As load current decreases, the efficiency of a PWM-based DC-DC converter decreases and the converter operates in the discontinuous conduction mode. To reduce the switching loss under the light load condition, a PFM mode is used to increase the efficiency in the DCM operation. The decreased switching frequency improves the efficiency, but worsens the output voltage ripple. In this paper, an analytical formula for the ripple is derived and used to optimize the switching frequency of a PFM-type converter under the light load condition.