A method for controlling a voltage step-up and step-down typed DC-DC converter that receives input of a direct current voltage such as a battery and supplies a controlled direct current voltage to a load is simplified.
It comprises a voltage step-down converter section consisting of a first switch 2, a first rectifying means 3 and an inductor 4, a voltage step-up converter section consisting of an inductor 4, a second switch 5 and a second rectifying means 6, and a control section consisting of an output capacitor 7, an error amplifying circuit 10, an oscillation circuit 11 and a pulse width control circuit 12, and ratio of a rising period or lowering period of an oscillation voltage Vt changes as the input and output voltage ratio becomes small in the step-down operation mode, and as the input and output voltage ratio becomes large in the voltage step-up operation mode, whereby the oscillation voltage Vt from the oscillation circuit 11 is compared with an error voltage Ve from the error amplifying circuit 10 and the duty ratio of each switch is adjusted to control the operation of voltage step-down, voltage step-up and step-down, and voltage step-up.