The ability to be in continuous listening mode and waiting for an event with zero power consumption can result in smaller batteries, or a battery life that is extended by years in some cases, according to researchers at the Electrical Energy Management Group of the University of Bristol. And so they have developed a low-voltage comparator circuit that could be used to reduce standby power to zero.

The circuit is a low-threshold analog voltage comparator with output buffer that only requires picowatts to activate other circuitry. One of the lead researchers, Bernard Stark, told EE Times Europe that the circuit, which is the subject of a patent application, uses multiple devices and thresholds to allow high Vin maximum of 20V and voltage thresholds for detection down to below 0.5V. There are also power-saving techniques to almost eliminate static current through the device. Off-state output leakage is below 100 picoamps.

Possible nullifications

"The voltage detector chip uses over a thousand times less energy than existing detectors to create a turn-on signal, merely five picojoules of energy and only around half a volt. Many sensors can provide this without requiring a power supply, therefore making listening effectively free," the University of Bristol said in a statement.

The voltage detector only requires a few picowatts to activate. In a wireless sensor node for the IoT the voltage detector could be powered by the initial signal coming from the primary sensor. This could then be used to turn on the rest of the node to capture sensor data, process and transmit it.

For equipment like security alarms, activity monitors and other IoT devices, the energy to keep the device alive and listening, can far outweigh the energy used to react. In these cases, it is especially important to eliminate listening power in order to increase battery life and make a system that is less environmentally wasteful.

Voltage detector ICs

A sensor can be used to generate a small voltage of 500mV or 650mV and the voltage detector is then able to trigger an open-drain output that activates a switch. The circuit uses only a few picoamps from the sensor, thereby permitting the use of conventional sensors, rather than more powerful and bulky energy harvesters. The detector is suitable for input signals with input voltage gradients from 0 to 10V/ms. At voltage gradients higher than 10V/ms, the threshold increases. Detection hysteresis prevents output oscillations.The university has two voltage detector ICs available as samples, the UB20M and the UB20L both implemented in 180nm HVCMOS by AMS AG.

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