The present invention discloses a DC-to-DC step-up converter to which a
logic control unit is added in order to reduce the ripples of a DC output
voltage and improve the power quality. The converter of the present
invention comprises a step-up circuit, a ring oscillator, a divider
circuit, a PFM (pulse frequency modulation) comparator and a logic
control unit. The step-up circuit is used to step up a source voltage to
generate a DC output voltage. The ring oscillator is used to generate an
oscillator output signal. The divider circuit receives the DC output
voltage to generate a feedback voltage. The PFM comparator compares the
feedback voltage with a reference voltage to generate a comparator output
signal to control outputting of the oscillator output signal. The logic
control unit is used to make the conductive period of a MOS transistor in
the step-up circuit identical to the high-level period of an output
signal of the ring oscillator to reduce the noise and the ripple of the
DC output voltage of the converter.