Circuit for voltage regulation and voltage regulating method

Disclosed is a circuit for voltage regulation, the circuit including: an amplifier operable to generate an amplifier output signal according to a reference voltage and a negative feedback voltage; an adaptive pre-driver operable to generate a bias current according to the amplifier output signal or according to the amplifier output signal and a current-dependent signal that varies with the variation of an output current, in which the bias current varies with the variation of the output current; a driving circuit operable to generate an output voltage and the output current according to the amplifier output signal; and a negative feedback circuit operable to generate the negative feedback voltage according to the output voltage. Since the bias current varies with the variation of the output current, the output impedance of the adaptive pre-driver varies with the variation of the output current so that the stability of the circuit is improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a regulator and a regulating method, especially to a circuit for voltage regulation and a voltage regulating method.

2. Description of Related Art

An electronic product usually has a power source such as a battery or a power supply. However, a high-frequency interference from the outside or interferences of other frequencies from the inside of the electronic product may cause the output voltage of the power source unstable so that the performance of an IC inside the electronic product may be affected. In order to prevent such problems, a low dropout regulator (LDO) is introduced for providing a stable output voltage.

A general LDO includes an amplifier, a transistor and a feedback circuit. The amplifier is configured to generate an amplifier output signal according to a reference voltage and a feedback voltage. The transistor is coupled between a power source terminal and an output terminal and configured to regulate the output current of the transistor according to the amplifier output signal so as to regulate the output voltage of the output terminal. The feedback circuit is configured to generate the feedback voltage according to the output voltage. Although the above-described LDO is operable to provide a stable output voltage, if the LDO is required to output a large current when necessary (i.e., if the output current of the transistor is a large current due to a heavy load), the transistor should have a high driving capability; consequently, the circuit area of the transistor is very large and the parasitic capacitance of the transistor in the aspect of the amplifier is very large as well, and thus the large parasitic capacitance causes the frequency response of the LDO unstable. This kind of LDO is found in the following literature: US patent of patent publication number US 20020005711 A1.

In order to solve the problems of the aforementioned LDO, some technique sets a pre-driver between the amplifier and the transistor so as to increase the stability of the LDO by the setting of the output impedance of the pre-driver. However, since the output impedance of the pre-driver is fixed, this technique cannot cope with a circumstance that the output current of the transistor varies dramatically; in other words, this technique can stabilize the LDO when the load is light (i.e., the output current of the transistor is very small), but cannot stabilize the LDO when the load is heavy. The above-described technique is found in the following literature: U.S. Pat. No. 6,246,221 B1.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a circuit for voltage regulation and a voltage regulating method capable of preventing the problems of the prior art.

An embodiment of the circuit for voltage regulation of the present invention includes an amplifier, an adaptive pre-driver, a driving circuit and a feedback circuit. The amplifier is configured to generate an amplifier output signal according to a reference voltage and a negative feedback voltage. The adaptive pre-driver is configured to generate a bias current according to the amplifier output signal or according to the amplifier output signal and a current-dependent signal that varies with the variation of an output current, in which the bias current varies with the variation of the output current. The driving circuit is configured to generate an output voltage and the output current according to the amplifier output signal. The negative feedback circuit is configured to generate the negative feedback voltage according to the output voltage. Since the bias current varies with the variation of the output current, the output impedance of the adaptive pre-driver, which is dependent on the bias current, and the frequency response of the circuit for voltage regulation affected by the output impedance vary with the variation of the output current, and thereby the stability of the circuit for voltage regulation is improved.

The voltage regulating method of the present invention is carried out by the circuit for voltage regulation of the present invention or the equivalent thereof. An embodiment of the voltage regulating method includes the following steps: generating an amplifier output signal according to a reference voltage and a negative feedback voltage; generating an output voltage and an output current according to the amplifier output signal; generating a bias current according to the amplifier output signal or according to the amplifier output signal and a current-dependent signal that varies with the variation of the output current, in which the bias current varies with the variation of the output current; and generating the negative feedback voltage according to the output voltage. Since the bias current varies with the variation of the output current, the output impedance dependent on the bias current and the frequency response of the circuit for voltage regulation affected by the output impedance vary with the variation of the output current, and thereby the stability of the circuit for voltage regulation is improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is written by referring to terms acknowledged in this industrial field. If any term is defined in the following description, such term should be explained accordingly.

