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

A DC-DC converter comprising:

• a converter circuit which includes a switching element and switches the switching element by means of a rectangular-wave signal to convert an input voltage into an output voltage having a value different from that of the input voltage;
• an output voltage detection circuit for detecting the value of the output voltage to output a detection voltage;
• and
• a duty-ratio setting circuit for outputting to the switching element the rectangular-wave signal in which the on-duty ratio is set on the basis of the detection voltage so that the output, voltage would become a predetermined value and for carrying out feedback control,
• wherein the duty-ratio setting circuit generates the rectangular-wave signal having a predetermined on-duty ratio when the on-duty ratio of the rectangular-wave signal set on the basis of the detection voltage reaches substantially 100% and the feedback control becomes impossible.

An electric appliance comprising:

• a DC-DC converter including
• a converter circuit which includes a switching element and switches the switching element by means of a rectangular-wave signal to convert an input voltage into an output voltage having a value different from that of the input voltage,
• an output voltage detection circuit for detecting the value of the output voltage to output a detection voltage, and
• a duty-ratio setting circuit for generating on the basis of the detection voltage the rectangular-wave signal in which the on-duty ratio is set so that the output voltage would be controlled at a predetermined value and for outputting the rectangular-wave signal to the switching element,
• wherein the duty-ratio setting circuit includes
• a control-voltage generator circuit for inputting the first reference voltage and the detection voltage to output the control voltage corresponding to a difference therebetween,
• a first-pulse generator circuit for inputting the control voltage to output a first rectangular pulse signal in which the on-duty ratio changes monotonously with reference to the change of the control voltage,
• a second-pulse generator circuit for generating a second rectangular pulse signal having a predetermined on-duty ratio, and
• a selector circuit for selecting one of the first pulse signal and the second pulse signal to output the one as the rectangular-wave signal, in which the first pulse signal is selected and outputted when the control voltage is biased to a first direction compared with the second reference voltage while the second pulse signal is selected and outputted when the control voltage is biased to a second direction compared with the second reference voltage, the first direction being a direction that the on-duty ratio of the first pulse signal becomes smaller as the control voltage is changed, and the second direction being a direction that the on-duty ratio becomes larger to the contrary;
• a battery for supplying the DC-DC converter with the input voltage;
• and
• a main body of the electric appliance driven by using the output voltage of the DC-DC converter.

A duty-ratio setting circuit, which is used for a DC-DC converter comprising a converter circuit which includes a switching element and switches the switching element by means of a rectangular-wave signal to convert an input voltage into an output voltage having a value different from that of the input voltage and an output voltage detection circuit for detecting the value of the output voltage to output a detection voltage, for setting on the basis of the detection voltage an on-duty ratio of the rectangular-wave signal so that the output voltage would be controlled at a predetermined value, the duty-ratio setting circuit comprising:

• a control-voltage generator circuit for inputting a first reference voltage and the detection voltage to output the control voltage corresponding to a difference therebetween;
• a first-pulse generator circuit for inputting the control voltage to output a first rectangular pulse signal in which the on-duty ratio changes monotonously in accordance with the change of the control voltage;
• a second-pulse generator circuit for generating a second rectangular pulse signal having a predetermined on-duty ratio;
• and
• a selector circuit for selecting and outputting one of the first pulse signal and the second pulse signal, in which the first pulse signal is selected and outputted when the control voltage has a value biased to a first direction with reference to the second reference voltage
• while the second pulse signal is selected and outputted when the control voltage has a value biased to a second direction with reference to the second reference voltage,
• the first direction being a direction that the on-duty ratio of the first pulse signal becomes smaller as the control voltage is changed, and the second direction being a direction that the on-duty ratio becomes larger to the contrary.

An electric appliance comprising:

• a DC-DC converter comprising
• a duty-ratio setting circuit, which is used for the DC-DC converter comprising a converter circuit which includes a switching element and switches the switching element by means of a rectangular-wave signal to convert an input voltage into an output voltage having a value different from that of the input voltage and an output voltage detection circuit for detecting the value of the output voltage to output a detection voltage, for setting on the basis of the detection voltage an on-duty ratio of the rectangular-wave signal so that the output voltage would be controlled at a predetermined value,
• the duty-ratio setting circuit comprising
• a control-voltage generator circuit for inputting a first reference voltage and the detection voltage to output the control voltage corresponding to a difference therebetween,
• a first-pulse generator circuit for inputting the control voltage to output a first rectangular pulse signal in which the on-duty ratio changes monotonously in accordance with the change of the control voltage,
• a second-pulse generator circuit for generating a second rectangular pulse signal having a predetermined on-duty ratio and
• a selector circuit for selecting and outputting one of the first pulse signal and the second pulse signal, in which the first pulse signal is selected and outputted when the control voltage has a value biased to a first direction with reference to the second reference voltage while the second pulse signal is selected and outputted when the control voltage has a value biased to a second direction with reference to the second reference voltage, the first direction being a direction that the on-duty ratio of the first pulse signal becomes smaller as the control voltage is changed, and the second direction being a direction that the on-duty ratio becomes larger to the contrary, the converter circuit and
• the output voltage detection circuit;
• a battery for supplying the DC-DC converter with the input voltage;
• and
• a main body of the electric appliance driven by using the output voltage of the DC-DC converter.