This patent specification describes a DC-DC converter which includes a switch configured to control an output voltage, a driver circuit configured to drive the switch, a flip-flop configured to control the driver circuit, an error amplifier configured to compare a feedback voltage of the output voltage with a reference voltage, a detector configured to sense an output current, a control voltage generator configured to generate a control voltage being ramped and a PWM comparator configured to compare the control voltage with an output voltage of the error amplifier and output a reset signal to the flip-flop. Ramping speed of the control voltage is changed in accordance with an external-control voltage input at an external-control terminal.