Patent Publication Number: US-10775822-B2

Title: Circuit for voltage regulation and voltage regulating method

Description:
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. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiments that are illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an embodiment of the circuit for voltage regulation of the present invention. 
         FIG. 2  shows an embodiment of the adaptive pre-driver of  FIG. 1 . 
         FIG. 3 a    shows an embodiment of the buffer circuit of  FIG. 2 . 
         FIG. 3 b    shows an embodiment of the buffer circuit of  FIG. 2 . 
         FIG. 4  shows an embodiment of the adaptive current source of  FIG. 2 . 
         FIG. 5  shows an embodiment of the voltage regulating method of the present invention. 
     
    
    
     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. 1  shows an embodiment of the circuit for voltage regulation of the present invention. As shown in  FIG. 1 , the circuit  100  for voltage regulation includes an amplifier  110 , an adaptive pre-driver  120 , a driving circuit  130  and a negative feedback circuit  140 . The amplifier  110  is configured to generate an amplifier output signal V AMP  according to a reference voltage V REF  and a negative feedback voltage V F . The adaptive pre-driver  120  is configured to generate a bias current I B  according to a current-dependent signal S I , in which the current-dependent signal S I  varies with the variation of an output current I OUT  synchronously or asynchronously and thereby the bias current I B  varies with the variation of the output current I OUT  synchronously or asynchronously; people of ordinary skill in the art can appreciate how to generate/use a signal (e.g., the amplifier output signal V AMP  or a signal derived from the output signal I OUT ) varying with the variation of the output current I OUT . Consequently, the output impedance of the adaptive pre-driver  120 , which is dependent on the bias current I B , varies with the variation of the output current I OUT . In this embodiment, as the output current I OUT  increases or decreases, the impedance of the adaptive pre-driver  120  changes proportionally (i.e., increases as the output current I OUT  increases, or decreases as the output current I OUT  decreases). The driving circuit  130  includes a transistor (e.g., PMOS transistor) or the equivalent thereof. The driving circuit  130  is coupled between a terminal of a power source voltage V IN  and an output terminal  132  and configured to output an output voltage \T OUT  and the output current I OUT  through the output terminal  132 ; in addition, as shown in  FIG. 1 , the driving circuit  130  may be configured to generate the current-dependent signal S I  that varies with the output current I OUT , but the present invention is not limited thereto. The output voltage V OUT  and the output current I OUT  are outputted to a load R L  (e.g., one or more internal circuits that are integrated into an integrated circuit, along with the circuit  100 ) so that the output current I OUT  is dependent on the output voltage V OUT  and the equivalent impedance of the load R L . In addition, the output terminal  132  can be optionally coupled with a capacitor C L  (e.g., an external capacitor that is set on a printed circuit board) to stabilize the output voltage V OUT . Each of the load R L  and the capacitor C L  can be included in the circuit  100  or set outside the circuit  100 . The negative feedback circuit  140  is coupled between the output terminal  132  and the amplifier  110  and configured to generate the negative feedback voltage V F  according to the output voltage V OUT . In this embodiment, the negative feedback circuit  140  includes two voltage-dividing resistors R 1 , R 2  and the resistances of the two resistors R 1 , R 2  could be the same or different. People carrying out the present invention can determine the resistances of the two resistors R 1 , R 2  or use a known or self-developed negative feedback circuit to replace the negative feedback circuit  140 . 
       FIG. 2  shows an embodiment of the adaptive pre-driver  120 . As shown in  FIG. 2 , the adaptive pre-driver  120  includes a buffer circuit  210  and an adaptive current source  220 . The buffer circuit  210  can be a diode-connected MOS circuit as shown in  FIG. 3 a    or the equivalent of the diode-connected MOS circuit as shown in  FIG. 3 b   . The buffer circuit  210  not only receives the aforementioned power source voltage V IN  but also connects with the amplifier  110  and the driving circuit  130  so as to receive the amplifier output signal V AMP  and output the bias current I B  according to the amplifier output signal V AMP . The adaptive current source  220  is configured to control the bias current I B  according to the current-dependent signal S I , in which at least a part of the bias current I B  passes through the buffer circuit  210  and thus the output impedance of the buffer circuit  210  is dependent on the at least a part of the bias current I B . In this embodiment, when the at least a part of the bias current I B  increases or decreases, the output impedance of the buffer circuit  210  changes inversely proportionally (i.e., decreases as the at least a part of the bias current I B  increases, or increases as the at least a part of the bias current I B  decreases). 
