Power consumption is always a major concern in the field of electronics, especially as the circuits controlling these electronics shrink in size while also growing in complexity. Utilizing a fairly new, ultra-low power technique known as a sub-threshold voltage mode for transistors operating in the circuit, the company named Psikick has developed what they call a “revolutionary” wireless sensor. Subthreshold sensors have significant power supply voltage (VDD) reduction, it is said to be 100 to 1000 times more power efficient than any other recently designed sub-threshold wireless sensor networks. The Department of Electrical Engineering at Texas A&M University published a paper in Circuits and Systems, 2005 titled, “Low power current mode ADC for CMOS sensor IC,” stating that they had created an integrated sensor utilizing sub-threshold and current mode techniques for low-power operation. The power consumption of their integrated circuit was under 6μW. According to the claims made by the Charlottesville-based company Psikick, this would mean that their wireless sensor technology only consumes between 6 and 60 nanowatts of power. Some of the benefits to such a low power design could include battery-less operation of the sensor, low-power voltage sources such as can be collected from wind, vibration, thermal gradients, solar, piezo actuation, and RF (radio frequency) energy scavenging. The applications for such a device could be limitless, useful in everything from medicine to athletics or the military. Such low powered sensors could be used indefinitely to measure patients heart rates or brain waves from the comfort of their own homes. They could be used in avionics to measure conditions outside the aircraft with little to no power input, or to measure the movements and heart rates of athletes for optimum performance on their field.
Another letter by the Dept. of Electronics & Computer Engineering at the University of Colorado at Boulder, published in Power Electronics in Dec. 2010, titled, “Custom IC for Ultra-low Power RF Energy Scavenging,” describes what makes these low power energy sources possible. They presented a custom integrated circuit including an ultralow power RF rectifying antenna power source and a microbattery for maximum power collection. This energy scavenger circuit operated a “boost converter in pulsed fixed-frequency discontinuous conduction mode to present a positive resistance to the rectifying antenna.” Their subthreshold current source was in the 200 nA range, placing their supply voltage in a range from 2.5 V to 4.15 V. This resulted in a power consumption of between 1.5 and 30 μW, with a higher conversion efficiency at higher voltages. This IC was made two years prior to Psikick’s wireless sensor, utilizing very similar technologies (CMOS, RF scavenging, subthreshold processes, etc.). Although the integrated circuit was designed for RF energy scavenging, the low-power “boost converter” mentioned is also responsible for the application of some other power sources mentioned, like wind, vibration, and temperature. On the homepage for the Psikick sensor, the Psikick team brags that their sensor is “Fully integrated and silicon-proven,” generally meaning that the technologies involved have proven to work as expected. However, no actual numbers are ever mentioned relating to supply voltages, dynamic or static power consumption, or subthreshold currents, with the exception of the 100 to 1000 times lower power claim. While this makes it difficult to determine any of the various technologies employed in the sensor’s development beyond speculation, the hype that Psikick has stirred up relating to their new technology definitely makes their design sound promising.
When a CMOS transistor operates at the sub-threshold voltage many problems start to be relevant. Problems that are normally insignificant or nonexistent when the transistors operate at normal conditions. When VDD is reduced, the dynamic energy also is reduced but the transistor leakage over longer time periods increases the leakage energy. Therefore it is necessary to balance both energies and find the best point of operation, normally at VDD (around 300-500 mV). Another problem is that the pMOS and nMOS thresholds are imbalanced and may lead to the need to change the circuit design to correct this difference. Circuits with several series and parallel transistors are another problem due to the stack effect, the series transistor will have less current when ON than the parallel transistors when OFF, making it necessary to raise the source current. Several parallel transistors are also a problem when the circuit is designed to operate as a static structure, since it will increase the leakage current and make the parallel transistor have a greater current when OFF than the series transistors when ON. Dynamic circuits should also be avoided, because the transistors logic works with subthreshold leakage current and this current gradually discharges the dynamic nodes.
Psikick is aiming to produce an application standard product for each vertical market. This helps on the production side of the chips but it also takes up extra space on the chip size level when they are going for more functions that will not be needed for every job. However this is not the main focus of Psikick since the goal was reduction in power consumed. With the amount of power savings, the use for these chips in The Internet of Things will be interesting to see. With the tiny size of these chips, the potential uses are going to seem limitless. Some such uses might be industrial process control, infrastructure monitoring, precision agriculture, medical biosensing, consumer wearables, smart homes/grids/cities, and many more according to the Forbes article about the new startup.
Some disadvantages to subthreshold processing may include a lowered processing power and a much slower speed. This is due to the much lower power constraints of a subthreshold circuit, and so the chip runs at only a “few tens of megahertz at most,” according to the article by Forbes. This is much slower than the gigahertz speeds at which most of us are used to having our chips run. On the plus side, however, there are many uses for which we do not need a significant amount of processing power but rather require minimal processes under extremely extended intervals. In a case such as this, being able to power a device, some type of monitor or sensor, for days, weeks, or maybe even years on end without ever stopping for a recharge or a reboot is a world-changing technology. Sending patients with bad hearts back home and monitoring their vitals via a subthreshold wireless sensor could not only save lives, but provide comfort while doing it. The opportunities for this new company and their circuit design are potentially limitless, and it will be exciting to see the spread of this new technology.
Article in question for reference: These Energy-Saving, Batteryless Chips Could Soon Power The Internet Of Things
By Adam Westman, Christian Sasso and Tanner Helton
The University of Mississippi Electrical Engineering Department introduced a Digital CMOS/VLSI Design course this semester. As part of this course, students researched a contemporary issue and wrote a blog article about their findings for presentation on SemiWiki. Your feedback is greatly appreciated.