Abstract:
Provided is a voltage regulator that is excellent in transient response characteristics even if a resistance of a resistor in a phase compensation circuit is large. In the voltage regulator, the resistor of the phase compensation circuit is so configured as to change the resistance thereof according to a voltage across both ends of the resistor. In a transient state in which an output voltage of an error amplifier circuit changes, the resistance of the resistor in the phase compensation circuit is decreased, to thereby improve the transient response characteristics of the voltage regulator.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a voltage regulator that outputs a constant output voltage, and more particularly, to a phase compensation circuit that stabilizes an operation of a voltage regulator. 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 3  is a circuit diagram illustrating a conventional voltage regulator. 
         [0005]    The conventional voltage regulator includes an output transistor  21  that outputs an output voltage Vout, a voltage divider circuit  22  that divides the output voltage Vout, and a reference voltage circuit  23  that generates a reference voltage. The conventional voltage regulator also includes an error amplifier circuit  24  that controls the output transistor  21  so as to hold the output voltage Vout constant on the basis of the output voltage of the voltage divider circuit  22  and the reference voltage, and a phase compensation circuit  20  that is disposed between the output transistor  21  and the error amplifier circuit  24 , and compensates a phase of an output terminal  20   d  of the phase compensation circuit  20 . The phase compensation circuit  20  has a phase compensation capacitor  20   a  and a phase compensation resistor  20   b  (for example, see JP 2005-215897 A). 
         [0006]    In the phase compensation circuit  20  of the voltage regulator, a resistance of the phase compensation resistor  20   b  may be set to be large in order to achieve a stable operation of the voltage regulator. 
         [0007]    When the output voltage Vout of the voltage regulator changes, an output voltage of the error amplifier circuit  24  also changes. In a transient state in which the output voltage of the error amplifier circuit  24  changes, when the resistance of the phase compensation resistor  20   b  is large, it takes time to charge and discharge a gate of the output transistor  21 . 
         [0008]      FIGS. 4A and 4B  are graphs each illustrating an input voltage and an output voltage, respectively, of the phase compensation circuit of the conventional voltage regulator. 
         [0009]    When an input voltage V 1  of the phase compensation circuit  20  changes as illustrated in  FIG. 4A , an output voltage V 2  of the phase compensation circuit  20  changes as illustrated in  FIG. 4B . When the resistance of the phase compensation resistor  20   b  is small, the output voltage V 2  changes as indicated by a dotted line in  FIG. 4B . On the other hand, when the resistance of the phase compensation resistor  20   b  is large, the output voltage V 2  changes as indicated by a solid line in  FIG. 4B . 
         [0010]    That is, there arises such a problem that transient response characteristics of the phase compensation circuit  20  are deteriorated, and therefore the transient response characteristics of the voltage regulator are deteriorated. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention has been made in view of the above-mentioned problem, and therefore an object of the present invention is to provide a voltage regulator that is excellent in transient response characteristics even if a resistance of a phase compensation resistor is large. 
         [0012]    In order to solve the above-mentioned problem, in the voltage regulator according to the present invention, a resistor of a phase compensation circuit is so configured as to change the resistance thereof according to a voltage across both ends of the resistor. In a transient state in which an output voltage of an error amplifier circuit changes, the resistance of the resistor in the phase compensation circuit is decreased, to thereby improve the transient response characteristics of the voltage regulator without sacrificing a performance of the phase compensation circuit. 
         [0013]    In the voltage regulator according to the present invention, in the transient state in which the output voltage of the error amplifier circuit changes, the resistance of the resistor in the phase compensation circuit is decreased, to thereby improve the transient response characteristics of the phase compensation circuit. Accordingly, the resistance of the resistor in the phase compensation circuit can be set to be large, and the transient response characteristics of the voltage regulator are excellent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]    In the accompanying drawings: 
           [0015]      FIG. 1  is a circuit diagram illustrating a voltage regulator according to the present invention; 
           [0016]      FIGS. 2A and 2B  are graphs each illustrating an input voltage and an output voltage, respectively, of a phase compensation circuit in the voltage regulator according to the present invention; 
           [0017]      FIG. 3  is a circuit diagram illustrating a conventional voltage regulator; and 
           [0018]      FIGS. 4A and 4B  are graphs each illustrating an input voltage and an output voltage, respectively, of a phase compensation circuit in the conventional voltage regulator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]      FIG. 1  is a circuit diagram illustrating a voltage regulator according to the present invention. 