       FIG. 4  shows an embodiment of the adaptive current source  220 . In  FIG. 4  the driving circuit  130  is a PMOS transistor, the buffer circuit  210  is a diode-connected MOS circuit, the adaptive current source  220  includes a detecting circuit  410  and a current mirror  420 . The detecting circuit  410  is coupled between the terminal of the power current voltage V IN  and the current mirror  420  and configured to generate a detection current I S  according to the current-dependent signal S I  which is the amplifier output signal V Amp  here. The current mirror  420  is configured to generate a mirror current I M  as the bias current I B  according to the detection current I S . According to  FIG. 4 , when the amplifier output signal V AMP  (i.e., the current-dependent signal S I  here) decreases due to a heavy load, the output current I OUT  of the driving circuit  130  and the detection current I S  of the detecting circuit  410  increase; meanwhile, the mirror current I M  (i.e., the bias current I B ) increases as the detection current I S  increases; therefore, the at least a part of the bias current I B , which passes through the buffer circuit  210 , increases, and this leads to the decrease of the equivalent impedance of the buffer circuit  210  and has a pole contributed by the equivalent impedance of the buffer circuit  210  and the equivalent capacitance of the driving circuit  130  (including the parasitic capacitance) be moved to a position of higher frequency; accordingly, the pole does not fall within the gain bandwidth of the circuit  100  because 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 circuit  100  is enhanced. Similarly, when the amplifier output signal V AMP  (i.e., the current-dependent signal S I  here) increases due to a light load, the output current I OUT  of the driving circuit  130  and the detection current I S  of the detecting circuit  410  decrease; meanwhile, the mirror current I M  (i.e., the bias current I B ) decreases as the detection current I S  decreases; therefore, the at least a part of the bias current I B , which passes through the buffer circuit  210 , decreases and this leads to the increase of the equivalent impedance of the buffer circuit  210  and has a pole contributed by the equivalent impedance of the buffer circuit  210  and the equivalent capacitance of the driving circuit  130  (including the parasitic capacitance) be moved to a position of lower frequency; since the gain bandwidth of the circuit  100  is 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 circuit  100 , the phase margin is still enough and the stability of the circuit  100  is ensured. 
     It should be noted that although in the embodiment of  FIG. 4 , the current-dependent signal S I  is the amplifier output signal V AMP , 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 I OUT  can be treated as the current-dependent signal S I  on condition that the adaptive current source  220  is capable of changing the bias current I B  according to the current-dependent signal S I . 
     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 in  FIG. 5  and includes the following steps:
     step S 510 : generating an amplifier output signal according to a reference voltage and a negative feedback voltage. This step can be carried out by the aforementioned amplifier  110  or the equivalent thereof.   step S 520 : generating an output voltage and an output current according to the amplifier output signal. This step can be carried out by the aforementioned driving circuit  130  or the equivalent thereof.   step S 530 : 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-driver  120  or the equivalent thereof.   step S 540 : generating the negative feedback voltage according to the output voltage. This step can be carried out by the aforementioned negative feedback circuit  140  or 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. 
     The aforementioned descriptions represent merely the preferred embodiments of the present invention, without any intention to limit the scope of the present invention thereto. Various equivalent changes, alterations, or modifications based on the claims of present invention are all consequently viewed as being embraced by the scope of the present invention.