A high-voltage, energy-efficient, 4-electrode output stage for implantable neural stimulator

In order to raise energy efficiency of implantable neural stimulators, an output stage circuit with a self-adaptive stimulating voltage is proposed. The output stage consists of a digital to analog converter (DAC), a single-ended primary inductance converter (SEPIC), a switch array and a current detection module. The SEPIC enables stimulating voltage to vary according to the stimulus current on tissue load or DAC digital input. It improves the output energy efficiency by dynamically cutting down the unnecessary stimulating voltage headroom on the current driver, especially when the stimulus current is small. Simulation showed that the stimulating voltage could change from 1.1V to 18.7V with less than 25mV ripple and the stimulus current could change from 0.7mA to 12.1mA with 0.1mA resolution. The output stage could work smoothly under the voltage supply range between 3.3V and 4.1V. The output efficiency of the proposed work could keep higher than 60% when stimulus current was above 1.4mA and even higher than 70% when stimulus current was above 2.9mA. The output stage is being fabricated on a silicon chip using CSMC 1μm HV technology, occupying 3.46mm×1.94mm.