A Single-Inductor-Based High-Voltage Transmit Beamformer for Wearable Ultrasound Devices Achieving 88% fCV2 Power Reduction

Compared to its implant-based electrical counterpart, ultrasonic vagus nerve stimulation alters neuronal activity through mechanical ultrasound waves, emerging as a promising non-invasive therapy for managing drug-resistant epilepsy [1]. To enable immediate intervention following seizure detection, a low-power, compact wearable device is sought to send continuous low- or medium-intensity focused ultrasound waves to targeted areas, such as the cervical vagus nerve (Fig. 35.1.1). The integration of transducer arrays and ASICs supporting high-voltage (HV) pulsing enables on-chip electrical transmit (TX) beamforming with sufficient acoustic pressure at the focal spot and allows for the miniaturization of such devices, but it also poses challenges in power consumption. A conventional class-D pulser (Fig. 35.1.1) exhibits excessive $\text{fCV}^{2}$ power loss due to the charging and discharging of the transducer's capacitance. A resonant pulser, in contrast, uses an inductor to recycle the associated energy, ideally reducing $\text{fCV}^{2}$ loss to 0. However, previous works [2], [3] would require multiple off-chip inductors to implement TX beamforming, making them unsuitable for our application. Moreover, the use of body-diode conduction for energy recycling significantly reduces power efficiency [2]. This paper presents a resonant TX beamformer architecture that employs a single inductor to pulse groups of transducer elements at different phases, thus enabling the required efficient continuous-wave TX beamforming.