Abstract:
Provided is a voltage regulator capable of suppressing fluctuation in a limited current. The voltage regulator includes: a first differential amplifier circuit configured to compare a voltage based on an output voltage and a reference voltage to each other, to thereby output a first voltage; a second differential amplifier circuit configured to compare the first voltage and a second voltage to each other, to thereby output a third voltage; a first transistor configured to receive the third voltage at a gate thereof such that the output voltage is generated at a drain thereof; a second transistor, which includes a gate connected in common to the gate of the first transistor and has a predetermined size ratio to the first transistor; and a voltage generating unit, which includes one end connected to a drain of the second transistor and is configured to generate the second voltage at the one end.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2016-051497 filed on Mar. 15, 2016, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a voltage regulator, and more particularly, to a voltage regulator having an overcurrent protection function. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 4  is a circuit diagram for illustrating a related-art voltage regulator  300 . 
         [0006]    The related-art voltage regulator  300  includes a power supply terminal  301 , a ground terminal  302 , a reference voltage source  310 , an error amplifier circuit  311 , resistors  312 ,  317 ,  318 , and  319 , an NMOS transistor  316 , PMOS transistors  313 ,  314 , and  315 , and an output terminal  320 . 
         [0007]    The PMOS transistor  315  has a source connected to the power supply terminal  301 , and a drain connected to the output terminal  320  and one end of the resistor  318 . The resistor  318  has another end connected to one end of the resistor  319  and a non-inverting input terminal of the error amplifier circuit  311 . The resistor  319  has another end connected to the ground terminal  302 . The PMOS transistor  314  has a source connected to the power supply terminal  301 , and a drain connected to one end of the resistor  317  and a gate of the NMOS transistor  316 . The PMOS transistor  313  has a source connected to the power supply terminal  301 , a drain connected to a gate of the PMOS transistor  315 , a gate of the PMOS transistor  314 , and an output of the error amplifier circuit  311 . The resistor  312  has one end connected to the power supply terminal  301 , and another end connected to a gate of the PMOS transistor  313  and a drain of the NMOS transistor  316 . The error amplifier circuit  311  has an inverting input terminal connected to one end of the reference voltage source  310 . The reference voltage source  310  has another end connected to the ground terminal  302 . The NMOS transistor  316  has a source connected to the ground terminal  302 . 
         [0008]    The related-art voltage regulator  300  operates such that, through a negative feedback circuit forming of the error amplifier circuit  311 , the PMOS transistor  315 , and the resistors  318  and  319 , a voltage at the one end of the resistor  319  is equal to a voltage VREF at the reference voltage source  310 . 
         [0009]    When a current that flows to a load (not shown) connected to the output terminal  320  increases in this state, a drain current I 1  of the PMOS transistor  315  increases. Then, a drain current I 2  of the PMOS transistor  314 , which is formed to have a predetermined size ratio to the PMOS transistor  315 , also increases. The current I 2  is supplied to the resistor  317  such that a voltage Vx is generated at the one end of the resistor  317 . When the voltage Vx increases to exceed a threshold of the NMOS transistor  316 , the NMOS transistor  316  is turned on, to thereby generate a drain current. The drain current of the NMOS transistor  316  is supplied to the resistor  312 , such that a voltage at the other end thereof decreases, to thereby turn on the PMOS transistor  313 . When the PMOS transistor  313  is turned on, a gate voltage of the PMOS transistor  315  increases, thereby limiting the drain current I 1 . 
         [0010]    Now, when a resistance value of the resistor  317  is represented by R 1 , the size ratio between the PMOS transistors  315  and  314  is represented by K, and a threshold voltage of the NMOS transistor  316  is represented by |VTHN|, a limited current I 1   m  of the current I 1  is expressed by Expression (1). 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                      
                     
                         
                     
