Linear voltage regulating circuit with undershoot minimization and method thereof

[0001] This invention generally relates to voltage regulation, and more particularly, to a linear voltage regulating circuit with undershoot minimization and a method thereof.

[0002] 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 MP_X coupled between a supply voltage V_CC and an output voltage V_OUT; an amplifier A₁ coupled to the pass transistor MP_X for controlling the response of the pass transistor MP_X by comparing a reference voltage V_REF and a feedback voltage V_FB; a feedback circuitry connected between the output node V_OUT and the amplifier A₁ for delivering the feedback voltage V_FB. Additionally, the output voltage V_OUT, inducing a load current I_LOAD, is coupled to a load device modeled by a load resistor R_ESR and a load capacitor C_L.

[0003] 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 MP_X in time. A large current from the MP_X therefore results, and acts to immediately charge the load capacitor C_L to increase the output voltage V_OUT. 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 V_OUT should still be high enough to turn off the pass transistor MP_X. 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 I_LOAD transitioning between a light load and heavy load, the output current will only be supplied from the load capacitor C_L. This consequently decreases the output voltage V_OUT.

[0004] When the output voltage V_OUT is lower than the desired voltage level, the regulator loop can be activated to restore the output voltage V_OUT to the desired level. However, due to loop bandwidth limitations, the output voltage V_OUT will supply an undershot voltage to the load device before the pass transistor MP_X can be turned on. Moreover, in turning on the pass transistor MP_X that is initially turned off, the large gate capacitance of the pass transistor MP_X will consume a large amount current. This acts to further worsen the undershot output voltage V_OUT. An undershoot output voltage V_OUT can therefore seriously hinder the operation of a load device.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0016] 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.

[0017] 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 V_in, and its gate connected to an error amplifier 212. Additionally, a feedback circuit, as shown in FIG. 2, couples the output voltage V_out 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.

[0018] 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 V_p, 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 V_p 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 V_p, the second current mirror module 250 is disabled without mirroring any current.

[0019] A load device is coupled to the output of the linear regulator 210 and powered by the regulated output voltage V_out and its accompanying output current. For simplicity, the load device is represented by an equivalent RC circuit, comprising a resistor R_L and a capacitor C_out coupled in parallel.

[0020] 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 C_out, thus increasing V_out. Subsequently, V_f also increases. However, due to the slew rate of the error amplifier 212, the pass voltage V_p does not increase quickly enough in response to the increased feedback voltage V_f. Therefore, after a single loop delay, the error amplifier 212 produces a pass voltage V_p high enough to turn off the pass transistor MP. It should be noted that the output voltage V_out 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.

[0021] 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 C_out and regulates the output voltage V_out. 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 V_out 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 V_out 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 V_in. 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.

[0022] 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 V_p, 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 V_p into two currents, I₂₁ and I₂₂, each flowing through the first and second converting nodes N₁ and N₂ respectively.

[0023] 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 V_in. 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 V_in 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 V_p, the second current mirror module 350 is disabled without mirroring any current.

[0024] 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 R_L and a capacitor C_out coupled in parallel.

[0025] 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 C_out, thus increasing V_out. Subsequently, V_f also increases. However, due to the slew rate of the error amplifier 312, the pass voltage V_p does not increase quickly enough to respond to the increased feedback voltage V_f. Therefore, after a single loop delay period, the error amplifier 312 will produce a pass voltage V_p high enough to turn off the pass transistor MP. It should be noted that the output voltage V_out 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 V_in, 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 C_out, and regulates the output voltage V_out. 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 V_out 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.

[0026] 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.

[0027] 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.

[0028] 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 V_p 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 V_out to recover from the overshoot condition into the under regulation condition. After the output voltage V_out 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.

[0029] 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.

[0030] 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 V_p 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 V_out to recover from the overshoot condition into the under regulation condition. After the output voltage V_out 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.

[0031] 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.

[0032] 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:

• • [0033] Step 610: Provide a voltage regulator having a first output producing the regulated output voltage and a second output producing a pass voltage;
  • [0034] 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;
  • [0035] Step 630: Couple a capacitive device to the first converting node;
  • [0036] 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
  • [0037] 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.

[0038] 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.

[0039] 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.