This letter proposes an ultralow power single-inductor multiple-output converter for an implantable stimulation system. This converter minimizes the number of switches turning on and off to deliver power to outputs of 0.8, 1.8, and 20 V, and it uses a soft-switching technique for all switching nodes. It also operates with a self-oscillating controller only when outputs require power. As a result, it has a peak efficiency of 92.3% when the converter generates the outputs of 0.8 and 1.8 V, of which load currents are 2 and 0.5 mA, respectively, from a battery voltage of 3 V. The converter consumes a quiescent power of less than 3.0 μW.