Abstract:
A safe and low-cost voltage regulator is provided through simplification of a circuit configuration of a protection circuit. The voltage regulator has a configuration in which current output of the protection circuit, which serves as a signal indicating that the protection circuit has started to operate, is input to a control circuit configured to improve responsiveness of the voltage regulator.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2016-007340 filed on Jan. 18, 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 including a protection circuit. 
         [0004]    The present application claims priority based on Japanese Patent Application No. 2016-007340 filed in Japan on Jan. 18, 2016, the disclosures of which are incorporated herein by reference in their entirety. 
         [0005]    2. Description of the Related Art 
         [0006]    In general, electronic devices such as cellular phones are configured to operate with rechargeable batteries. A voltage regulator is provided to the battery such that an output voltage to the electronic device does not fluctuate and the electronic device stably operates, even when a charged state of the battery fluctuates. Further, the voltage regulator, which is configured to prevent the output voltage to the electronic device from fluctuating such that the electronic device stably operates even when a load of the electronic device sharply fluctuates, is provided with a control circuit for achieving a more stable output voltage of the voltage regulator in some cases. 
         [0007]      FIG. 3  is a circuit diagram of a related-art voltage regulator  30 . The voltage regulator  30  includes an output transistor  31  configured to output an output voltage VOUT, voltage-dividing resistors  32  for dividing the output voltage VOUT such that a divided voltage VFB is output, an error amplifier circuit  35  configured to operate to match the divided voltage VFB with a reference voltage VREF, and control the output transistor  31  such that the output voltage VOUT is constant, a protection circuit  33  configured to protect the voltage regulator  30 , and a control circuit  34  configured to detect fluctuation in output voltage VOUT to operate to stabilize the output voltage VOUT. 
         [0008]    The protection circuit  33  is configured to detect failure including, for example, a case in which an excessive current flows through the output transistor  31 , and output a signal at a power supply voltage level to perform control for turning off the output transistor  31 . 
         [0009]    The control circuit  34  is configured to detect decrease in output voltage VOUT to perform control for turning off the output transistor  31 . 
         [0010]    Here, the control circuit  34  is configured to stop its operation when receiving the signal at the power supply voltage level that indicates that the protection circuit  33  has started to operate. Thus, when the output voltage VOUT is decreased through control by the protection circuit  33 , the output voltage VOUT is not increased through operation of the control circuit  34 . As a result, safety of the voltage regulator is increased (for example, see Japanese Patent Application Laid-open No. 2008-204018). 
         [0011]    However, the protection circuit  33  only has an analog signal having an intermediate potential, as means for detecting failure, such as a voltage. Thus, for the protection circuit  33 , a circuit configured to convert an output signal being an analog signal into a digital signal is required. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention has been made in order to solve the problem described above, and achieves a safe and low-cost voltage regulator through simplification of the circuit configuration of a protection circuit. 
         [0013]    In order to solve the related-art problem, a voltage regulator according to one embodiment of the present invention has the following configuration. 
         [0014]    The voltage regulator includes: an output transistor configured to output an output voltage; a voltage divider circuit configured to generate a divided voltage by dividing the output voltage, and output the divided voltage; an error amplifier circuit configured to compare a reference voltage and the divided voltage to each other, to thereby control a gate of the output transistor such that the output voltage is constant; a protection circuit configured to protect circuits of the voltage regulator by controlling the gate of the output transistor, when a predetermined condition is satisfied; and a control circuit configured to operate to stabilize the output voltage when the output voltage fluctuates, and stop operation of the control circuit when receiving a signal indicating that the protection circuit has started to operate, the signal indicating that the protection circuit has started to operate being current output. 
         [0015]    According to the voltage regulator of the present invention, the current output of the protection circuit serves as an output signal thereof that indicates that the protection circuit has started to operate, and hence the circuit configuration of the protection circuit can be simplified. As a result, there is provided an effect that the safe and low-cost voltage regulator can be achieved. 
         [0016]    In addition, the output current of the protection circuit, which indicates that the protection circuit has started to operate, is a drain current of the transistor whose gate and source are common with those of the transistor configured to control the gate of the output transistor of the voltage regulator, or a current that is generated by the current mirror circuit configured to receive the drain current as input. As a result, there is provided an effect that the safe voltage regulator can be achieved through addition of a few elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a circuit diagram for illustrating an example of a voltage regulator according to an embodiment of the present invention. 
           [0018]      FIG. 2  is a circuit diagram for illustrating a voltage regulator according to another embodiment of the present invention. 
