Active Charge Balancer with 6.6 to 40 V Quad-Rail Power Supply Compliance for Neural Stimulators

Abstract

Closed loop charge balancing is a major concern in secure electrical stimulation since it reduces the risk of electrode dissolution and tissue destruction, which might arise after unbalanced stimulation pulses. This CMOS integrated active charge balancer is based on a consequence-based technique that instantaneously removes excess charges between two consecutive stimulation pulses. For high adaptability to various stimulation scenarios, this balancer provides a widely adaptive power supply compliance that ranges from 6.6 to 40 V, without affecting its instantaneous compensation behavior and efficacy. In order to reach a supply compliance as high as 40 V, a design is elaborated that overcomes the technological high voltage limitations and protects the active components of the used 0.35 μm CMOS process. Thus, an adaptive quad-rail concept has been developed to divide the supply voltage into three subranges. The presented charge balancing circuit consists of a fully differential operational transconductance amplifier and an advanced class-B stage, designed in accordance with the quad-rail concept. For arbitrary applications, further flexibility is provided by incorporating three compensation current limitations of ±500 μA, ±300 μA and ±200 μA. Additionally, two safety limits, ±50 mV and ±100 mV, have been adopted. The power dissipation at 40 V is 19.6 μW only. The layout occupies 0.21 mm2.