Patent Publication Number: US-2015061623-A1

Title: Voltage regulator of low-drop-output type and operation method of the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of Korean Patent Application No. 10-2013-0106268 filed on Sep. 4, 2013, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to voltage regulator of a low-drop-output type and to an operation method of the same. 
     2. Description of the Related Art 
     In general, with continually evolving mobile communications technologies, a power amplifier is used for amplifying an output of a RF signal in a RF stage of a mobile communication terminal. In particular, a power amplifier using CMOS technologies suitable for light, thin, simple and integrated devices are being actively developed. 
     In order to stably operate such a power amplifier, a voltage regulator may be used in the power amplifier. A voltage regulator of a low-drop-output (LDO) type, inter alia, may be used, which may down-convert a supply voltage, may be simply designed, and may reduce current consumption. 
     Typical voltage regulators of the LDO type may include an operational amplifier, a PMOS transistor and a resistor circuit unit. 
     The operational amplifier, upon receiving a reference voltage in an inverting input terminal thereof, provides a first voltage, equal to the reference voltage, to a non-inverting input terminal thereof. 
     The PMOS transistor includes its source connected to the supply voltage terminal, and its gate and drain connected to the output terminal of the operation amplifier, and may be operated according to an output voltage from the operation amplifier to provide a current corresponding to the amplitude of the output voltage from the operational amplifier. 
     The resistor circuit unit includes a plurality of resistors connected to one another in series between the drain of the PMOS transistor and a ground, and may provide an output voltage obtained by multiplying the first voltage provided at the non-inverting input terminal of the operational amplifier by a fixed resistance ratio. 
     In the existing voltage regulator of the LDO type, an output voltage may vary as the supply voltage varies, and thus the voltage may vary in the non-inverting input terminal of the operation amplifier. In this case, an input offset may occur when the voltages between the inverting terminal and the non-inverting terminal of the operational amplifier do not match. 
     Therefore, in the existing voltage regulator of the LDO type, if an offset occurs in the operational amplifier, the voltage regulator operates unstably, fails to provide a desired gain or phase margin, and so on. 
     Patent Document 1 referenced below relates to a semiconductor device having an embedded regulator. However, the document does not teach removing an offset occurring when the supply voltage is varied. 
     RELATED ART DOCUMENT 
     (Patent Document 1) Japanese Patent Laid-Open Publication No. 2000-0066744 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides a voltage regulator of a low-drop-output type capable of operating stable by way of removing an offset occurring when a supply voltage varies, and an operation method of the same. 
     According to an aspect of the present invention, there is provided a voltage regulator of a low-drop-output type, including: an error amplifying unit providing a gate signal according to a voltage difference between a reference voltage and a feedback voltage; a semiconductor switch regulating a current between a supply voltage terminal and a ground according to the gate signal; a voltage detecting unit detecting the supply voltage to provide a detected voltage; a feedback control unit providing a feedback control signal according to the detected voltage; and a feedback voltage regulating unit connected between the semiconductor switch and the ground to regulate the feedback voltage according to the feedback control signal. 
     The feedback voltage regulating unit may include first and second resistor circuit parts connected between the semiconductor switch and the ground, wherein the feedback voltage is provided at a connection node between the first and second resistor circuit parts, and wherein at least one of the first and second resistor circuit parts has a resistance value varying according to the feedback control signal. 
     According to another aspect of the present invention, there is provided a voltage regulator of a low-drop-output type, comprising: an error amplifying unit providing a gate signal according to a voltage difference between a reference voltage and a feedback voltage; a semiconductor switch regulating a current between a supply voltage terminal and a ground according to the gate signal; a voltage detecting unit detecting the supply voltage to provide a detected voltage; a feedback control unit providing a feedback control signal according to the detected voltage; and a feedback voltage regulating unit connected between the semiconductor switch and the ground to regulate the feedback voltage according to the feedback control signal, wherein the feedback voltage regulating unit includes first and second resistor circuit parts connected between the semiconductor switch and the ground, wherein the feedback voltage is provided at a connection node between the first and second resistor circuit parts, and wherein the first resistor circuit part has a resistance value varying according to the feedback control signal. 