           [0019]      FIG. 3  is a circuit diagram of a related-art voltage regulator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Now, embodiments of the present invention are described with reference to the drawings. 
         [0021]      FIG. 1  is a circuit diagram for illustrating an example of a voltage regulator according to an embodiment of the present invention. 
         [0022]    The voltage regulator of this embodiment includes an output transistor  11  being a PMOS transistor configured to output an output voltage VOUT, voltage-dividing resistors  15  for dividing the output voltage VOUT such that a divided voltage VFB is output, an error amplifier circuit  12  configured to operate to match the divided voltage VFB with a reference voltage VREF, and control the output transistor  11  such that the output voltage VOUT is constant, a protection circuit  13  configured to protect the voltage regulator, and a control circuit  14  configured to detect fluctuation in power supply voltage or output voltage VOUT to operate to stabilize the output voltage VOUT by controlling the error amplifier circuit  12 . 
         [0023]    The protection circuit  13  includes a determination circuit  53  configured to determine whether or not a predetermined condition is satisfied under which protection of the voltage regulator is required, a PMOS transistor  54  having a gate that is controlled by the determination circuit  53  and being configured to control a gate voltage of the output transistor  11 , and a PMOS transistor  55  having a gate that is controlled by the determination circuit  53  and being configured to output a current as output of the protection circuit  13  when the predetermined condition is satisfied. 
         [0024]    The control circuit  14  is configured to stop its operation when receiving the output current of the protection circuit  13  that indicates that the protection circuit  13  has started to operate due to satisfaction of the predetermined condition. 
         [0025]    The voltage regulator as described above is configured to operate as follows to enhance its safety when the protection circuit operates. 
         [0026]    The protection circuit  13  determines, with the determination circuit  53 , whether or not the predetermined condition is satisfied under which protection of the voltage regulator is required, such as an overcurrent state or an overheated state, and increases the gate voltage of the output transistor  11  of the voltage regulator with a current of the PMOS transistor  54  that is caused to flow through control of the gate of the PMOS transistor  54 , to thereby decrease a current flowing through the output transistor  11 . As a result, the output voltage VOUT decreases. 
         [0027]    At this time, if the control circuit  14  detects the decrease in output voltage VOUT to control the error amplifier circuit  12 , thereby operating to return the output voltage VOUT to a predetermined voltage, the error amplifier circuit  12  operates to decrease the gate voltage of the output transistor  11 . Consequently, the current flowing through the output transistor  11  increases to impair the safety of the voltage regulator. However, because the control circuit  14  of the voltage regulator of this embodiment is configured to stop its operation when receiving the output current output from the PMOS transistor  55  that indicates that the protection circuit  13  has started to operate, a current controlled by the protection circuit  13  flows through the output transistor  11 . 
         [0028]    As described above, according to the voltage regulator of this embodiment, current output of the protection circuit  13  serves as an output signal thereof that indicates that the protection circuit  13  has started to operate. Thus, even when the protection circuit only has an analog signal having an intermediate potential of a power supply voltage and a ground voltage, there is no need to provide a circuit configured to generate from the analog signal a digital signal only taking the power supply voltage or the ground voltage except for a transition state of the signal. As a result, there is provided an effect that the safe voltage regulator may be achieved through addition of a few elements. 
         [0029]    In particular, the output current of the protection circuit  13 , which indicates that the protection circuit  13  has started to operate, is a drain current of the transistor  55  whose gate and source are in common with those of the transistor  54  configured to control the gate of the output transistor  11  of the voltage regulator. As a result, the safe voltage regulator may be achieved through addition of only one element. 
         [0030]      FIG. 2  is a circuit diagram for illustrating a voltage regulator according to another embodiment of the present invention. A protection circuit  13  of the voltage regulator of  FIG. 2  includes a current mirror circuit  56 . An output current indicating that the protection circuit  13  has started to operate is an output current of the current mirror circuit  56  configured to receive a drain current of a PMOS transistor  55  as input. 
         [0031]    As generally and widely known among those skilled in the art, the current mirror circuit  56  has a function of performing conversion of a current value, and a function of converting a direction of current from a source current into a sink current, or from a sink current into a source current. 
         [0032]    As described above, according to the voltage regulator of this embodiment, an appropriate amount of current may be output in an appropriate direction depending on the circuit form of the control circuit  14  configured to operate to stabilize the output voltage VOUT, or on a contact point of the control circuit  14  at which the output current that is output from the protection circuit  13  is input.