Patent ID: 7400120

Claim:
A constant voltage control device that is supplied with an electric power from an external power supply of which output voltage is inconstant, and applies plural different types of stabilized voltages to an electronic control device, the constant voltage control device comprising a first switching element generating a first output voltage, a second switching element that is cascade-connected to said first switching element and generates a second output voltage, and a third switching element acting as a by-pass power supply circuit with respect to said first switching element; wherein: said first switching element is a transistor of which conduction state is negative-feedback controlled by a first comparator/amplifier comparing a voltage proportional to said first output voltage with a reference voltage so that said first output voltage may be a first predetermined voltage; said second switching element is a transistor of which conduction state is negative-feedback controlled by a second comparator/amplifier comparing a voltage proportional to said second output voltage with a reference voltage so that said second output voltage may be a second predetermined voltage, being a value smaller than said first predetermined voltage; and said third switching element is a transistor in which a ratio between a closed-circuit period and a switching cycle is controlled via a duty-factor control circuit, and which makes the control of suppressing a flowing current through said first switching element by forming a shunt circuit when the flowing current through said first switching element is not less than a predetermined value; said first switching element further comprises a current detection resistor connected in series; said by-pass power supply circuit further comprises a choke coil and a balancing resistor connected in series to an output circuit of said third switching element; an output voltage from said choke coil is smoothed by a smoothing capacitor, and a current return diode forming a current return circuit including said choke coil and said smoothing capacitor when said third switching element is brought in an open circuit; and said balancing resistor is connected between said smoothing capacitor and an output terminal of said first switching element or said second switching element.