     According to another aspect of the present invention, there is provided a voltage regulator of a low-drop-output type, comprising: an error amplifying unit providing a gate signal according to a voltage difference between a reference voltage and a feedback voltage; a semiconductor switch regulating a current between a supply voltage terminal and a ground according to the gate signal; a voltage detecting unit detecting the supply voltage to provide a detected voltage; a feedback control unit providing a feedback control signal according to the detected voltage; and a feedback voltage regulating unit connected between the semiconductor switch and the ground to regulate the feedback voltage according to the feedback control signal, wherein the feedback voltage regulating unit includes first and second resistor circuit parts connected between the semiconductor switch and the ground, wherein the feedback voltage is provided at a connection node between the first and second resistor circuit parts, and wherein the second resistor circuit part has a resistance value varying according to the feedback control signal. 
     The error amplifying unit may include an operational amplifier having an inverting input terminal to receive the reference voltage, an non-inverting input terminal to receive the feedback voltage, and an output terminal connected to the semiconductor switch to provide the gate signal, and wherein the gate signal has a level corresponding to the voltage difference between the reference voltage and the feedback voltage. 
     The semiconductor switch may include a PMOS transistor having its source connected to the supply voltage terminal, its gate connected to an output terminal of the error amplifying unit, and a drain connected to the feedback voltage regulating unit, wherein the PMOS transistor regulates a source-drain current according to the gate signal. 
     The voltage detecting unit may include at least two detection resistors, first and second detection resistors connected between the supply voltage terminal and the ground, wherein the detected voltage is provided at a connection node between the first and second detection resistors. 
     The feedback control unit may provide a feedback control signal including first to nth control signals, states of the first to nth control signals being determined according to an amplitude of the detected voltage. 
     The feedback control unit may include: a comparator comparing the detected voltage with first to nth reference voltages to provide first to Nth comparison signals, respectively, wherein the first to Nth comparison signals have a high level if the detected voltage is higher than corresponding reference voltages and have a low level otherwise, and a decoder decoding the first to Nth comparison signals into first to nth control signals of the feedback control signals to provide the feedback voltage regulating unit with the decoded signals. 
     The first resistor circuit part may include first to Nth switched resistors connected to one another in parallel to be switched on or off according to the feedback control signal, wherein each of the first to Nth switched resistors includes a switch and a resistor connected to each other in series. 
     According to another aspect of the present invention, there is provided an operation method of a voltage regulator of a low-drop-output type having an error amplifying unit providing a gate signal according to a voltage difference between a reference voltage and a feedback voltage and a semiconductor switch regulating a current between a supply voltage terminal and a ground according to the gate signal, the method including: detecting, by a voltage detecting unit, the supply voltage to provide a detected voltage; providing, by a feedback control unit, a feedback control signal according to the detected voltage; and regulating, by a feedback voltage regulating unit connected between the semiconductor switch and the ground, the feedback voltage according to the feedback control signal. 
     The error amplifying unit may include an operational amplifier having an inverting input terminal to receive the reference voltage, an non-inverting input terminal to receive the feedback voltage, and an output terminal connected to the semiconductor switch to provide the gate signal, and wherein the gate signal has a level corresponding to the voltage difference between the reference voltage and the feedback voltage. 
     The semiconductor switch may include a PMOS transistor having its source connected to the supply voltage terminal, its gate connected to an output terminal of the error amplifying unit, and a drain connected to the feedback voltage regulating unit, wherein the PMOS transistor regulates a source-drain current according to the gate signal. 
     The providing of the feedback control signal may include: comparing, by the feedback control unit, the detected voltage with first to nth reference voltages to provide first to Nth comparison signals, respectively; and decoding the first to Nth comparison signals into first to nth control signals of the feedback control signals to provide the feedback voltage regulating unit with the decoded signals, wherein the first to Nth comparison signals have a high level if the detected voltage is higher than corresponding reference voltages and have a low level otherwise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a voltage regulator according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating an example of the feedback voltage regulating unit according to the embodiment of the present invention; 
         FIG. 3  is a circuit diagram illustrating another example of the feedback voltage regulating unit according to the embodiment of the present invention; 
         FIG. 4  is a diagram illustrating feedback control signals from the feedback control unit according to the embodiment of the present invention; 
         FIG. 5  is a diagram illustrating an implementation example of the feedback control unit according to the embodiment of the present invention; 
         FIG. 6  is a circuit diagram illustrating an implementation example of the feedback voltage regulating unit according to the embodiment of the present invention; 
         FIG. 7  is a circuit diagram illustrating another implementation example of the feedback voltage regulating unit according to the embodiment of the present invention; 
         FIG. 8  is a diagram illustrating an example of the feedback control unit according to the embodiment of the present invention; 
         FIG. 9  is a circuit diagram illustrating an example of the feedback voltage regulating unit according to the embodiment of the present invention; 
         FIG. 10  is a flowchart illustrating an operation method of the voltage regulator according to an embodiment of the present invention; and 
         FIG. 11  is a flowchart illustrating a process of providing a feedback control signal according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Throughout the drawings, the same or like reference numerals will be used to designate the same or like elements. 
