Patent ID: 7196504

Claim:
A constant-voltage circuit having an input terminal pulled up to an input voltage and an output terminal outputting an output voltage, the constant-voltage circuit comprising: a first transistor configured to control an output current flowing from the input terminal to the output terminal in accordance with a first control signal; a first control circuit configured to control the first transistor by outputting the first control signal such that the output voltage output from the output terminal is substantially equal to a predetermined voltage; a second control circuit having a response property faster than the first control circuit to a variation of the output voltage and configured to cause the first transistor to increase the output current for a predetermined time period, regardless of the first control signal, when the output voltage varied to an extent greater than a predetermined output voltage variation value, the second control circuit including: a second transistor configured to control an operation of the first transistor in accordance with a second control signal, and a differential amplifier including a non-inverting input terminal connected to a bias voltage, an inverting input terminal connected to the non-inverting input terminal via a resistor and to the output terminal via a capacitor, and a differential pair of third and fourth transistors, the third transistor configured to have a current drive capability variably set to determine the predetermined output voltage variation value, and configured to control an operation of the second transistor by outputting the second control signal such that a voltage at the inverting input terminal is substantially equal to the bias voltage.