A PWM controller for adjusting an output voltage of a buck and boost converter includes a first saw wave generator, which generates a first saw wave in accordance with the level of the output voltage. A first comparator coupled to the first saw wave generator compares the first saw wave with a first reference voltage and generates a first pulse. A peak hold circuit coupled to the first saw wave generator holds a peak value of the first saw wave. A second saw wave generator coupled to the peak hold circuit generates a second saw wave having a lower limit value that is the peak value of the first saw wave. A second comparator coupled to the second saw wave generator compares the second saw wave with the first reference voltage and generates a second pulse.