Charge Balance Circuit for Constant Current Neural Stimulation with Less than 8 nC Residual Charge

Abstract

Charge balancing is a major concern in functional electrical stimulation. Any excess charge accumulation over time leads to electrolysis with the electrode dissolution and tissue destruction. Therefore, charge balance circuits are used for mitigating the effects of charge accumulation in tissues. This work introduces an active synchronous charge balance circuit for constant current neural stimulators that operates without requiring negative supply for remaining charge detection. The charge balance circuit detects the residual charge by monitoring the electrode voltages just before the stimulation. If the voltage difference between the electrodes is above a certain threshold, a balance current is generated to achieve net zero charge at the electrode. Balancing current and the main stimulation current are injected simultaneously, preventing any interference in other electrodes. The charge balance circuit is dynamically disabled to reduce the system power when the charge detection is not active. The circuit can operate for the stimulation currents up to 1.4 mA and hold the electrode charge under 8 nC/phase while consuming only 6.36 μW power.