The present invention discloses a circuit for voltage regulation and a voltage regulating method capable of adaptively adjusting a frequency response in accordance with the variation of an output current and thereby improving the stability of voltage regulation.

FIG. 1shows an embodiment of the circuit for voltage regulation of the present invention. As shown inFIG. 1, the circuit100for voltage regulation includes an amplifier110, an adaptive pre-driver120, a driving circuit130and a negative feedback circuit140. The amplifier110is configured to generate an amplifier output signal VAMPaccording to a reference voltage VREFand a negative feedback voltage VF. The adaptive pre-driver120is configured to generate a bias current IBaccording to a current-dependent signal SI, in which the current-dependent signal SIvaries with the variation of an output current IOUTsynchronously or asynchronously and thereby the bias current IBvaries with the variation of the output current IOUTsynchronously or asynchronously; people of ordinary skill in the art can appreciate how to generate/use a signal (e.g., the amplifier output signal VAMPor a signal derived from the output signal IOUT) varying with the variation of the output current IOUT. Consequently, the output impedance of the adaptive pre-driver120, which is dependent on the bias current IB, varies with the variation of the output current IOUT. In this embodiment, as the output current IOUTincreases or decreases, the impedance of the adaptive pre-driver120changes proportionally (i.e., increases as the output current IOUTincreases, or decreases as the output current IOUTdecreases). The driving circuit130includes a transistor (e.g., PMOS transistor) or the equivalent thereof. The driving circuit130is coupled between a terminal of a power source voltage VINand an output terminal132and configured to output an output voltage \TOUTand the output current IOUTthrough the output terminal132; in addition, as shown inFIG. 1, the driving circuit130may be configured to generate the current-dependent signal SIthat varies with the output current IOUT, but the present invention is not limited thereto. The output voltage VOUTand the output current IOUTare outputted to a load RL(e.g., one or more internal circuits that are integrated into an integrated circuit, along with the circuit100) so that the output current IOUTis dependent on the output voltage VOUTand the equivalent impedance of the load RL. In addition, the output terminal132can be optionally coupled with a capacitor CL(e.g., an external capacitor that is set on a printed circuit board) to stabilize the output voltage VOUT. Each of the load RLand the capacitor CLcan be included in the circuit100or set outside the circuit100. The negative feedback circuit140is coupled between the output terminal132and the amplifier110and configured to generate the negative feedback voltage VFaccording to the output voltage VOUT. In this embodiment, the negative feedback circuit140includes two voltage-dividing resistors R1, R2and the resistances of the two resistors R1, R2could be the same or different. People carrying out the present invention can determine the resistances of the two resistors R1, R2or use a known or self-developed negative feedback circuit to replace the negative feedback circuit140.

FIG. 2shows an embodiment of the adaptive pre-driver120. As shown inFIG. 2, the adaptive pre-driver120includes a buffer circuit210and an adaptive current source220. The buffer circuit210can be a diode-connected MOS circuit as shown inFIG. 3aor the equivalent of the diode-connected MOS circuit as shown inFIG. 3b. The buffer circuit210not only receives the aforementioned power source voltage VINbut also connects with the amplifier110and the driving circuit130so as to receive the amplifier output signal VAMPand output the bias current IBaccording to the amplifier output signal VAMP. The adaptive current source220is configured to control the bias current IBaccording to the current-dependent signal SI, in which at least a part of the bias current IBpasses through the buffer circuit210and thus the output impedance of the buffer circuit210is dependent on the at least a part of the bias current IB. In this embodiment, when the at least a part of the bias current IBincreases or decreases, the output impedance of the buffer circuit210changes inversely proportionally (i.e., decreases as the at least a part of the bias current IBincreases, or increases as the at least a part of the bias current IBdecreases).

