DC-DC converter, duty-ratio setting circuit and electric appliance using them

摘要

A DC-DC converter in which no operational instability occurs and ranges of input and output voltages are wide, an electric appliance using such DC-DC converter, and a duty-ratio setting circuit in which no operational instability occurs and ranges of input and output voltages of a converter circuit can be widen in controlling the converter circuit are provided. A DC-DC converter 10 has a converter circuit 110, an output voltage detection circuit 120 and a duty-ratio setting circuit 20. An error amplifier circuit 21 compares Vref1 with Vd to output a control voltage Vfb. A pulse-width modulator circuit 25 compares a triangular-wave voltage Vct with the control voltage Vfb to output a first pulse signal PWO. A second-pulse generator circuit 26 outputs a second pulse signal PSO having a predetermined on-duty ratio. A selector circuit 27 selects the first pulse signal PWO or the second pulse signal PSO according to the control voltage Vfb to output the selected signal to a transistor M1 as a rectangular-wave signal PS.