       FIG. 1  is a block diagram of a voltage regulator according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the voltage regulator according to the embodiment of the present invention may include an error amplifying unit  100 , a semiconductor switch  200 , a voltage detecting unit  300 , a feedback control unit  400 , and a feedback voltage regulating unit  500 . 
     The voltage regulator may further include a reference voltage generating unit  50  to generate a reference voltage V ref . 
     The error amplifying unit  100  may generate a gate signal SG according to a voltage difference between a predetermined reference voltage Vref and a feedback voltage Vfb to provide the semiconductor switch  200  with a gate signal SG. 
     As an exemplary implementation, the error amplifying unit  100  may include an operational amplifier OP 1  having an inverting input terminal to receive the reference voltage Vref, a non-inverting input terminal to receive the feedback voltage Vfb, and an output terminal connected to the semiconductor switch  200  to provide the gate signal SG. 
     The operation amplifier OP 1  may provide the semiconductor switch  200  with a gate signal SG having a level corresponding to a voltage difference between a reference voltage Vref received at the inverting input terminal and a feedback voltage Vfb received at the non-inverting input terminal. 
     For example, if the reference voltage Vref is greater than the feedback voltage Vfb, the operational amplifier OP 1  may provide the semiconductor switch  200  with a gate signal SG having a level sufficient to switch on the semiconductor switch  200 . If the reference voltage V ref  is not greater than the feedback voltage Vfb, the operational amplifier OP 1  may provide the semiconductor switch  200  with a gate signal SG having a level sufficient to switch off the semiconductor switch  200 . 
     The semiconductor switch  200  may regulate a current between a supply voltage VDD terminal and a ground according to the gate signal SG. Here, the semiconductor switch  200  may be a switching element such as a transistor. For example, when the semiconductor switch  200  is switched on, a current flow is determined by a total resistance from the supply voltage VDD terminal to the ground and the supply voltage VDD. 
     The semiconductor switch  200  may include a PMOS transistor PMOS 1  having its source connected to the supply voltage VDD terminal, its gate connected to the output terminal of the error amplifying unit  100 , and its drain connected to the feedback voltage regulating unit  500 . 
     Here, the PMOS transistor PMOS 1  may regulate a source-drain current according to the gate signal SG. For example, the PMOS transistor PMOS 1  is turned on when the gate signal SG is in a low level and is turned off when the gate signal SG is at a high level. 
     The voltage detecting unit  300  may detect the supply voltage VDD to provide a detected voltage Vd. 
     As an exemplary implementation, the voltage detecting unit  300  may include at least two resistors, i.e., a first detection resistor R 31  and a second detection resistor R 32  connected between the supply voltage VDD terminal and the ground. 
     The detected voltage Vd may be provided at a connection node between the first detection resistor R 31  and the second detection resistor R 32 . For example, in the case that the ratio of the resistance between the first detection resistor R 31  and the second detection resistor R 32  was 5:1, that the supply voltage VDD was 6V, and that the reference voltage V ref  was 2.5V, the detected voltage Vd would be 1V. 
     The feedback control unit  400  may provide a feedback control signal SC according to the detected voltage Vd. The feedback control signal SC may have different signals to regulate the feedback voltage Vfb from the feedback voltage regulating unit  500  depending on the amplitude of the detected voltage Vd. 
     The feedback voltage regulating unit  500  may be connected between the semiconductor switch  200  and the ground to regulate the feedback voltage Vfb according to the feedback control signal SC. For example, the feedback voltage regulating unit  500  may include a resistor circuit in which a resistor that determines the feedback voltage Vfb is varied according to the feedback control signal SC. The resistor circuit may regulate the feedback voltage Vfb. 
     In addition, the feedback voltage regulating unit  500  includes a resistor circuit connected between the semiconductor switch  200  and the ground, by which an output voltage Vout amplified from the feedback voltage is provided at the non-inverting input terminal of the operational amplifier OP 1   
       FIG. 2  is a circuit diagram of an example of the feedback voltage regulating unit according to the embodiment of the present invention, and  FIG. 3  is a circuit diagram of another example of the feedback voltage regulating unit according to the embodiment of the present invention. 