         [0020]    The voltage regulator according to the present invention includes a phase compensation circuit  10 , an output transistor  11 , a voltage divider circuit  12 , a reference voltage circuit  13 , an error amplifier circuit  14 , a power supply terminal  15 , an output terminal  16 , and a ground terminal  17 . The phase compensation circuit  10  includes a phase compensation capacitor  10   a , phase compensation resistors  10   b  and  10   c , a control transistor  10   d , an input terminal  10   e , an input terminal  10   f , and an output terminal  10   g.    
         [0021]    The phase compensation circuit  10  has the input terminal  10   e  connected to an output terminal of the error amplifier circuit  14 , the input terminal  10   f  connected to the power supply terminal  15 , and the output terminal  10   g  connected to a gate of the output transistor  11 . The output transistor  11  has a source and a back gate connected to the power supply terminal  15 , and a drain connected to the output terminal  16 . The voltage divider circuit  12  is disposed between the output terminal  16  and the ground terminal  17 , and an output terminal of the voltage divider circuit  12  is connected to a non-inverting input terminal of the error amplifier circuit  14 . The reference voltage circuit  13  is disposed between an inverting input terminal of the error amplifier circuit  14  and the ground terminal  17 . 
         [0022]    The phase compensation capacitor  10   a  has one end connected to the input terminal  10   e  of the phase compensation circuit  10 , and another end connected to the output terminal  10   g  of the phase compensation circuit  10 . The phase compensation resistor  10   b  has one end connected to the input terminal  10   e  of the phase compensation circuit  10 , and another end connected to a gate of the control transistor  10   d . The phase compensation resistor  10   c  has one end connected to the gate of the control transistor  10   d , and another end connected to the output terminal  10   g  of the phase compensation circuit  10 . The control transistor  10   d  has a source connected to the input terminal  10   e  of the phase compensation circuit  10 , a drain connected to the output terminal  10   g  of the phase compensation circuit  10 , and a back gate connected to the input terminal  10   f  of the phase compensation circuit  10 . 
         [0023]    The voltage regulator described above operates as follows. 
         [0024]    The output transistor  11  outputs an output voltage Vout. The voltage divider circuit  12  divides the output voltage Vout. The reference voltage circuit  13  generates a reference voltage. The error amplifier circuit  14  outputs a control signal that controls the output transistor  11  so as to hold the output voltage Vout constant on the basis of the output voltage of the voltage divider circuit  12  and the reference voltage. 
         [0025]    When the output voltage Vout decreases, the output voltage of the voltage divider circuit  12  also decreases. When the output voltage of the voltage divider circuit  12  is lower than the reference voltage, an output voltage of the error amplifier circuit  14  and an input voltage V 1  of the phase compensation circuit  10  decrease. Control is made to decrease a gate voltage of the output transistor  11  and increase the output voltage Vout according to the control signal through the phase compensation circuit  10 . Further, when the output voltage Vout increases, the gate voltage of the output transistor  11  increases, and the output voltage Vout decreases under the control. Accordingly, control is made to keep the output voltage Vout constant. 
         [0026]    Subsequently, a description is given of an operation of the phase compensation circuit  10  of the voltage regulator according to the present invention. The phase compensation circuit  10  compensates a phase of the control signal which is output from the error amplifier circuit  14 . In particular, a capacitance of the phase compensation capacitor  10   a  and resistances of the phase compensation resistors  10   b  and  10   c  are so set as not to oscillate the voltage regulator. 
         [0027]    First, a description is given of a transient state in a case where a voltage change of the output voltage Vout is small. 
