22.6 A 22V compliant 56µW active charge balancer enabling 100% charge compensation even in monophasic and 36% amplitude correction in biphasic neural stimulators

Abstract

Functional electrical stimulation (FES) is a technique that stimulates nerves by electrical charge, but carries the risk of charge accumulation, voltage pile-up, electrode corrosion and finally tissue destruction. Using biphasic stimulus current pulses, the main transferred charge is compensated by reversing the current direction. However, due to PVT variations in integrated circuits mismatch in the biphasic waveform always occurs. Charge balancing (CB) has thus become an integral part of FES to ensure safe chronic stimulation [1]. This paper presents a CMOS integrated, 22V compliant active charge balancer accomplishing "cause and consequence" based compensation, ensuring both instantaneous and long-term balanced condition. By inherently incorporating an idle window, the consequence-based compensation Inter-Pulse Charge Control (IPCC) offers a self-regulated and reference-free removal of charges without interfering with naturally evoked action potentials. Compared to other approaches, its simple architecture at 1.4μA current consumption and its ability of compensating a stimuli mismatch of up to ±100% with respect to the idle window, make it suitable as independent balancer offering autonomous CB for arbitrary and even monophasic neural stimulators. The main feature of the cause-based Offset Compensation (OC) is up to ±36% biphasic stimuli mismatch correction via an adjustable PI-controller providing an overall G m of 1.5nS at only 860nA consumption. Figure 22.6.1 illustrates a neural stimulation setup with a biphasic stimulation current source, the presented active CB circuits, and the equivalent circuit model of an electrode-tissue interface [1]. An exemplary curve of the electrode voltage V E around common mode (CM) during stimulation is depicted, in which an unbalanced stimulus consecutively raises the remaining DC potential of V E . To prevent irreversible electrochemical reactions a voltage safety window is defined that must not be exceeded [1]. The illustration of electrode current I E shows that the consequence-based CB approaches aim to reduce the electrode potential after each stimulus, whereas the cause-based methods try to symmetrize the anodic and cathodic charge of the stimulus. The latter typically utilizes long-term integration of the electrode voltage [1,2], but might show a settling process with overshoots during startup. For reliability reasons, most of today's certified medical devices are equipped with passive CB, mostly DC-blocking capacitors, despite their disadvantage in size, settling time, and uncontrolled balancing. Active consequence-focused compensation based on the injection of a fixed amount of charge by short current pulses is proposed in [1,3,4]. However, the ensuing possibility of "an unwanted neural stimulation has not been proven yet. [...] the maximum amount of mismatch charge, which can be compensated, depends on the adjusted charge per pulse and the number of pulses allowed over time" [1], demanding a precise configuration of the system during long-term trials.The main advan...

Analysis of passive charge balancing for safe current-mode neural stimulation

“…The latter can be explained with Fig. 4, where curves of the tolerable mismatch errors are calculated with respect to the discharge time, from equations (8) and (15). It can be noted that for the electrode parameters above, the matching conditions between the stimulation currents are very relaxed (> 17%) for discharge times longer than 1 ms (∼ 0.3τ e ), when a limit for V C dl ∞ of ±100 mV is considered.…”

“…However, much smaller windows are often considered for safety (e.g., ±100 mV [7], or ±50 mV [14]). Though some implementations that aim at controlling the electrode potential already exist [7], [15], this approach is not as common as those that try to limit the DC current. However, as indicated in [3], it might be not only easier but also desirable to design pulse generators that avoid exceeding the water window, rather than those which ensure the stimulus is precisely charge-balanced.…”

A Multi-Channel Stimulator With High-Resolution Time-to-Current Conversion for Vagal-Cardiac Neuromodulation

“…Given the strict power constraints imposed on implantable devices, state-of-the-art neural integrated stimulators are required to drive multiple channels and operate power-efficiently. For example, using dynamic HV supplies [14] or merging the I-DAC directly into a HV output stage at the expense of chip area [15], [16].…”

A 24-Channel Neurostimulator IC With Channel-Specific Energy-Efficient Hybrid Preventive-Detective Dynamic-Precision Charge Balancing

“…Considering commonly used electrodes (C DL <100nF) and a relatively-high stimulation frequency of 100Hz [22], [29], [30], the 100nA error translates into a few millivolts voltage error. This shows that (i) some of the threshold values used in the literature (20-100mV range [22], [23], [26], [31], [32]) might only work for low-frequency stimulation or small C DL values; and (ii) the threshold value is dependent on both the application and the electrodes, so it must be programmable.…”