Step-down power supply circuit, power supply module, and step-down power supply circuit control method

Provided is a step-down power supply circuit (20), comprising: a first PMOS transistor (37), a source thereof connected to a voltage input (151), and a drain thereof connected to a voltage output (152); a control circuit (21) which operates by a voltage of an internal voltage line (153), and controls the gate voltage of the first PMOS transistor (37); a comparator (23) which operates by the voltage of the internal voltage line (153), and changes the output from a first state to a second state when the voltage of the voltage input (151) falls; switch circuits (26, 27) which connect the voltage input (151) to the internal voltage line (153) when the output of the comparator (23) is in the first state, and which connect the voltage output (152) to the internal voltage line (153) when the output of the comparator (23) is in the second state; and cutout circuits (31, 38) which, when the output of the comparator (23) is in the second state, cut out a current backflow path from the voltage output (152) to the voltage input (151) via the first PMOS transistor (37).