Active Charge Balancer With Adaptive 3.3 V to 38 V Supply Compliance for Neural Stimulators

Charge balancing has become a common key safety requirement of electrical stimulation systems. It reduces the risk of electrode dissolution and tissue damage, which might arise after unbalanced stimulation pulses. A CMOS integrated consequence-based active charge balancer is presented that instantaneously removes excess charges between two consecutive stimulation pulses. This balancer provides a highly adaptive power supply compliance from 3.3 V to 38 V, verified by measurements. In order to maintain the compensation behavior and efficacy over the entire supply range and to protect the active components at such high voltages, a quad-rail design is elaborated that overcomes the technological high voltage limitations of the employed 0.35 $\mu \text{m}$ CMOS process. The presented charge balancing circuit consists of a fully differential operational transconductance amplifier for monitoring and voltage level translation, as well as an advanced class-B stage as current driver, designed in accordance with the quad-rail concept. Further system flexibility is provided by incorporating three different compensation current limitations of $\pm 500~\mu \text{A}$ , $\pm 300~\mu \text{A}$ and $\pm 200~\mu \text{A}$ . Additionally, two safety limits of ±50 mV and ±100 mV have been adopted. The charge balancer is very power efficient, it only dissipates 6.3 $\mu \text{W}$ and 25 $\mu \text{W}$ , at 3.3 V and 38 V respectively.