     Referring to  FIGS. 2 and 3 , the feedback voltage regulating unit  500  may include a first resistor circuit part  510  and a second resistor circuit part  520  connected between the semiconductor switch  200  and the ground. The feedback voltage Vfb may be provided at a connection node between the first resistor circuit part  510  and the second resistor circuit part  520 . 
     The resistance value of at least one of the first resistor circuit part  510  and the second resistor circuit part  520  may vary according to the feedback control signal SC. 
     As shown in  FIG. 2 , the first resistor circuit part  510  may be a variable resistance circuit the resistance value of which varies according to the feedback control signal SC. 
     Alternatively, as shown in  FIG. 3 , the second resistor circuit part  520  may be a variable resistance circuit the resistance value of which varies according to the feedback control signal SC. 
       FIG. 4  is a diagram illustrating feedback control signals from the feedback control unit according to the embodiment of the present invention. Referring to  FIG. 4 , the feedback control unit  400  may provide feedback control signals SC including first to nth control signals SC 1  to SCN, the state of which is determined according to the amplitude of the detected voltage Vd. 
       FIG. 5  is a diagram illustrating an implementation example of the feedback control unit according to the embodiment of the present invention. 
     Referring to  FIG. 5 , the feedback control unit  400  may include a comparator unit  410  and a decoder  420 . 
     The comparator unit  410  may include first to nth comparators com 1  to comN that compare the detected voltage Vd with first to nth reference voltages V ref   1  to V refN  to provide first to Nth comparison signals S com1  to S comN , respectively. 
     The first to nth comparators com 1  to comN may provide first to Nth comparison signals Scom 1  to ScomN, respectively, each of which has a high level if the detected voltage Vd is higher than its reference voltage and otherwise has a low level. 
     The decoder  420  may decode the first to Nth comparison signals Scom 1  to ScomN into first to nth control signals SC 1  to SCN of the feedback control signal SC, respectively, and provide the decoded signals to the feedback voltage regulating unit  500 . 
     Now, exemplary implementations of the first resistor circuit part  510  and the second resistor circuit part  520  will be described with reference to  FIGS. 6 and 7 . As described above, at least one of the first resistor circuit part  510  and the second resistor circuit part  520  may be a variable resistance circuit with resistance values varying according to the feedback control signal SC. Exemplary implementations thereof will be described with reference to  FIGS. 6 and 7 . 
       FIG. 6  is a circuit diagram illustrating an implementation example of the feedback voltage regulating unit according to the embodiment of the present invention. 
     Referring to  FIG. 6 , the first resistor circuit part  510  may include first to Nth switched resistors  510 - 1  to  510 -N that are connected to each other in parallel and are switched on or off according to the feedback control signal SC. 
     Each of the first to Nth switched resistors  510 - 1  to  510 -N may include a switch and a resistor connected to each other in series. 
     For example, the first switched resistor  510 - 1  may include a switch SW 1  and a resistor R 51 - 1  connected to each other in series. The second switched resistor  510 - 2  may include a switch SW 2  and a resistor R 51 - 2  connected to each other in series. Further, the Nth switched resistor  510 -N may include a switch SWN and a resistor R 51 -N connected to each other in series. Here, the first to Nth switched resistors  510 - 1  to  510 -N may be switched on or off according to the first to nth control signals SC 1  to SCN, respectively. 
       FIG. 7  is a circuit diagram illustrating another implementation example of the feedback voltage regulating unit according to the embodiment of the present invention. 
     Referring to  FIG. 7 , the second resistor circuit part  520  may include first to Nth switched resistors  520 - 1  to  520 -N that are connected to each other in parallel and are switched on or off according to the feedback control signal SC. 
     Each of the first to Nth switched resistors  520 - 1  to  520 -N may include a switch and a resistor connected to each other in series. 
     For example, the first switched resistor  520 - 1  may include a switch SW 1  and a resistor R 52 - 1  connected to each other in series. The second switched resistor  520 - 2  may include a switch SW 2  and a resistor R 52 - 2  connected to each other in series. Further, the Nth switched resistor  520 -N may include a switch SWN and a resistor R 52 -N connected to each other in series. Here, the first to Nth switched resistors  520 - 1  to  520 -N may be switched on or off according to the first to nth control signals SC 1  to SCN, respectively. 
     Now, the operations of the feedback control unit  400  and the feedback voltage regulating unit  500  will be described with reference to  FIGS. 8 to 10 . 
       FIG. 8  is a diagram illustrating an example of the feedback control unit according to the embodiment of the present invention. 
