# Patent

## Title
Linear voltage regulating circuit with undershoot minimization and method thereof

## Abstract

A voltage regulating circuit for providing a regulated output voltage. The voltage regulating circuit includes a voltage regulator, a converting circuit, a capacitive device, a first current mirror module, and a second current mirror module. The voltage regulator has a first output producing the regulated output voltage and a second output producing a pass voltage. The converting circuit converts the pass voltage into a first current and a second current passing through a first converting node and a second converting node respectively, where the first current charges/discharges the capacitive device. The first current mirror module has a first current mirror path coupled to the first converting node and a second current mirror path coupled to the second converting node. The second current mirror module has a first current mirror path coupled to the second converting node and a second current mirror path coupled to the first output.

## Background
BACKGROUND
This invention generally relates to voltage regulation, and more particularly, to a linear voltage regulating circuit with undershoot minimization and a method thereof.
A regulator, coupled between a voltage supply source and a load device, is used to provide a sufficiently constant output current to maintain the drive of a load device. When the load device undergoes a rapid load current transition, where current draw or load impedance alternates between a heavy load and light load, a typical regulator can have several shortcomings. FIG. 1 illustrates such a typical voltage regulator 100 according to the related art. This related art voltage regulator 100 suffers from an undershoot problem when the load device undergoes a rapid transition between a heavy load and light load. The voltage regulator 100 includes a pass transistor MPX coupled between a supply voltage VCC and an output voltage VOUT; an amplifier A1 coupled to the pass transistor MPX for controlling the response of the pass transistor MPX by comparing a reference voltage VREF and a feedback voltage VFB; a feedback circuitry connected between the output node VOUT and the amplifier A1 for delivering the feedback voltage VFB. Additionally, the output voltage VOUT, inducing a load current ILOAD, is coupled to a load device modeled by a load resistor RESR and a load capacitor CL.
Due to loop bandwidth limitations in the load transient response of a transition from a heavy load to light load, the voltage regulator 100 is unable to turn off the pass transistor MPX in time. A large current from the MPX therefore results, and acts to immediately charge the load capacitor CL to increase the output voltage VOUT. This forces the voltage regulator 100 to enter a voltage overload condition. Upon stabilization of the voltage overload condition through the regulator loop, the output voltage VOUT should still be high enough to turn off the pass transistor MPX. However, the charge from the voltage overload stored in capacitor CL will undergo an exponential decay through the feedback network established by resistors R1 and R2. During the time interval between the removal of the output current load, and the appropriate response of the amplifier A1, the output voltage remains unregulated. Meanwhile, if the load device consumes the output current, such as in a case of load current ILOAD transitioning between a light load and heavy load, the output current will only be supplied from the load capacitor CL. This consequently decreases the output voltage VOUT.
When the output voltage VOUT is lower than the desired voltage level, the regulator loop can be activated to restore the output voltage VOUT to the desired level. However, due to loop bandwidth limitations, the output voltage VOUT will supply an undershot voltage to the load device before the pass transistor MPX can be turned on. Moreover, in turning on the pass transistor MPX that is initially turned off, the large gate capacitance of the pass transistor MPX will consume a large amount current. This acts to further worsen the undershot output voltage VOUT. An undershoot output voltage VOUT can therefore seriously hinder the operation of a load device.
U.S. Pat. No. 5,894,227 teaches a voltage regulator utilizing a comparator C1 to compare the gate voltage of the pass transistor and a reference voltage VTRIP in order to control a discharge transistor MPD. However, due to variations in fabrication processes, the reference voltage VTRIP may be set too high. This affects the operation of the discharge transistor MPD, and degrades the overall voltage regulation efficiency under a light load.
Other related art voltage regulators, such as that described in U.S. Pat. No. 5,966,004 and U.S. Pat. No. 6,201,375, utilize a regulator loop with an offset voltage to turn on the discharge transistor when the output voltage is higher than a reference voltage. Although the regulator loop may quickly discharge an initial output voltage, this voltage regulator still suffers from the same problem as described above. When the output voltage becomes lower than the reference voltage, the discharge path is identical to that mentioned in U.S. Pat. No 5,894,227. Since the discharge path still comprises a resistor network, recovery from an unregulated voltage condition may not be any faster due to the regulator loop.
SUMMARY
Therefore, it is an objective of the present invention to provide a linear voltage regulating circuit with undershoot minimization and a method thereof. This circuit is intended to quickly restore the output voltage from an overshoot condition, and to provide proper voltage regulation under normal operation.
According to an embodiment of the present invention, a voltage regulating circuit for providing a regulated output voltage is disclosed. The voltage regulating circuit comprises a linear voltage regulator having a first output producing the linear output voltage and a second output producing a pass voltage, a converting circuit for converting the pass voltage into a first current and a second current passing through a first converting node and a second converting node respectively, a capacitive device coupled to the first converting node, a first current mirror module comprising a first current mirror path coupled to the first converting node and a second current mirror path coupled to the second converting node, and a second current mirror module comprising a first current mirror path coupled to the second converting node and a second current mirror path coupled to the first output. The capacitive device is capable of maintaining the first current mirror module for a charging/discharging period to allow the output voltage to recover from an overshoot condition. The output will be restored into a regulated condition when a load device experiences a transition from a heavy load to light load.
These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a voltage regulator of the related art.
FIG. 2 illustrates a circuit diagram of a linear voltage regulating circuit according to a first embodiment of the present invention.
FIG. 3 illustrates a circuit diagram of a linear voltage regulating circuit according to a second embodiment of the present invention.
FIG. 4 illustrates a circuit diagram of a linear voltage regulating circuit according to a third embodiment of the present invention.
FIG. 5 illustrates a circuit diagram of a linear voltage regulating circuit according to a fourth embodiment of the present invention.
FIG. 6 is a flowchart illustrating a method for providing a regulated output voltage according to a fifth embodiment of the present invention.



