Abstract:
Provided is a voltage regulator having improved transient response characteristics even when a load current is switched from a light load to a heavy load. The voltage regulator includes, to a gate of a detection transistor constituting an output current detection circuit: a resistive element for interrupting the gate of the detection transistor from an output terminal of a differential amplifier circuit in an AC manner; and a capacitive element connected to an output terminal of the voltage regulator in an AC manner.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2012-051841 filed on Mar. 8, 2012, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage regulator, and more specifically, to an improvement of transient response characteristics when an output current fluctuates. 
     2. Description of the Related Art 
       FIG. 6  illustrates a conventional voltage regulator including an output current detection circuit. A differential amplifier circuit  104  compares an output voltage of a reference voltage circuit  103  and an output voltage of a voltage dividing circuit  106  to each other and controls a gate-source voltage of an output transistor  105 , to thereby obtain a desired voltage at an output terminal  102 . An output current detection circuit  107  includes a detection transistor  112 , an output current monitoring circuit  113 , and a control circuit  114 . 
     When the output terminal  102  of the voltage regulator decreases because of an increased load current, the differential amplifier circuit  104  operates so as to increase the gate-source voltage of the output transistor  105 . The output transistor  105  and the detection transistor  112  are transistors having the same characteristics but different K values, and are current-mirror connected to each other. Therefore, the detection transistor  112  allows a current Im corresponding to a load current of the output voltage  102  to flow. The output current monitoring circuit  113  converts the current Im flowing through the detection transistor  112  into a voltage, and outputs the voltage. In response to the voltage output from the output current monitoring circuit  113 , the control circuit  114  generates and outputs a control signal. In response to the control signal output from the control circuit  114 , the differential amplifier circuit  104  increases a bias current. 
     As described above, in the conventional voltage regulator, the output current detection circuit controls the bias current of the differential amplifier circuit  104  in accordance with the load current, and hence transient response characteristics are improved (see, for example, Japanese Patent Application Laid-open No. 2011-96210). 
     However, the conventional voltage regulator including the output current detection circuit detects the load current by an output signal of the differential amplifier circuit  104 , thereby controlling the bias current of the differential amplifier circuit  104 . Thus, it has been difficult to swiftly respond to a decrease in output voltage. In other words, there has been a problem in that, when the load current is switched from a light load to a heavy load, the bias current of the differential amplifier circuit  104  is reduced, and hence the transient response characteristics of the differential amplifier circuit  104  at the time of detecting the decrease in output voltage are poor. 
     SUMMARY OF THE INVENTION 
     In order to solve the above-mentioned problem, the present invention provides a voltage regulator including a resistive element, which is connected between a gate terminal of an output transistor and a gate terminal of a detection transistor, and a capacitive element, which is connected between an output terminal of the voltage regulator and the gate terminal of the detection transistor. 
     According to the voltage regulator of the present invention, the detection transistor swiftly allows a current to flow in response to a decrease in output voltage caused by an increased load current. Thus, an output current detection circuit can increase a bias current of a differential amplifier circuit at high speed. In this way, the decrease in output voltage caused by an increased load can be suppressed, and hence transient response characteristics can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a circuit diagram illustrating a voltage regulator including an output current detection circuit according to a first embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating another example of the voltage regulator including the output current detection circuit according to the first embodiment of the present invention; 
         FIG. 3  is a circuit diagram illustrating a voltage regulator including an output current detection circuit according to a second embodiment of the present invention; 
         FIG. 4  is a circuit diagram illustrating a voltage regulator including an output current detection circuit according to a third embodiment of the present invention; 
         FIG. 5  is a circuit diagram illustrating an example of a voltage detection circuit according to the second and third embodiments of the present invention; and 
         FIG. 6  is a circuit diagram illustrating a conventional voltage regulator including an output current detection circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a circuit diagram illustrating a voltage regulator including an output current detection circuit according to a first embodiment of the present invention. The voltage regulator in this embodiment includes a reference voltage circuit  103 , a differential amplifier circuit  104 , an output transistor  105 , a voltage dividing circuit  106 , an output current detection circuit  107 , a resistor  151 , and a capacitor  152 . The output current detection circuit  107  includes a detection transistor  112 , an output current monitoring circuit  113 , and a control circuit  114 . 
     Next, connections of the circuit components of the voltage regulator in this embodiment are described. 
     The reference voltage circuit  103  has an output terminal connected to an inverting input terminal of the differential amplifier circuit  104 . The voltage dividing circuit  106  is provided between an output terminal  102  and a Vss terminal  100 , and has an output terminal connected to a non-inverting input terminal of the differential amplifier circuit  104 . The differential amplifier circuit  104  has an output terminal connected to a gate of the output transistor  105 . The resistor  151  is provided between the output terminal of the differential amplifier circuit  104  and a gate of the detection transistor  112 . The capacitor  152  is provided between the gate of the detection transistor  112  and the output terminal  102 . The output transistor  105  has a source connected to a Vin terminal and a drain connected to the output terminal  102 . The detection transistor  112  has a source connected to the Vin terminal and a drain connected to the output current monitoring circuit  113 . The output current monitoring circuit  113  has an output terminal connected to the control circuit  114 . The control circuit  114  has an output terminal connected to an operating current control terminal of the differential amplifier circuit  104 . 
     Next, the operation of the voltage regulator in this embodiment is described. 
