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MIT demos picowatt radio for Internet of Things

Turn it off. No, REALLY off

MIT boffins working on low-power radio have given the International Solid State Circuits Conference (ISSCC) an insight into how standby power can be cut in Internet of Things devices.

It's a thorny problem: if, for example, a sensor is waiting to be polled so it can answer with a snippet of data, the radio has to be in a state where it can be woken up. On the other hand, as MIT electrical engineering professor Anantha Chandrakasan says, when the device is off, you want the least possible leakage currents.

As Chandrakasan notes, even in the off-state, semiconductors allow small currents to flow across the gates, and it's that leakage he's looking to eliminate.

To do that, his group has experimented with using charge pumps to provide a small reverse-polarity charge to transistors.

For example, in an NPN transistor, a positive voltage on the gate is what allows current to flow across the semiconductor. To block the leak current, the MIT group's charge pump applies a small negative charge to the gate when the device is idle. This, the university's release explains, drives electrons away from the transistor and makes it a better insulator when it's idling.

The university describes the operation of the charge pump thus: “When the charge pump is exposed to the voltage that drives the chip, charge builds up in one of the capacitors. Throwing one of the switches connects the positive end of the capacitor to the ground, causing a current to flow out the other end. This process is repeated over and over. The only real power drain comes from throwing the switch, which happens about 15 times a second.”

MIT's picowatt radio chip

Power down: MIT's ultra-low-power radio

Even with the power needed for switching, the university says tests on a prototype made by TSMC found the design “spent only 20 picowatts of power to save 10,000 picowatts in leakage”.

The other key part of the work described in the snappily-titled A +10dBm 2.4GHz Transmitter with Sub-400pW Leakage and 43.7% System Efficiency was to break down the radio system design so that most of the circuit can run at the lowest possible voltage.

Here, the problem is different: the higher the radio frequency you want to drive the transmitter at, the higher the voltage you need and more power it's going to consume.

It's cheaper to fabricate a radio chip as a single unit, meaning that the operating voltage of the whole module is a function of what the transmitter needs. Instead, to drive down power, the Chanrakesan team used voltage doubler circuits of capacitors and inductors around the transmitter to give it a higher operating voltage, while leaving the rest of the device at a lower voltage.

MIT says the device is suitable for Bluetooth and 802.15.4 transmissions.

Texas Instruments and Shell contributed funding to the research. ®

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