DETAILED DESCRIPTION
Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.
FIG. 2 illustrates a circuit diagram of a linear voltage regulating circuit 200 according to a first embodiment of the present invention. The linear voltage regulating circuit 200 comprises a linear regulator 210, a converting circuit 220, a capacitive device (in this embodiment the capacitive device is implemented by a capacitor C1), a first current mirror module 240, and a second current mirror module 250. The linear regulator 210 comprises a pass transistor (PMOS transistor) MP having its drain connected to a voltage divider built by two resistors R11, R12, its source connected to a first reference voltage Vin, and its gate connected to an error amplifier 212. Additionally, a feedback circuit, as shown in FIG. 2, couples the output voltage Vout and the error amplifier 212. Since the operation of the linear regulator 210 is well known to those skilled in this art, further description is omitted for brevity.
The converting circuit 220 comprises a plurality of transistors M11, M12, M13, M14, where transistor M11 is a PMOS transistor and transistors M12, M13 and M14 are NMOS transistors. As shown in FIG. 2, the gate of the transistor M11 is connected to the gate of the pass transistor MP. Therefore, as the pass transistor MP is turned on from the pass voltage Vp, transistor M11 will also turned on. In the converting circuit 220, the transistors M12 and M13 establish a current mirror to serve as a first current generator for a first current I11, and transistors M12 and M14 establish another current mirror to serve as a second current generator for producing a second current I12. In summary, the converting circuit 220 is used to convert the pass voltage Vp into two currents I11 and I12, each flowing through a first and second converting node N1 and N2, respectively. The capacitive device, implemented by the capacitor C1, has one of its ends coupled to the converting node N11 and the other end coupled to ground. The capacitor C1 is implemented to have a large capacitance. The first current mirror module 240, which mirrors the first current I11 to generate a third current I13, comprises two transistors M15 and M16, where transistor M15 is diode-connected resulting in capacitor C1 being coupled to the first converting node N11. Please note that the current mirror ratios of the aforementioned current mirrors are properly designed such that the second current I12 is greater than the third current I13. Therefore, the voltage level at the second converting node N12 is pulled down approximately to ground voltage due to the second current I12, and transistors M17 and M18 in the second current mirror module 250 accordingly being turned off. In other words, as the pass transistor MP is turned on due to the pass voltage Vp, the second current mirror module 250 is disabled without mirroring any current.
A load device is coupled to the output of the linear regulator 210 and powered by the regulated output voltage Vout and its accompanying output current. For simplicity, the load device is represented by an equivalent RC circuit, comprising a resistor RL and a capacitor Cout coupled in parallel.
Under a load transient response of the linear regulator 210, when the transition from a heavy load to a light load occurs, the large output current that passes through the load device will suddenly decreases to become the small or zero output current. The current flowing through MP is forced to flow to the capacitor Cout, thus increasing Vout. Subsequently, Vf also increases. However, due to the slew rate of the error amplifier 212, the pass voltage Vp does not increase quickly enough in response to the increased feedback voltage Vf. Therefore, after a single loop delay, the error amplifier 212 produces a pass voltage Vp high enough to turn off the pass transistor MP. It should be noted that the output voltage Vout is charged to an overshoot output voltage immediately because of the loop delay time. As the pass transistor MP is turned off, transistors M11, M12, M13, and M14 are turned off accordingly. The diode-connected transistor M15 and transistor M16 in the first current mirror module 240, however, remain on and generate the third current I13 due to the capacitive device (i.e. the capacitor C1). Since transistor M13 is turned off, the current passing through the transistor M15 is forced to charge the capacitor C1 via the current path built between the gate and drain of the transistor M15.
According to the present invention, the value of capacitor C1 is large enough to maintain the first current mirror module 240 being activated for a charging period. Because transistor M14 is turned off, the voltage level at the second converting node N12 is no longer pulled down to the ground voltage, and the second current mirror module 250 is activated to induce a discharge current I14 in response to the received second current I12. The discharge current I14 then discharges the capacitor Cout and regulates the output voltage Vout. In this embodiment, the discharge current I14 is designed to be a percentage of the output current provided to the load device and is in proportion to the output current provided to the load device operating under a heavy load condition. This is because the larger the output current in heavy load condition, the higher the peak of the output voltage Vout when the transition from heavy load to light load occurs. Therefore, since the discharge current I14 depends upon the output current in heavy load condition, the linear voltage regulating circuit 200 can quickly recover from the undershoot condition to the under regulation condition. Please note that the capacitance of capacitor C1 should be properly designed such that the second current mirror module 250 remains on until the output voltage Vout has recovered from the overshoot status to the under regulation condition. After the charging period expires, transistors M15 and M16 are turned off because the gate voltage is pulled to approach Vin. Since there is no current flowing into the transistor M17, discharge current I14 is not induced and the linear voltage regulating circuit 200 enters into a steady light load condition.
