Implantable medical devices are becoming smaller and more deeply implanted in the human body for various applications (i.e., neurostimulation, drug delivery, bone fracture monitoring). Therefore, an efficient ultrasound power transfer link is needed to charge these devices. However, this is challenging because each ultrasound transducer has limited angular sensitivity. This work proposes a low-power telemetry protocol that can reliably feedback the power sent to the implant with backscattered ultrasound. The protocol works by sending two consecutive interrogation signals and connecting a circuit on the receiver that modulates only one of the two signals. The modulated signal can be decoded with an external ultrasound probe. In this work, the circuit was built, verified, and compared with simulation results. It was shown that the telemetry protocol could accurately localize the receiving ultrasound element at sub-mm precision at a 10 cm depth. ... Read more