Constant voltage circuit capable of quickly responding to a sudden change of load current

A constant voltage circuit having an input terminal receiving an input voltage and an output terminal outputting an output voltage through a voltage conversion based on the input voltage, comprising:

• an output transistor which allows a current to flow from the input terminal to the output terminal in accordance with a control signal input to a gate of the output transistor;
• a reference voltage generator configured to generate and output first and second reference voltages;
• a voltage detector configured to detect a voltage output from the output terminal and to generate and output a proportional voltage in proportion to the voltage detected;
• and
• error amplifying circuit configured to control the output transistor to equalize the proportional voltage substantially with the first reference voltage, the error amplifying circuit including
• a first error amplifier configured to control operations of the output transistor so as to equalize the proportional voltage substantially with the first reference voltage, and
• a second error amplifier having a faster response speed than the first error amplifier relative to changes of the output voltage output from the output terminal and configured to cause the output transistor to increase an output current for a predetermined time period in response to a rapid decrease of the output voltage output from the output terminal at a speed faster than a predetermined speed, the second error amplifier including
• a control transistor which controls the operations of the output transistor in response to a control signal input to a gate of the control transistor,
• a differential amplifying circuit having first and second input terminals, configured to receive the second reference voltage to the first input terminal and to control the control transistor to equalize a voltage input to the second input terminal with the second reference voltage,
• a capacitor connected between the another input terminal of the differential amplifying circuit and the output terminal,
• a resistor connected between the first and second input terminals of the differential amplifying circuit,
• a proportion current generator configured to generate and output a current in proportion to a current output from the output transistor, and
• a current mirror circuit including an output-side transistor, configured to generate and output a current in proportion to the output current from the proportion current generator, and to change an impedance of the output-side transistor in accordance with an output current from the proportion current generator so as to control a gate voltage of the control transistor to control the operations of the control transistor to control a gain of the second error amplifier.