FIG. 4shows an embodiment of the adaptive current source220. InFIG. 4the driving circuit130is a PMOS transistor, the buffer circuit210is a diode-connected MOS circuit, the adaptive current source220includes a detecting circuit410and a current mirror420. The detecting circuit410is coupled between the terminal of the power current voltage VINand the current mirror420and configured to generate a detection current ISaccording to the current-dependent signal SIwhich is the amplifier output signal VAmphere. The current mirror420is configured to generate a mirror current IMas the bias current IBaccording to the detection current IS. According toFIG. 4, when the amplifier output signal VAMP(i.e., the current-dependent signal SIhere) decreases due to a heavy load, the output current IOUTof the driving circuit130and the detection current ISof the detecting circuit410increase; meanwhile, the mirror current IM(i.e., the bias current IB) increases as the detection current ISincreases; therefore, the at least a part of the bias current IB, which passes through the buffer circuit210, increases, and this leads to the decrease of the equivalent impedance of the buffer circuit210and has a pole contributed by the equivalent impedance of the buffer circuit210and the equivalent capacitance of the driving circuit130(including the parasitic capacitance) be moved to a position of higher frequency; accordingly, the pole does not fall within the gain bandwidth of the circuit100because the gain bandwidth is also moved to a position of higher frequency due to the heavy load, the phase margin increases and the stability of the circuit100is enhanced. Similarly, when the amplifier output signal VAMP(i.e., the current-dependent signal SIhere) increases due to a light load, the output current IOUTof the driving circuit130and the detection current ISof the detecting circuit410decrease; meanwhile, the mirror current IM(i.e., the bias current IB) decreases as the detection current ISdecreases; therefore, the at least a part of the bias current IB, which passes through the buffer circuit210, decreases and this leads to the increase of the equivalent impedance of the buffer circuit210and has a pole contributed by the equivalent impedance of the buffer circuit210and the equivalent capacitance of the driving circuit130(including the parasitic capacitance) be moved to a position of lower frequency; since the gain bandwidth of the circuit100is also moved to a position of more lower frequency due to the light load, the pole does not fall within the gain bandwidth of the circuit100, the phase margin is still enough and the stability of the circuit100is ensured.

It should be noted that although in the embodiment ofFIG. 4, the current-dependent signal SIis the amplifier output signal VAMP, this is not a limitation to the implementation of the present invention. People of ordinary skill in the art can appreciate that a signal capable of varying with the variation of the output current IOUTcan be treated as the current-dependent signal SIon condition that the adaptive current source220is capable of changing the bias current IBaccording to the current-dependent signal SI.

The voltage regulating method of the present invention is carried out by the circuit for voltage regulation of the present invention or the equivalent thereof. An embodiment of the voltage regulating method is shown inFIG. 5and includes the following steps:step S510: generating an amplifier output signal according to a reference voltage and a negative feedback voltage. This step can be carried out by the aforementioned amplifier110or the equivalent thereof.step S520: generating an output voltage and an output current according to the amplifier output signal. This step can be carried out by the aforementioned driving circuit130or the equivalent thereof.step S530: generating a bias current according to the amplifier output signal or according to the amplifier output signal and a current-dependent signal that varies with the variation of the output current, in which the bias current varies with the variation of the output current. This step can be carried out by the aforementioned adaptive pre-driver120or the equivalent thereof.step S540: generating the negative feedback voltage according to the output voltage. This step can be carried out by the aforementioned negative feedback circuit140or the equivalent thereof.

Since those of ordinary skill in the art can appreciate the detail and the modification of the voltage regulating method of the present invention according to the disclosure of the circuit for voltage regulation of the present invention, which implies that the features of the circuit for voltage regulation can be applied to the voltage regulating method in a reasonable manner, repeated and redundant description is omitted here while the requirements of enablement and written description are still fulfilled.

It should be noted that people of ordinary skill in the art can implement the present invention by selectively using some or all of the features of any embodiment in this specification or selectively using some or all of the features of multiple embodiments in this specification as long as such implementation is practicable, which implies that the present invention can be carried out flexibly. It should also be noted that in the embodiments of this specification, “executing an operation according to a signal” or the like can be interpreted as receiving the signal to execute the operation or receiving the derivative of the signal to execute the operation, in which the derivative of the signal could be an amplified/attenuated/delayed/reversed signal of the signal and can be determined by those carrying out the present invention in accordance with their demand.

To sum up, the circuit for voltage regulation and the voltage regulating method of the present invention can adaptively adjust a frequency response according to the variation of an output current and thereby increase the stability of voltage regulation.