Patent ID: 7075277

Claim:
A DCâ€”DC converter of voltage step-up and step-down type comprising a voltage step-down converter section having a first switch, a voltage step-up converter section having a second switch and a control section for turning ON and OFF said first switch and said second switch, respectively, for receiving a direct current input voltage and outputting a direct current output voltage to a load, wherein said control section comprises: an error amplifying circuit for comparing said direct current output voltage with a predetermined voltage and for outputting an error voltage, an oscillation circuit for generating an oscillation voltage cyclically changing between a first setting voltage and a second setting voltage lower than said first setting voltage, a ratio of rising period or a ratio of lowering period per one cycle of said oscillation voltage increasing as the difference between said error voltage and said first setting voltage increases, when said error voltage is higher than said first setting voltage, and the ratio of rising period or the ratio of lowering period per one cycle of said oscillation voltage increasing as the difference between said error voltage and said second setting voltage increases, when said error voltage is lower than said second setting voltage, and a pulse width control circuit for controlling the ON and OFF periods of said first switch and the ON and OFF periods of said second switch so as to carry out control of a voltage step-down operation mode wherein said second switch is fixed at OFF state and said first switch is turned ON and OFF, or to carry out control of a voltage step-up operation mode wherein said first switch is fixed to ON state and said second switch is turned ON and OFF, in a case where said error voltage is compared with said oscillation voltage and said error voltage does not coincide with said oscillation voltage, and to carry out control of a voltage step-up and step-down operation mode wherein both said first switch and said second switch are turned ON and OFF in a case where said error voltage coincides with said oscillation voltage occasionally.