This paper presents a tri-mode digital buck converter in 0.18-μm CMOS technology for photovoltaic energy harvesting. The on-chip gate-boosted digital pulsewidth modulation (DPWM) improves the conversion efficiency at heavy load conditions. Pulse-frequency modulation (PFM) along with digital self-tracking zero current detection is proposed to avoid reverse current at light load. The asynchronous mode (AM) operation further reduces the controller loss and improves the conversion efficiency at ultra-light load conditions. By applying DPWM, PFM, and AM at different load conditions, the proposed converter provides a maximum conversion efficiency of 92% with output power ranging from 50 nW to 10 mW. In addition, the proposed buck converter achieves more than 70% efficiency from 400 nW to 10 mW output power.