     The gate of the output transistor  105  is separated from the output terminal of the differential amplifier circuit  104  in an AC manner by the resistor  151 , and hence the output transistor  105  is coupled to the output terminal  102  in an AC manner via capacitive coupling of the capacitor  152 . 
     When a load  108  fluctuates from a light load to a heavy load, a current flowing from the output terminal  102  to the load  108  increases to decrease a voltage of the output terminal  102 . In this case, the gate of the detection transistor  112  can receive the decrease in output voltage of the output terminal  102  due to the action of the resistor  151  and the capacitor  152 . Therefore, without waiting for control of a gate-source voltage of the output transistor  105  performed by the differential amplifier circuit  104 , a current is allowed to flow through the output current monitoring circuit  113  by the detection transistor  112 . As a result, a bias current of the differential amplifier circuit  104  can be increased via the control circuit  114 . After that, the detection transistor  112  supplies a current to the output current monitoring circuit  113  based on the voltage used for the differential amplifier circuit  104  to control the output transistor  105  in accordance with the output voltage of the voltage dividing circuit  106 . As a result, a bias current of the differential amplifier circuit  104  corresponding to the load  108  is allowed to flow. 
     As described above, the voltage regulator in this embodiment controls the gate of the detection transistor  112  in response to the fluctuation in output voltage of the output terminal  102 , thereby being capable of controlling the bias current of the differential amplifier circuit  104  swiftly in response to the fluctuation in output current. Thus, the transient response characteristics can be improved. 
     Note that, as illustrated in  FIG. 2 , a pre-driver  201  which is current-mirror connected to the detection transistor  112  may be added in parallel to the output transistor  105 . 
     With this configuration, when the output current fluctuates from a light load to a heavy load, a gate-source voltage of the pre-driver  201  becomes larger at the time of the decrease in output because of capacitive coupling of the capacitor  152 . Thus, an output current can be supplied from the pre-driver. Therefore, the voltage regulator operates so as to pull up the output voltage  102  by the current supplied from the pre-driver  201  to the output. Thus, the transient response characteristics can be improved more. 
     Second Embodiment 
       FIG. 3  is a circuit diagram illustrating a voltage regulator including an output current detection circuit according to a second embodiment of the present invention. The voltage regulator in this embodiment is obtained by adding a voltage detection circuit  301  to the circuit in the first embodiment. The voltage detection circuit  301  is provided between the output terminal  102  and the Vss terminal  100 , and has an output terminal connected to the gate of the detection transistor  112 . 
     Next, the operation of the voltage regulator in the second embodiment is described. 
     When a load  108  fluctuates from a light load to a heavy load, in response to the fluctuation in output voltage of the output terminal  102 , the voltage detection circuit  301  outputs a voltage and a current for directly pulling down a gate voltage of the detection transistor  112 . Therefore, a current is allowed to flow through the output current monitoring circuit  113  by the detection transistor  112 . As a result, the bias current of the differential amplifier circuit  104  can be increased via the control circuit  114 . In this way, the bias current of the differential amplifier circuit  104  can be increased faster than in the first embodiment, and hence the transient response characteristics can be improved more. 
     In this case, the voltage detection circuit  301  only needs to operate so that the output terminal may be a voltage of the Vss terminal when the decrease in voltage of the output terminal  102  is detected. For example, the voltage detection circuit  301  may be formed of a circuit as illustrated in  FIG. 5 . 
     The voltage detection circuit  301  illustrated in  FIG. 5  includes depletion mode NMOS transistors  501 ,  502 ,  503 , and  504 , a capacitor  505 , and a resistor  506 . An input terminal  510  is connected to the output terminal  102  of the voltage regulator, and an output terminal  511  is connected to the gate of the detection transistor  112 . 
     Note that, in the circuit of  FIG. 3 , the same effect can be obtained even without the capacitor  152 . 
     Further, the pre-driver  201  which is current-mirror connected to the detection transistor  112  may be added in parallel to the output transistor  105 . 
     Third Embodiment 
       FIG. 4  is a circuit diagram illustrating a voltage regulator including an output current detection circuit according to a third embodiment of the present invention. The voltage regulator in this embodiment is obtained by modifying the circuit in the second embodiment so that the output of the voltage detection circuit  301  is input to the control circuit  114  via a logic circuit  401  (for example, OR circuit). 
     Next, the operation of the voltage regulator in the third embodiment is described. 
     When a load  108  fluctuates from a light load to a heavy load, in response to the fluctuation in output voltage of the output terminal  102 , the voltage detection circuit  301  outputs a signal for increasing a bias current of the differential amplifier circuit  104  to the control circuit  114  via the logic circuit  401 . The logic circuit  401  performs OR operation (in the case of OR circuit) on the signal of the voltage detection circuit  301  and the output voltage of the output current monitoring circuit  113 , and outputs a signal to the control circuit  114 . As a result, the bias current of the differential amplifier circuit  104  can be increased via the control circuit  114 . In this way, the bias current of the differential amplifier circuit  104  can be increased faster than in the other embodiments, and hence the transient response characteristics can be improved more. 
     Note that, in the circuit of  FIG. 4 , the same effect can be obtained even without the resistor  151  and the capacitor  152 . 
     Further, the pre-driver  201  which is current-mirror connected to the detection transistor  112  may be added in parallel to the output transistor  105 .