Power consumption in neural stimulation devices such as cochlear implants, or retinal implants, is an important issue. These devices operate in volume constrained conditions and therefore have a drawback in terms of energy storage. This means that the converters used for the necessary voltage compliance in these devices must be as efficient as possible. This work describes a system implementing an adaptive converter together with a constant current stimulator. The adaptive converter utilizes a three-stage charge pump with a total of 600 pF on-chip MIM capacitors occupying 1 mm(2) area. Fed from a single supply, it can provide 3.3/6/9/12 V output with varying load current between 100 to 900 mu A. The converter changes its output, i.e., stimulator supply voltage depending on the digitally controlled stimulation current. This eliminates the unused voltage headroom and provides more efficient operation compared to constant voltage converters. In addition, reduced number of bulky off-chip flying capacitors make the converter appealing for volume constricted stimulators, such as fully implantable cochlear implants. An H-bridge was used to create the stimulation current, enabling single supply operation. Operational stage number and pulse frequency modulation was utilized to configure the output. In-vitro tests show a decrease of up to 79% in the power dissipation of the constant current stimulator compared to its constant 12 V operation.