a detailed description is omitted for brevity. Although the embodiment above depicts only the SOI structure for the PMOS transistor M 1 , it is possible to construct the entire circuitry of the low-pass filter  1  on the SOI substrate. 
     To recapitulate, the low-pass filter  1  according to this patent specification includes the capacitor C 1  connected between the output terminal LPOUT and ground, the first, PMOS transistor M 1  with its source terminal connected to the input terminal LPIN and its drain terminal connected to the output terminal LOUT, the first resistor R 1  connected between the source and gate terminals of the first transistor M 1 , and the first current source  2  connected between the gate terminal of the first transistor M 1  and ground, wherein biasing the first transistor M 1  with the first voltage generated across the first resistor R 1  supplied with the first current source  2  establishes a stable impedance to enable reliable filtering with an extremely low cutoff frequency substantially insensitive to process and environmental variations, which can be formed on a single semiconductor substrate for integration into a semiconductor integrated circuit. 
     Numerous additional modifications and variations are possible in light of the above teachings. For example, although several embodiments disclosed herein describe the low-pass filter  1  incorporated into a constant voltage circuit being a series voltage regulator, the low-pass filter  1  according to this patent specification is applicable to various electronic systems including switching voltage regulators and other constant voltage circuits. 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. 2009-165466 filed on Jul. 14, 2009 in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference herein.