Constant-voltage circuit capable of reducing time required for starting, semiconductor apparatus including constant-voltage circuit, and control method of constant-voltage circuit

A constant-voltage circuit (1) includes a switching regulator (2) and a series regulator (3) each configured to start operating according to a selection signal input from outside for exclusively causing one of the switching and series regulators to operate, and convert an input voltage (Vin) input to an input terminal (IN) to a rated voltage to output to an output terminal (OUT). The switching regulator performs a soft-start operation for raising an output voltage (Vout) to the rated voltage at a constant speed when starting operation. The series regulator regulates a current so that an output current does not exceed a predetermined value, simultaneously starts operating when the switching regulator starts operating, and automatically stops outputting a current to the output terminal when the soft-start operation is completed. A semiconductor apparatus provided with a predetermined function includes the constant-voltage circuit. A control method controls the constant-voltage circuit.