This paper presents a buck converter with a switch-on-demand modulator (SOM) for achieving a fast transient response, small voltage ripple, and high power efficiency over a wide load range. Switching power MOS on or off depending on the energy demand of the load circuit results in a hybrid operation of pulse width modulation (PWM) and pulse frequency modulation (PFM). The proposed buck converter uses 90nm CMOS process and can achieve a transient response time of less than 2µs and a voltage ripple of 18mV at a load current range of 10mA∼500mA with a power efficiency above 88%.