FIG. 3 illustrates a circuit diagram of a linear voltage regulating circuit 300 according to a second embodiment of the present invention. The linear voltage regulating circuit 300 comprises a linear regulator 310, a converting circuit 320, a capacitive device (in this embodiment the capacitive device is implemented by a capacitor C2), a first current mirror module 340, and a second current mirror module 350. The configuration of the linear regulator 310 shown in FIG. 3 is identical to that of the linear regulator 210 shown in FIG. 2, and as such further description is omitted for brevity. In this embodiment, the converting circuit 320 comprises two transistors; (PMOS transistors) M21 and M22 coupled to a first converting node N21 and a second converting node N22 respectively. As shown in FIG. 3, the gates of the transistors M21 and M22 are both connected to the gate of the pass transistor MP. Therefore, as the pass transistor MP is turned on due to the pass voltage Vp, the transistors M21 and M22 are turned to pass a first current I21 and a second current I22 respectively. In short, the converting circuit 320 is used to convert the pass voltage Vp into two currents, I21 and I22, each flowing through the first and second converting nodes N1 and N2 respectively.
The capacitive device, implemented by capacitor C2, has one of its ends coupled to the first converting node N21, and the other end coupled to a first reference voltage Vin. In addition, capacitor C2 has large value capacitance. The first current mirror module 340, which mirrors the first current I21 to generate a third current I23, comprises two transistors M23 and M24, where the transistor M23 is diode-connected to make capacitor C2 coupled to the first converting node N21. The current mirror ratio of the first current mirror module 340 is properly implemented such that the third current I23 is smaller than the second current I22. This results in the voltage level at the second converting node N22 being pulled up to approximately that of the first reference voltage Vin due to the second current I22, and transistors M25-M28 in the second current mirror module 350 being turned off accordingly. In other words, as the pass transistor MP is turned on due to the pass voltage Vp, the second current mirror module 350 is disabled without mirroring any current.
Similar to the previous exemplary embodiment, the load device coupled to the output of the linear regulator 310 is represented by an equivalent RC circuit including a resistor RL and a capacitor Cout coupled in parallel.
In the load transient response of the linear regulator 310, when the transition from a heavy load to light load occurs, a large output current passing through the load device suddenly decreases to become a small or zero output current. The current flowing through MP is forced to flow to the capacitor Cout, thus increasing Vout. Subsequently, Vf also increases. However, due to the slew rate of the error amplifier 312, the pass voltage Vp does not increase quickly enough to respond to the increased feedback voltage Vf. Therefore, after a single loop delay period, the error amplifier 312 will produce a pass voltage Vp high enough to turn off the pass transistor MP. It should be noted that the output voltage Vout is immediately charged to an overshoot output voltage because of the loop delay period. As the pass transistor MP is turned off, transistors M21 and M22 are also accordingly turned off. The diode-connected transistor M23 and transistor M24 in the first current mirror module 340 however still remain on, and generate a third current I23 to the capacitive device (i.e. the capacitor C2). Since transistor M21 is turned off, the current passing through transistor M23 is forced to discharge into capacitor C2. According to the present invention, the capacitance of capacitor C2 is large enough to maintain the first current mirror module 340 being on for the discharging period. Because transistor M22 is turned off, the voltage level at the second converting node N22 is no longer pulled up to the first reference voltage Vin, and the second current mirror module 350 is activated to induce a discharge current I24 in response to the received second current I23. The discharge current I24 then discharges the capacitor Cout, and regulates the output voltage Vout. Similar to the design of the above embodiment, the discharge current I24 is also configured to be proportional to the output current provided to the load device operating under a heavy load condition. As a result, the linear voltage regulating circuit 300 can quickly recover from the undershoot condition into the under regulation condition. Additionally, the capacitance of the capacitor C2 should be properly designed such that the second current mirror module 350 remains on until the output voltage Vout has recovered from the overshoot status to the under regulation condition. After the discharging period expires, the transistors M23 and M24 are turned off because the gate voltage is pulled down to approach the ground voltage. Since there is no current flowing into the transistor M25, discharge current I24 is not induced and the linear voltage regulating circuit 300 enters a steady light load condition.
The capacitive devices in the embodiments shown in FIG. 2 and FIG. 3, which require large capacitances, can be implemented by metal-insulator-metal (MiM) capacitors. However, larger capacitances require larger chip areas, which greatly increase the production cost. Therefore, the present invention further makes use of a capacitance boost technique for obtaining large capacitances using small chip area.