     Referring to  FIG. 8 , the comparator unit  410  of the feedback control unit  400  may include first to nth comparators com 1  to comN that compare the detected voltage Vd with first to 4th reference voltages V ref1  to V ref4  to provide first to Nth comparison signals S com1  to S com4 , respectively. 
     First, the operation of the comparator unit  410  will be described. The first to fourth comparators com 1  to com 4  may provide first to 4th comparison signals S com1  to S com4 , respectively, each of which has a high level if the detected voltage Vd is higher than its reference voltage and otherwise has a low level. 
     For example, if the detected voltage Vd is higher than the first reference voltage V ref1  (e.g., 1V), the first to fourth comparators com 1  to com 4  may output first to fourth comparison signals S com1  to S com4  of a high level, respectively. 
     For another example, if the detected voltage Vd is lower than the first reference voltage V ref1  (e.g., 1V) and higher than the second reference voltage V ref2  (e.g., 0.9V), the second to fourth comparators com 2  to com 4  may output the second to fourth comparison signals S com2  to S com4  of a high level, respectively, while the first comparison signal S com1  becomes a lower level. 
     For another example, if the detected voltage Vd is lower than the second reference voltage V ref2  (e.g., 0.9V) and higher than the third reference voltage V ref3  (e.g., 0.8V), the third and fourth comparators com 3  and com 4  may output the third and fourth comparison signals S com3  and S com4  having high levels, respectively, while the first and second comparison signals S com1  and S com2  are reduced to a lower level. 
     Further, if the detected voltage Vd is lower than the third reference voltage V ref3  (e.g., 0.8V) and higher than the fourth reference voltage V ref4  (e.g., 0.72V), the fourth comparator com 4  may output the fourth comparison signal S com4  of a high level, while the first to third comparison signals S com1  to S com3  become a lower level. 
     Second, the operation of the decoder  420  will be described. The decoder  420  may decode the first to fourth comparison signals S com1  to S com4  into first to fourth control signals SC 1  to SC 4 , respectively, and provide the decoded signals to the feedback voltage regulating unit  500 . 
     For example, if all of the first to fourth comparison signals S com1  to S com4  are a high level, only the first control signal SC 1  of the first to fourth control signals S com1  to S com4  is output at switching-on level. 
     For example, if the first to fourth comparison signals S com1  to S com4  are a L (low level), H, H and H, respectively, only the second control signal SC 2  of the first to fourth control signals S com1  to S com4  is output at switching-on level. 
     For another example, if the first to fourth comparison signals S com1  to S com4  are a L, L, H and H, respectively, only the third control signal SC 3  of the first to fourth control signals S com1  to S com4  is output at switching-on level. 
     If the first to fourth comparison signals S com1  to S com4  are a L, L, L and H, respectively, only the fourth control signal SC 4  of the first to fourth control signals S com1  to S com4  is output at switching-on level. 
       FIG. 9  is a circuit diagram illustrating an example of the feedback voltage regulating unit according to the embodiment of the present invention. 
     Referring to  FIG. 9 , the first resistor circuit part  510  may include first to fourth switched resistors  510 - 1  to  510 - 4  that are connected to each other in parallel and are switched on or off according to the feedback control signal SC. 
     Each of the first to fourth switched resistors  510 - 1  to  510 - 4  may include a switch and a resistor connected to each other in series. 
     For example, the first switched resistor  510 - 1  may include a switch SW 1  and a resistor R 51 - 1  connected to each other in series. The second switched resistor  510 - 2  may include a switch SW 2  and a resistor R 51 - 2  connected to each other in series. The third switched resistor  510 - 3  may include a switch SW 3  and a resistor R 51 - 3  connected to each other in series. The fourth switched resistor  510 - 4  may include a switch SW 4  and a resistor R 51 - 4  connected to each other in series. 
     Here, the first to fourth switched resistors  510 - 1  to  510 - 4  may be switched on or off according to the first to fourth control signals SC 1  to SC 4 , respectively. 
     For example, if the supply voltage VDD is 6V, when only the first control signal SC 1  of the first to fourth control signals SC 1  to SC 4  is at the switching-on level, only the first switched resistor  510 - 1  of the first to fourth switched resistor  510 - 1  to  510 - 4  is closed. At this time, if the ratio of the resistance values between the resistor R 51 - 1  included in the first switched resistor  510 - 1  and the resistor R 52  of the second resistor circuit part  520  is 1:1, the feedback voltage Vfb at the connection node between the first and second resistor circuit parts  510  and  520  is equal to the reference voltage V ref  (e.g., 2.5V), and the feedback voltage regulating unit  500  may provide the output voltage Vout of 5V, obtained by multiplying the feedback voltage Vfb by two times. 
