The converter is typically 50% more efficient than existing converters at these current levels and is aimed at sensors for the Internet of Things and wearable designs.
“Typically, converters have a quiescent power, which is the power that they consume even when they’re not providing any current to the load,” says Arun Paidimarri at MTL when the work was done and now at IBM Research. “So, for example, if the quiescent power is a microamp, then even if the load pulls only a nanoamp, it’s still going to consume a microamp of current. My converter is something that can maintain efficiency over a wide range of currents.”The step-down converter takes input voltages ranging from 1.2 to 3.3 V with an output of 0.7 to 0.9 V.
“In the low-power regime, the way these power converters work, it’s not based on a continuous flow of energy,” said Paidimarri. “It’s based on these packets of energy. You have these switches, and an inductor, and a capacitor in the power converter, and you basically turn on and off these switches.”
The control circuitry for the switches includes a circuit that measures the output voltage of the converter. If the output voltage is below some threshold — in this case, 0.9 volts — the controllers throw a switch and release a packet of energy. Then they perform another measurement and, if necessary, release another packet.
If no device is drawing current from the converter, or if the current is going only to a simple, local circuit, the controllers might release up to two hundred packets per second. But if the converter is feeding power to a radio, it might need to release a million packets a second.
The converter uses a variable clock, which can run the switch controllers at a wide range of rates. That, however, requires more complex control circuits. The circuit that monitors the converter’s output voltage contains a voltage divider to provide current from the output for measurement, but this can reduce the converter’s efficiency.So in the MIT researchers’ chip, the divider is activated only briefly for a measurement, reducing the quiescent power by 50% over even the best previously reported experimental low-power, step-down converter and a tenfold expansion of the current-handling range.
“This opens up exciting new opportunities to operate these circuits from new types of energy-harvesting sources, such as body-powered electronics,” said Anantha Chandrakasan, the Vannevar Bush Professor of Electrical Engineering and Computer Science at MIT.