                      
                     1 
                      
                     m 
                   
                   = 
                   
                     
                       K 
                       × 
                       VTHN 
                     
                     
                       R 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0011]    As described above, the related-art voltage regulator  300  has an overcurrent protection function, and an output current may be limited when the load is short-circuited, for example (see, for example, Japanese Patent Application Laid-open No. 2003-29856). 
         [0012]    However, the related-art voltage regulator  300  has a problem in that fluctuation in the limited current I 1   m  is large. This is because fluctuation in the threshold voltage VTHN affects the limited current I 1   m , as can be seen in Expression (1). 
         [0013]      FIG. 5  is a graph for showing a waveform of an output voltage VOUT relative to an output current IOUT of the related-art voltage regulator  300 . The dotted lines indicate a fluctuation range of the limited current. In general, the fluctuation in the threshold voltage VTHN is about ±0.1 from a center value of 0.6 V, and hence the fluctuation in the limited current I 1   m  caused by the threshold voltage VTHN is ±16.7%, which is a very large fluctuation. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention has been made in order to solve the above-mentioned problem, and provides a voltage regulator capable of suppressing fluctuation in a limited current. 
         [0015]    According to one embodiment of the present invention, there is provided a voltage regulator including: a first differential amplifier circuit configured to compare a voltage based on an output voltage and a reference voltage to each other, to thereby output a first voltage; a second differential amplifier circuit configured to compare the first voltage and a second voltage to each other, to thereby output a third voltage; a first transistor configured to receive the third voltage at a gate of the first transistor such that the output voltage is generated at a drain of the first transistor; a second transistor, which includes a gate connected in common to the gate of the first transistor and has a predetermined size ratio to the first transistor; and a voltage generating unit, which includes one end connected to a drain of the second transistor and is configured to generate the second voltage at the one end. 
         [0016]    According to the voltage regulator of the present invention, the first voltage, which is an output voltage of the first differential amplifier circuit, is a reference value for a limited current of a drain current of the first transistor, and the second voltage, which is generated by the second transistor and the voltage generating unit, is a value in proportion to the drain current of the first transistor. Those first and second voltages are compared to each other by the second differential amplifier circuit, which forms a negative feedback circuit with the second transistor and the voltage generating unit, to thereby achieve an overcurrent protection. At this time, fluctuation in the limited current, which is a criterion for determining an overcurrent, is almost completely dependent on fluctuation in the reference voltage. Therefore, for example, by generating the reference voltage using a voltage source in which fluctuation is significantly small, for example, a bandgap voltage source, the fluctuation in the limited current can be suppressed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a circuit diagram for illustrating a voltage regulator of a first embodiment of the present invention. 
           [0018]      FIG. 2  is a graph for showing a waveform of an output voltage VOUT relative to an output current of the voltage regulator of  FIG. 1 . 
           [0019]      FIG. 3  is a circuit diagram for illustrating a voltage regulator of a second embodiment of the present invention. 
           [0020]      FIG. 4  is a circuit diagram of the related-art voltage regulator. 
           [0021]      FIG. 5  is a graph for showing a waveform of the output voltage VOUT relative to an output current of the voltage regulator of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    Now, embodiments of the present invention are described with reference to the drawings. 
         [0023]      FIG. 1  is a circuit diagram for illustrating a voltage regulator  100  of a first embodiment of the present invention. 
         [0024]    The voltage regulator  100  of this embodiment includes a power supply terminal  101 , a ground terminal  102 , a first differential amplifier circuit  127 , a second differential amplifier circuit  128 , a voltage generating unit  129 , PMOS transistors  112  and  113 , a reference voltage source  114 , resistors  124  and  125 , and an output terminal  126 . 
         [0025]    The first differential amplifier circuit  127  includes PMOS transistors  115  and  116 , NMOS transistors  117  and  118 , and a current source  110 . 
         [0026]    The second differential amplifier circuit  128  includes NMOS transistors  119  and  120 , a current source  111 , and a resistor  121 . 
         [0027]    The voltage generating unit  129  includes a PMOS transistor  123  and a resistor  122 . 
         [0028]    The PMOS transistor  113  has a source connected to the power supply terminal  101 , and a drain connected to the output terminal  126  and one end of the resistor  125 . The PMOS transistor  112  has a source connected to the power supply terminal  101 , and a drain connected to one end of the voltage generating unit  129  (source of PMOS transistor  123 ) and a gate of the NMOS transistor  120 . The current source  111  has one end connected to the power supply terminal  101 , and another end connected to a drain of the NMOS transistor  119 , a gate of the PMOS transistor  112 , and a gate of the PMOS transistor  113 . The resistor  125  has another end connected to one end of the resistor  124  and a gate of the PMOS transistor  116 . The resistor  124  has another end connected to the ground terminal  102 . The PMOS transistor  123  has a gate connected to a drain thereof and one end of the resistor  122 . Another end of the resistor  122  (another end of voltage generating unit  129 ) is connected to the ground terminal  102 . The NMOS transistor  120  has a drain connected to the power supply terminal  101 , and a source connected to a source of the NMOS transistor  119  and one end of the resistor  121 . The resistor  121  has another end connected to the ground terminal  102 . The current source  110  has one end connected to the power supply terminal  101 , and another end connected to a source of the PMOS transistor  115  and a source of the PMOS transistor  116 . The PMOS transistor  115  has a gate connected to one end of the reference voltage source  114 , and a drain connected to a gate and a drain of the NMOS transistor  117 . The reference voltage source  114  has another end connected to the ground terminal  102 . The PMOS transistor  116  has a drain connected to a gate of the NMOS transistor  119  and a drain of the NMOS transistor  118 . The NMOS transistor  118  has a gate connected to the gate of the NMOS transistor  117 , and a source connected to the ground terminal  102 . The NMOS transistor  117  has a source connected to the ground terminal  102 . 
         [0029]    In the first differential amplifier circuit  127 , the gate of the PMOS transistor  115  and the gate of the PMOS transistor  116  are inputs, and the drain of the PMOS transistor  116  is an output. In the second differential amplifier circuit  128 , the gate of the NMOS transistor  119  and the gate of the NMOS transistor  120  are inputs, and the drain of the NMOS transistor  119  is an output. 
         [0030]    For illustrative purposes, a drain current of the PMOS transistor  113  is represented by I 1 , and a drain current of the PMOS transistor  112  is represented by I 2 . The PMOS transistor  112  has a predetermined size ratio to the PMOS transistor  113 , and is configured to operate as a replica element. Further, a voltage at the output terminal  126 , a gate voltage of the NMOS transistor  120 , a gate voltage of the NMOS transistor  119 , a voltage at the another end of the current source  110 , a voltage at the one end of the resistor  121 , and a voltage at the one end of the reference voltage source  114  are represented by VOUT, VG 2 , VG 1 , VS 1 , VS 2 , and VREF, respectively. Further, a resistance value of the resistor  122  is represented by R, a voltage at the one end of the resistor  124  is represented by VFB, and a voltage at the another end of the current source  111  is represented by VGATE. 
         [0031]    Next, operation of the voltage regulator  100  having the above-mentioned configuration is described. 
         [0032]    A first state in which a load current supplied to the output terminal  126  is much smaller than the limited current is described. 
         [0033]    In this case, the current I 1  and the current I 2 , which is determined by the size ratio between the PMOS transistor  113  and the PMOS transistor  112 , each have a small current value. Further, the current I 2  is supplied to the voltage generating unit  129 , and hence the voltage VG 2 , which is generated at the one end of the voltage generating unit  129 , also has a small value. When the voltage VG 2  is below a threshold of the NMOS transistor  120 , the NMOS transistor  120  is off. 
         [0034]    In this situation, the first differential amplifier circuit  127  compares the voltage VREF and the voltage VFB to each other, and then amplifies a difference therebetween to output the voltage VG 1 . In the second differential amplifier circuit  128 , the NMOS transistor  120  is off. Thus, the voltage VG 1  is amplified by the NMOS transistor  119 , the resistor  121 , and the current source  111  such that the voltage VGATE is output. The PMOS transistor  113  receives the voltage VGATE at the gate thereof to generate the drain current I 1 , and then supplies the drain current I 1  to a load (not shown) connected to the output terminal  126 . 
         [0035]    The voltage VOUT is divided by the resistor  125  and the resistor  124  so that the divided voltage is input to the first differential amplifier circuit  127 . Through the loop as described above, a negative feedback functions and the first differential amplifier circuit  127  operates such that the voltage VREF and the voltage VFB become equal to each other. 
         [0036]    A second state in which the load current increases as compared to the first state is described. 
         [0037]    When a current that flows to the load (not shown) connected to the output terminal  126  increases, the current I 1  of the PMOS transistor  113  and the current I 2  of the PMOS transistor  112  each increase. As a result, the voltage VG 2  also increases, to thereby turn on the NMOS transistor  120 . Thus, the drain current of the NMOS transistor  120  is supplied to the resistor  121 , and the voltage VS 2  rises. 
         [0038]    It may be thought that the NMOS transistor  119  is turned off because a gate-source voltage thereof reduces. However, due to the function of the negative feedback, the NMOS transistor  119  is not turned off. In particular, through the function of the negative feedback, the voltage regulator  100  operates such that the voltage VREF and the voltage VFB become equal to each other. Thus, when the voltage VS 2  rises, the voltage VG 1  is increased by a corresponding amount. As a result, a predetermined voltage difference is maintained between the gate and the source of the NMOS transistor  119 . In other words, even if the load current increases to thereby increase the voltage VG 2 , the predetermined voltage VOUT may be obtained. 
         [0039]    A third state in which the load current further increases as compared to the second state such that the overcurrent protection function is put into operation is described. 
         [0040]    When the current that flows to the load (not shown) connected to the output terminal  126  further increases, the voltage VG 1  rises in the same mechanism as in the second state, but an upper limit of a voltage value of the voltage VG 1  is limited by the voltage VS 1 . The voltage VS 1  is determined by a sum of the voltage VREF and an absolute value |VGSP 1 | of the gate-source voltage of the PMOS transistor  115 , and is expressed by Expression (2). 
         [0000]      VS1=VREF+|VGSP1|  (2)
 