FIG. 4 illustrates a circuit diagram of a linear voltage regulating circuit 400 according to a third embodiment of the present invention. The linear voltage regulating circuit 400 comprises a linear regulator 210, a converting circuit 220, a capacitive device 430, a first current mirror module 240, and a second current mirror module 250. The linear voltage regulating circuit 400 shown in FIG. 4 appears similar to the linear voltage regulating circuit 200 shown in FIG. 2. The key difference is the inclusion of a capacitive device 430, which is not a single capacitor having a large capacitance value. In this embodiment, the capacitive device 430 includes a plurality of transistors M41-M43 and a capacitor C3 having small capacitance, where the diode-connected transistor M42 and the transistor M43 form a current mirror. The aspect ratio (W/L) of the transistor M42 is K1, and the aspect ratio (W/L) of the transistor M43 is K2, where the ratio of K2/K1 is defined to be K (K>1) in order to implement the capacitance boost. The operation of the capacitance boost is detailed as follows.
In the load transient response of the linear regulator 210 of the linear voltage regulating circuit 400, during the transition from heavy load to light load, the boosted pass voltage Vp acts to turn off transistor M41. As described above, transistors M15 and M16 still remain on. In addition, transistors M42 and M43 are turned on to form a current mirror, where the current passing through transistor M43 is K times as great as the current passing through transistor M42. Since these two current mirror paths share the same current source, (i.e. the drain current outputted from the transistor M15) the equivalent capacitive load viewed by the transistor M15 is substantially equal to (1+K)*C3. In this embodiment, K is defined to be significantly greater than one. The equivalent capacitive load viewed by transistor M15 therefore is substantially equal to K*C3. Please note that capacitor C3 has a small capacitance such that the chip area for implementing the capacitive device 430 is small. Accordingly, the gate voltage of transistors M15 and M16 is slowly increased because of the large capacitance load of value K*C3. Therefore, the capacitive device 430 is capable of maintaining the first current mirror module 240 being turned on during a charging period to allow the output voltage Vout to recover from the overshoot condition into the under regulation condition. After the output voltage Vout is restored to the under regulation condition, transistor M41, which is a long-channel transistor, is turned on and its drain current becomes equal to the drain current of transistor M42. As a result, no further current is provided to charge the capacitor C3.
FIG. 5 illustrates a circuit diagram of a linear voltage regulating circuit 500 according to a fourth embodiment of the present invention. The linear voltage regulating circuit 500 comprises a linear regulator 310, a converting circuit 320, a capacitive device 530, a first current mirror module 340, and a second current mirror module 350. The linear voltage regulating circuit 500 shown in FIG. 5 is similar to the linear voltage regulating circuit 300 of FIG. 3. The key difference is the capacitive device 530, which is not simply a single capacitor having large capacitance. In this embodiment, the capacitive device 530 includes a plurality of transistors M51-M53, a capacitor C4 having small capacitance, and a diode-connected transistor M52 coupled to transistor M53 which form a current mirror. The aspect ratio (W/L) of the transistor M52 is K1, and the aspect ratio (W/L) of the transistor M53 is K2, where the ratio of K2/K1 is defined to be K (K>1) in order to implement the capacitance boost. The operation of the capacitance boost is detailed as follows.
In the load transient response of the linear regulator 310 of the linear voltage regulating circuit 500, when the transition from a heavy load to a light load occurs, the boosted pass voltage Vp turns off transistors M21 and M22. As described above, transistors M23 and M24 still remain on. As a result, the gate voltage of transistor M51 is pulled down to approach ground voltage, causing transistor M51 to turn off. However, transistors M52 and M53 are turned on to form a current mirror, where the current passing through the transistor M53 is K times the current passing through the transistor M52. Since these two current mirror paths share the same current source, (i.e. the drain current outputted from the transistor M23) the equivalent capacitive load viewed by transistor M23 is substantially equal to (1+K)*C4. In this embodiment, K is defined to be significantly greater than one. The equivalent capacitive load viewed by the transistor M15 therefore simplifies to approximate K*C4. Please note that capacitor C4 has a small capacitance such that the chip area for implementing the capacitive device 530 is small. Accordingly, the gate voltage of transistors M23 and M24 is slowly decreased because due to the large capacitance K*C4. Therefore, the capacitive device 530 is capable of maintaining the first current mirror module 340 to remain on for the discharging period, allowing the output voltage Vout to recover from the overshoot condition into the under regulation condition. After the output voltage Vout enters the under regulation condition, transistor M51, which is a long-channel transistor, is turned on and its source current becomes equal to the source current of transistor M52. As a result, no current is provided to discharge capacitor C4.
Please note that the circuit configurations of the above embodiments shown in FIG. 2 to FIG. 5 are only for illustrative purposes, and are not meant to provide limitations of the present invention.
A method for providing a regulated output voltage is further disclosed, as shown in FIG. 6, used to facilitate the device described above. Provided that substantially the same result is achieved, the steps of process 600 below need not be in the exact order shown and need not be contiguous, that is, other steps can be intermediate. The method comprises:

    
    
        Step 610: Provide a voltage regulator having a first output producing the regulated output voltage and a second output producing a pass voltage;
        Step 620: Convert the pass voltage into a first current and a second current, and pass the first current and the second current at a first converting node and a second converting node, respectively;
        Step 630: Couple a capacitive device to the first converting node;
        Step 640: Couple a first current mirror path to the first converting node and a second current mirror path to the second converting node, wherein the first current mirror path corresponds to the second current mirror path; and
        Step 650: Couple a third current mirror path to the second converting node and a fourth current mirror path to the first output, wherein the third current mirror path corresponds to the fourth current mirror path.
    
    


Briefly summarized, the present disclosure provides a capacitive device that is capable of maintaining a first current mirror module remaining on for a charging/discharging period, and a method thereof. This allows the output voltage to recover from an overshoot condition, and enter an under regulation condition when the load device has a transition from a heavy load to a light load.
Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

## Claims

1. A voltage regulating circuit for providing a regulated output voltage, comprising:
a voltage regulator having a first output producing the regulated output voltage and a second output producing a pass voltage;
a converting circuit, coupled to the voltage regulator, for converting the pass voltage into a first current and a second current, wherein the converting circuit has a voltage input node coupled to the second output for receiving the pass voltage, a first converting node, and a second converting node, the first current flows through the first converting node, and the second current flows through the second converting node;
a capacitive device coupled to the first converting node;
a first current mirror module comprising a first current mirror path coupled to the first converting node and a second current mirror path coupled to the second converting node; and
a second current mirror module comprising a first current mirror path coupled to the second converting node and a second current mirror path coupled to the first output.

2. The voltage regulating circuit of claim 1 wherein the voltage regulator comprises:
an error amplifier having a first input coupled to a first reference voltage, a second input, and an error output coupled to the second output;
a pass transistor having a gate coupled to the second output, a first electrode coupled to a second reference voltage, and a second electrode coupled to the first output; and
a feedback circuit coupled between the first output and the second input.