     Or, if the supply voltage VDD is 5.4V, when only the second control signal SC 2  of the first to fourth control signals SC 1  to SC 4  is the switching-on level, only the second switched resistor  510 - 2  of the first to fourth switched resistor  510 - 2  to  510 - 4  is closed. 
     At this time, if the ratio of the resistance values between the resistor R 51 - 2  included in the second switched resistor  510 - 2  and the resistor R 52  of the second resistor circuit part  520  is 1:0.9, the feedback voltage Vfb at the connection node between the first and second resistor circuit parts  510  and  520  is equal to the reference voltage V ref  (e.g., 2.5V), and the feedback voltage regulating unit  500  may provide the output voltage Vout of 4.75V, obtained by multiplying the feedback voltage Vfb by 1.9 times. 
     For another example, if the supply voltage VDD is 4.8V, when only the third control signal SC 3  of the first to fourth control signals SC 1  to SC 4  is the switching-on level, only the third switched resistor  510 - 3  of the first to fourth switched resistor  510 - 1  to  510 - 4  is closed. 
     At this time, if the ratio of the resistance values between the resistor R 51 - 3  included in the third switched resistor  510 - 3  and the resistor R 52  of the second resistor circuit part  520  is 1:0.8, the feedback voltage Vfb at the connection node between the first and second resistor circuit parts  510  and  520  is equal to the reference voltage V ref  (e.g., 2.5V), and the feedback voltage regulating unit  500  may provide the output voltage Vout of 4.5V, obtained by multiplying the feedback voltage Vfb by 1.8 times. 
     For another example, if the supply voltage VDD is 4.32V, when only the fourth control signal SC 4  of the first to fourth control signals SC 1  to SC 4  is the switching-on level, only the fourth switched resistor  510 - 4  of the first to fourth switched resistor  510 - 1  to  510 - 4  is closed. 
     At this time, if the ratio of the resistance values between the resistor R 51 - 4  included in the fourth switched resistor  510 - 4  and the resistor R 52  of the second resistor circuit part  520  is 1:0.7, the feedback voltage Vfb at the connection node between the first and second resistor circuit parts  510  and  520  is equal to the reference voltage V ref  (e.g., 2.5V), and the feedback voltage regulating unit  500  may provide the output voltage Vout of 4.25V, obtained by multiplying the feedback voltage Vfb by 1.7 times. 
       FIG. 10  is a flowchart illustrating the operation method of the voltage regulator according to the embodiment of the present invention, and  FIG. 11  is a flowchart illustrating the process of providing the feedback control signal according to the embodiment of the present invention. 
     The operation method of the voltage regulator according to the embodiment of the present invention will be described with reference to  FIGS. 1 to 11 . 
     In describing the operation method of the voltage regulator according to the embodiment of the present invention, the above descriptions with reference to  FIGS. 1 to 9  may be equally applied, and thus the same descriptions will not be repeated. 
     Referring to  FIG. 10 , in the operation method of the voltage regulator of a low-drop-output type mentioned earlier, the voltage detecting unit  300  may detect the supply voltage VDD to provide the detected voltage Vd (S 100 ). 
     Then, the feedback control unit  400  may provide the feedback control signal SC according to the detected voltage Vd (S 200 ). 
     The feedback voltage regulating unit  500  connected between the semiconductor switch  200  and the ground may regulate the feedback voltage Vfb according to the feedback control signal SC (S 300 ). 
     The providing of the feedback control signal SC (S 200 ) will be described with reference to  FIG. 11 . 
     First, the feedback control unit  400  may compare the detected voltage Vd with each of the first to nth reference voltages V ref1  to V refN  to provide first to Nth comparison signals S com1  to S comN  (S 210 ). 
     Then, the first to Nth comparison signals S com1  to S comN  may be decoded into the first to nth control signals SC 1  to SCN, respectively, and to be provided to the feedback voltage regulating unit  500  (S 210 ). 
     At this time, the first to Nth comparison signals S com1  to S comN  may have a high level if the detected voltage Vd is higher than a corresponding reference voltage and has a low level otherwise. 
     As set forth above, according to the embodiments of the present invention, an offset occurring when a supply voltage varies is removed, so that stable operation is maintained even at a low supply voltage. 
     While the present invention has been shown and described in connection with the embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.