         [0041]    When the voltage VG 2  becomes equal to the voltage VS 1 , the gate-source voltage of the NMOS transistor  119  decreases. Thus, when the drain current of the NMOS transistor  119  decreases, the voltage VGATE increases, thereby limiting the drain current I 1  of the PMOS transistor  113 . When an absolute value of a gate-source voltage of the PMOS transistor  123  is represented by |VGSP 2 |, and the size ratio between the PMOS transistors  113  and  112  is represented by K, the voltage VG 2  at this time is expressed by Expression (3). 
         [0000]    
       
         
           
             
               
                 
                   
                     VG 
                      
                     
                         
                     
                      
                     2 
                   
                   = 
                   
                     
                       
                         
                           I 
                            
                           
                               
                           
                            
                           1 
                           × 
                           R 
                         
                         K 
                       
                       + 
                     
                     | 
                     
                       VGSP 
                        
                       
                           
                       
                        
                       2 
                     
                     | 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0042]    As described above, when the drain current I 1  of the PMOS transistor  113  is limited, the voltage VS 1  and the voltage VG 2  are equal to each other, and the absolute values VGSP 1  and VGSP 2  are substantially equal to each other. Thus, from Expression (2) and Expression (3), a limited current I 1   m  of the current I 1  is expressed by Expression (4). 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                      
                     
                         
                     
                      
                     1 
                      
                     m 
                   
                   = 
                   
                     
                       K 
                       × 
                       VREF 
                     
                     R 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0043]    As described above, the limited current I 1   m  of the current I 1  is determined, and the overcurrent protection function is put into operation. It is understood from Expression (4) that the limited current I 1   m  is in proportion to the voltage VREF. 
         [0044]      FIG. 2  is a graph for showing a waveform of the output voltage VOUT relative to an output current IOUT of the voltage regulator  100  of this embodiment. The dotted lines indicate a fluctuation range of the limited current I 1   m . When the reference voltage source  114  is configured as a bandgap voltage source, fluctuation in the voltage VREF is about ±3%. Thus, fluctuation in the limited current I 1   m  caused by the fluctuation in the voltage VREF may be suppressed to ±3%. 
         [0045]    As described above, in the voltage regulator  100  of this embodiment, the fluctuation in the limited current I 1   m  may be made much smaller than that in the related-art voltage regulator  300 . 
         [0046]    Next, with reference to  FIG. 3 , a voltage regulator  200  of a second embodiment of the present invention is described. 
         [0047]    The voltage regulator  200  of this embodiment is different from the voltage regulator  100  of the first embodiment in that the voltage generating unit  129  has a different configuration. That is, as illustrated in  FIG. 3 , the voltage generating unit  129  is formed of the resistor  122  having one end connected to the drain of the PMOS transistor  112 , and another end connected to the ground terminal  102 . 
         [0048]    Other configurations are the same as those of the voltage regulator  100  of  FIG. 1 . Thus, the same components are denoted with the same symbols and overlapping descriptions are omitted as appropriate. 
         [0049]    Operation of the voltage regulator  200  of this embodiment is described. A difference in operation from the voltage regulator  100  of the first embodiment is described as in the description of the difference in configuration. In the operation of the voltage regulator  200  of this embodiment, the voltage VG 2  in the third state is different from that in the voltage regulator  100  of the first embodiment, and is expressed by Expression (5) instead of Expression (3). 
         [0000]    
       
