A 22 V Compliant 56 <inline-formula> <tex-math notation="LaTeX">${\mu}$ </tex-math> </inline-formula>W Twin-Track Active Charge Balancing Enabling 100% Charge Compensation Even in Monophasic and 36% Amplitude Correction in Biphasic Neural Stimulators

Abstract

Closed-loop charge balancers make a considerable contribution to a safe and secure electrical stimulation and additionally present an alternative to bulky blocking capacitors. This CMOS-integrated, 22 V compliant Twin-Track active charge balancer accomplishes both instantaneous and long-term balanced conditions. Long-term charge balancing is achieved by a cause-based PI-controlled offset compensation of the remaining electrode voltage. A $G_{\mathrm{ m}}$ of only 1.5 nS offers a large (8 ms) time constant with only a 12 pF capacitor and leads to the first integrated PI-controller for offset compensation in neural implants. Instantaneous autonomous balancing is realized by a second concept, the consequence-based Inter-Pulse Charge Control. It uses a class-B architecture to supply the compensation currents. Thus, no additional references are required to define the safety window of the remaining electrode voltage. It consumes 31.8 ${\mu }\text{W}$ in the idle state, while the delivered output power can be as high as 11 mW. The two complementary approaches can be combined to one system, providing the opportunity to achieve a better performance. The system consumes 56 ${\mu }\text{W}$ and features a 36% mismatch correction in biphasic stimuli amplitude, as well as an autonomous 100% charge removal at maximum compensation amplitudes of 500 ${\mu }\text{A}$ , which makes it a suitable complement to monophasic stimulators.