A digitally controlled Buck-Boost converter uses a fully synthesized constant ON/OFF time-based fractional-N controller to regulate the output over a 3.3V-to-5.5V input voltage range. The proposed architecture does not use either a high resolution digital pulse width modulator or an analog to digital converter. Fabricated in a 500nm CMOS process, the prototype achieves a peak efficiency of 92% at 500mA load current. The use of a smaller inductor and no external compensation capacitor makes the proposed solution highly cost effective. © 2011 IEEE.