         
           
             
               
                 
                   
                     VG 
                      
                     
                         
                     
                      
                     2 
                   
                   = 
                   
                     
                       I 
                        
                       
                           
                       
                        
                       1 
                       × 
                       R 
                     
                     K 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0050]    The voltage VS 1  is the same as in Expression (2). Further, the voltage VS 1  and the voltage VG 2  are equal to each other in the third state, and hence the limited current I 1   m  of the current I 1  is expressed by Expression (6) from Expression (2) and Expression (5). 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                      
                     
                         
                     
                      
                     1 
                      
                     m 
                   
                   = 
                   
                     
                       K 
                       R 
                     
                      
                     
                       ( 
                       
                         
                           VREF 
                           + 
                         
                         | 
                         
                           VGSP 
                            
                           
                               
                           
                            
                           1 
                         
                         | 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0051]    The limited current I 1   m  of the current I 1  is determined in this way, and the overcurrent protection function is put into operation. It is understood from Expression (6) that the limited current I 1   m  of this embodiment is in proportion to a sum of the voltage VREF and the absolute value |VGSP 1 | of the gate-source voltage of the PMOS transistor  115 . 
         [0052]    When the reference voltage source  114  is configured as the bandgap voltage source, the voltage of the voltage VREF and fluctuation thereof is 1.2 V±0.036 V. Here, when the absolute value |VGSP 1 | is 0.6 V±0.1 V, a voltage of a sum of the values is 1.8 V±0.136 V. As a result, the fluctuation in the limited current I 1   m  caused by fluctuation in the sum of the voltage VREF and the absolute value |VGSP 1 | may be suppressed to ±7.6%. 
         [0053]    As described above, even when the voltage generating unit  129  is formed of only the resistor  122 , the fluctuation in the limited current I 1   m  may be significantly suppressed as compared to the related-art voltage regulator  300 . In general, the resistance value R has a negative temperature coefficient in many cases and the absolute value |VGSP 1 | also has a negative temperature coefficient. Thus, it is also possible to balance out those coefficients to improve temperature characteristics. 
         [0054]    As described above, in the voltage regulator  200  of this embodiment, the fluctuation in the limited current I 1   m  may be reduced and the temperature characteristics may be improved as compared to the related-art voltage regulator  300 . 
         [0055]    The embodiments of the present invention have been described above, but the present invention is not limited to the above-mentioned embodiments. It is to be understood that various modifications can be made to the present invention without departing from the gist thereof. 
         [0056]    For example, in the example described in the first embodiment, the voltage generating unit  129  is formed of the PMOS transistor  123  and the resistor  122  connected in series. Further, the PMOS transistor  123  is arranged on the PMOS transistor  112  side, and the resistor  122  is arranged on the ground terminal  102  side. However, the resistor  122  may be arranged on the PMOS transistor  112  side, and the PMOS transistor  123  may be arranged on the ground terminal  102  side. 
         [0057]    Further, in the embodiments, the examples in which MOS transistors are used in the voltage regulator are described. However, bipolar transistors or the like may be used. 
         [0058]    Further, in the embodiments, a circuit configuration in which the polarities of the PMOS transistors and the NMOS transistors are reversed may be used.