         [0028]    When a voltage drop of the output voltage Vout is small, a voltage difference between the input voltage V 1  and an output voltage V 2  of the phase compensation circuit  10  is small. Accordingly, since the control transistor  10   d  is off, the phase compensation circuit  10  is configured such that the phase compensation capacitor  10   a,  and the phase compensation resistors  10   b  and  10   c  are connected in parallel to each other. 
         [0029]    Subsequently, a description is given of a transient state in a case where the voltage change of the output voltage Vout is large. 
         [0030]    When the voltage drop of the output voltage Vout is large, the input voltage V 1  of the phase compensation circuit  10  largely decreases. In this situation, when the resistance of the phase compensation circuit  10  is high, the voltage difference between the input voltage V 1  and the output voltage V 2  is large. The voltage difference is divided by the phase compensation resistors  10   b  and  10   c , and then applied to the gate of the control transistor  10   d , whereby the control transistor  10   d  turns on. Therefore, the phase compensation circuit  10  is configured such that the phase compensation capacitor  10   a , the phase compensation resistors  10   b  and  10   c , and the control transistor  10   d  are connected in parallel. In this state, since the control transistor  10   d  turns on, the resistance of a resistor between the input terminal  10   e  and the output terminal  10   g  of the phase compensation circuit  10  becomes small. That is, transient response characteristics of the phase compensation circuit  10  become excellent. Further, when an increase in the voltage of the output voltage Vout is large, the transient response characteristics of the phase compensation circuit  10  become excellent by turning on the control transistor  10   d  in the same manner as that described above. 
         [0031]      FIGS. 2A and 2B  are graphs each illustrating the input voltage and the output voltage, respectively, of the phase compensation circuit in the voltage regulator according to the present invention. 
         [0032]    According to the phase compensation circuit of the present invention, when the input voltage V 1  of the phase compensation circuit  10  changes as illustrated in  FIG. 2A , the output voltage V 2  of the phase compensation circuit  10  changes at high speed as illustrated in  FIG. 2B , as compared with  FIG. 4B . 
         [0033]    In this example, it is assumed that the input voltage of the phase compensation circuit  10  is V 1 , the output voltage is V 2 , and a threshold value of the control transistor  10   d  is Vthp. Then, when the resistances of the phase compensation resistors  10   b  and  10   c  are equal to each other, a condition under which the control transistor  10   d  turns on is given by Expression 1. 
         [0000]      | V 1 −V 2|/2 &gt;|Vthp|   (1). 
         [0034]    When the resistances of the phase compensation resistors  10   b  and  10   c  are equal to each other, timing at which the control transistor  10   d  turns on is equal between when the output voltage Vout decreases and when the output voltage Vout increases. That is, in the transient state in which the output voltage Vout changes, transient response characteristics of the control transistor  11  are identical between when the output voltage Vout decreases and when the output voltage Vout increases. 
         [0035]    Further, it is assumed that the resistance of the phase compensation resistor  10   b  is R 1 , and the resistance of the phase compensation resistor  10   c  is R 2 . Then, in a case where the resistances of the phase compensation resistor  10   b  and the phase compensation resistor  10   c  are different from each other, the condition under which the control transistor  10   d  turns on is represented by Expression 2 when the output voltage Vout decreases, and by Expression 3 when the output voltage Vout increases. 
         [0000]      ( V 2 −V 1)× R 1/( R 1 +R 2)&gt;| Vthp|   (2) 
         [0000]      ( V 1 −V 2)× R 2/( R 1 +R 2)&gt;| Vthp|   (3) 
         [0036]    As described above, when the resistances of the phase compensation resistor  10   b  and the phase compensation resistor  10   c  differ from each other, adjustment can be made such that the transient response characteristics in the case where the output voltage Vout increases are excellent, or the transient response characteristics in the case where the output voltage Vout decreases are excellent. 
         [0037]    The back gate of the control transistor  10   d  is connected to the power supply terminal  15 . Alternatively, the back gate can be connected to a node whose voltage is higher than the voltage of the source and the drain other than the power supply terminal  15 . 
         [0038]    Further, the control transistor  10   d  is a PMOS transistor, but May be an NMOS transistor. In this case, the back gate of the control transistor  10   d  is connected to a node whose voltage is lower than the voltage of the source and the drain.