Super Tiny Bluetooth Transmitter Powered for 11 Years

IoT devices are typically battery powered, with expected service intervals measured in months or a few years. In some cases, like the tire pressure sensors on automobiles, the entire device is replaced instead of the battery. What if IoT devices required such minute amounts of power that the primary source of loss experienced by the battery was self-discharge, rather than draw? A research team, led by assistant professor David Wentzloff at the University of Michigan, have just about gotten to that point.

The team successfully fabricated a millimeter scale, standalone BLE chip. The chip would be the cornerstone of BLE “motes,” mote being shorthand for remote or micromote. The device was announced and demonstrated at the 2019 International Solid-State Circuits Conference in February of this year.

Conventionally, a radio transmitter has three elements, an antenna, a tunable oscillator and an amplifier to boost the oscillators output. To drive the size of the chip down, the team combined the radio transmitter’s RF oscillator with the antennae. This eliminated the need for an amplifier. The team dubbed this combined unit a “power oscillator.”

As the antenna is used a as an element in the power oscillators resonant tank circuit, the antenna is much larger than what it might have been had the team gone with a conventional transmitter topology. Though the antenna is larger, in this case being a 14mm long PCB trace, the Q-factor of the power oscillator is five times what it would have been has they etched the inductive element into the chip. Q-factor is a figure-of-merit that describes how much energy it takes to keep an oscillating element oscillating at a desired frequency and amplitude. This high Q-factor contributes tremendously to the chip’s nearly non-existent power requirements. Drawing 0.6mW from a 5.8mm coin cell battery, the battery is estimated to last for 11 years! At that rate, the battery might die of old age before the BLE chip sucks it dry.

For more details, you will have to read the team’s paper, titled “Analysis and Design of an Ultra-Low-Power Bluetooth Low-Energy Transmitter With Ring Oscillator-Based ADPLL and 4x Frequency Edge Combiner.”

As a final thought. It might be possible to power devices like that indefinitely, with tiny on-board MEMs generators or maybe millimeter-sized Seebeck devices. Buildings, vehicles, clothing, your own body, almost anything could be “seeded” with IoT motes. If some of the devices are destroyed or become inoperative due to other means, it won’t matter. Their fellow motes are legion and will continue to report temperature, movement, strain, or whatever it is that they were designed to do.