According to embodiments, in a case that a source voltage is lowered and a potential difference between the source voltage and a ripple terminal voltage is below a constant potential difference, the ripple terminal voltage is lowered until the ripple terminal voltage reaches a first threshold value when a lowered value of the source voltage is equal to or larger than a predetermined voltage that exceeds the first threshold value by the constant potential difference, and the ripple terminal voltage is lowered until the ripple terminal voltage reaches a second threshold value smaller than the first threshold value when the lowered value of the source voltage is below the predetermined voltage.