An adaptive step-up/down DC-DC converter with tri-band tri-mode digital control is presented. Three control methods jointly and seamlessly optimize the converterpsilas operation performances at the start-up state, steady state and dynamic voltage/load transient state. A compact automatic substrate switching circuit is used for adaptive substrate biasing, consuming only 88-muW power. A low-power self-biased current sensing technique is also employed with 98.4% sensing accuracy. The adaptive step-up/down voltage conversion offers a wide output range for flexible dynamic voltage scaling in VLSI systems. The converter was designed with a standard 0.35-mum digital CMOS N-well process. The entire die area is 1.3 mm2. With a 2.5-V input power supply, the converter precisely provides an adjustable power output with a voltage ranging from 0.9 to 3.0 V and a power ranging from 11 to 800 mW. The maximum efficiency of 96.5% is measured at 0.9-V output and 45-mW load power, while the efficiency remains above 50% over the entire power range. Load transient response is shorter than 5 mus when the load current changes by 100% from 87.5 to 175 mA. It functions well when the supply voltage varies between 1.8 and 3.3 V, with a line regulation of 20.4 mV/V. The design provides a high-performance solution to on-chip adaptive power supply designs.