3. The voltage regulating circuit of claim 1 wherein the converting circuit further comprises:
a transistor having a gate coupled to the second output, a first electrode coupled to a reference voltage, and a second electrode;
a first current generator, coupled to the first converting node and the second electrode, for generating the first current; and
a second current generator, coupled to the second converting node and the second electrode, for generating the second current.

4. The voltage regulating circuit of claim 3 wherein the first and second current generators are current mirrors having a common diode-connected transistor.

5. The voltage regulating circuit of claim 1 wherein the capacitive device is a single capacitor.

6. The voltage regulating circuit of claim 1 wherein the capacitive device comprises:
a third current mirror module comprising a first current mirror path, and a second current mirror path coupled to the first converting node, wherein a current mirror ratio of the second current mirror path of the third current mirror module to the first current mirror path of the third current mirror module is greater than one;
a capacitor, coupled between the first current mirror path of the third current mirror module and the first converting node; and
a transistor having a gate coupled to the second output, a first electrode coupled to a reference voltage, and a second electrode coupled to the first current mirror path of the third current mirror module.

7. The voltage regulating circuit of claim 6 wherein the transistor is a long-channel transistor.

8. The voltage regulating circuit of claim 1 wherein the converting circuit comprises:
a first transistor having a gate coupled to the second output, a first electrode coupled to a reference voltage, and a second electrode coupled to the first converting node; and
a second transistor having a gate coupled to the second output, a first electrode coupled to the reference voltage, and a second electrode coupled to the second converting node.

9. The voltage regulating circuit of claim 1 wherein the capacitive device comprises:
a third current mirror module comprising a first current mirror path, and a second current mirror path coupled to the first converting node, wherein a current mirror ratio of the second current mirror path of the third current mirror module to the first current mirror path of the third current mirror module is greater than one;
a capacitor, coupled between the first current mirror path of the third current mirror module and the first converting node; and
a transistor having a gate coupled to the first converting node, a first electrode coupled to a reference voltage, and a second electrode coupled to the first current mirror path of the third current mirror module.

10. The voltage regulating circuit of claim 9 wherein the transistor is a long-channel transistor.

11. The voltage regulating circuit of claim 1 wherein the second current is greater than the first current.

12. A method for providing a regulated output voltage, comprising:
(a) providing a voltage regulator having a first output producing the regulated output voltage and a second output producing a pass voltage;
(b) converting the pass voltage into a first current and a second current, and passing the first current and the second current at a first converting node and a second converting node, respectively;
(c) coupling a capacitive device to the first converting node;
(d) coupling a first current mirror path to the first converting node and a second current mirror path to the second converting node, wherein the first current mirror path corresponds to the second current mirror path; and
(e) coupling a third current mirror path to the second converting node and a fourth current mirror path to the first output, wherein the third current mirror path corresponds to the fourth current mirror path.

13. The method of claim 12 wherein step (b) is performed by:
providing a transistor having a gate coupled to the second output;
mirroring a current passing through the first transistor to generate the first current; and
mirroring the current passing through the first transistor to generate the second current.

14. The method of claim 12 wherein the capacitive device is a single capacitor.

15. The method of claim 12 wherein step (c) further comprises:
providing the capacitive device a current mirror module comprising a first current mirror path, and a second current mirror path coupled to the first converting node, wherein a current mirror ratio of the second current mirror path of the current mirror module to the first current mirror path of the current mirror module is greater than one;
providing the capacitive device a capacitor, coupled between the first current mirror path of the current mirror module and the first converting node;
enabling the current mirror module for charging/discharging the capacitor when the regulated output voltage enters an overshoot condition; and
stopping the current mirror module from charging/discharging the capacitor when the regulated output voltage enters an under regulation condition.

16. The method of claim 12 wherein step (b) is performed by:
providing a first transistor, having a gate coupled to the second output, for outputting the first current; and
providing a second transistor, having a gate coupled to the second output, for outputting the second current.

17. The method of claim 16 wherein step (c) further comprises:
providing the capacitive device a current mirror module comprising a first current mirror path, and a second current mirror path coupled to the first converting node, wherein the current mirror ratio of the second current mirror path of the current mirror module to the first current mirror path of the current mirror module is greater than one;
providing the capacitive device a capacitor, coupled between the first current mirror path of the current mirror module and the first converting node;
enabling the current mirror module for charging/discharging the capacitor when the regulated output voltage enters a overshoot condition; and
stopping the current mirror module from charging/discharging the capacitor when the regulated output voltage enters an under regulation condition.

18. The method of claim 12 wherein the second current is greater than the first current.