A Programmable Energy Recycling Resonant Pulser for Miniature Wearable Ultrasound Applications

Abstract

This work presents an ASIC designed for portable (wearable) ultrasound (US) imaging systems. The ASIC employs a new type of energy-efficient high-voltage (HV) transmit pulser able to generate pulses up to 30V directly from a low-voltage battery supply to excite an ultrasound transducer. HV operation is a necessity in US imaging transceiver design in order to generate sufficiently large pressure waves inside of the body and hence obtain high roundtrip SNR. Traditionally, external HV supplies are used to supply the transmit pulser with 10’s of volts. Currently, for portable designs, on-chip HV DC-DC converters are employed to generate the HV supply, and large off-chip decoupling capacitors are required to regulate the HV supply. The goal of this work is to circumvent these additional (very large) conversion losses and increase the end-to-end efficiency of portable US imaging systems significantly.

The pulser uses only a single off-chip component, an inductor, to produce HV half-sine wave pulses to excite a US transducer. It uses the resonance energy transfer from the energy stored on the inductor to create a pulse with the capacitance associated with the US transducer, hence the name ”resonant pulser”. The resulting system architecture leads to a small area-efficient design. The resonant operation of the pulser makes it possible to recycle residual reactive energy left on the transducer back to the source resulting in an energy-efficient design.

Two prototype chips have been taped out in TSMC 180nm BCD Gen2 technology. One of the ASICs implements a single pulser design which has been optimized for power efficiency, while the second ASIC implements 2 transmit channels and the ability to receive echoes. The two-channel ASIC has been measured both electrically and acoustically with CMUT transducers. It successfully generates programmable pulse amplitudes with a 1V accuracy. The ASIC is the first able to directly create an HV pulse from a low voltage supply, and the first reported portable design to use only a single off-chip component. Besides the pulser being 63% more efficient compared to a Class-D pulser, it saves a considerable amount of power considering the omission of a HV supply.