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issue: December 2004 APPLIANCE Magazine

Electronics Report
Low-Power Supervisory Circuits


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Austriamicrosystems AG’s AS1904/5/6 family of ultra-low-power supervisory circuits is said to be ideal for appliances that stay in standby mode for long periods of time.

The AS1904 ultra-low supervisory circuit from austriamicrosystems is part of the AS1904/5/6 product family, which the company claims offers the world’s lowest power consumption.

With a typical current consumption of only 150 nA at 3.3 V, the devices can be used in portable and battery-powered equipment such as smoke detectors, heart rate monitors, glucose meters, mobile measurement equipment, and industrial data acquisition systems such as bar code readers and scanners.

The product family offers three devices with different output drivers. The AS1904 has a push-pull driver with an active low reset. The AS1905 uses the same output stage, but has an active high reset. The AS1906 comes equipped with an open-drain output with an active low reset. Each device offers four typical, factory-trimmed, time-out periods of 5, 20, 100, and 500 ms. The reset threshold is also factory-trimmed between 2.2 and 3.1 V with 1.5-percent tolerance.

The supervisory circuits monitor the supply voltage of systems using a microprocessor and initiate a reset as soon as the voltage decreases below a predefined threshold. The state of the reset is active for a predefined time after the supply voltage has risen above the threshold.

“The accuracy of the threshold and the duration of the reset are significant for the appliance producer,” notes Manfred Kogler, a product manager at austriamicrosystems (Schloss Premstätten, Austria). “These features, together with the low-power consumption, create a full package, which gives customers maximum flexibility in their own designs.”

In a smoke detector, for example, the only power-consuming device is the supervisory circuit, which is on continuously, so it is the most critical part of the system. Therefore, reducing power consumption is critical to the effectiveness of the end product.

Mr. Kogler admits that it was challenging to design the low-power devices in such a small form factor. However, using its knowledge base, the company was able to decrease the package size to only 2.5 by 3 mm while also reducing the power consumption. “We use analog mixed signal design and have all of the capabilities in house,” Mr. Kogler explains. “Additionally, by integrating power control functions on a chip, the overall device can be smaller through elimination of additional components.”

He adds that in low-power designs such as the AS1904/5/6, total current consumption is in the region of nanoAmpere, so all leakage currents and parasitics must be taken into consideration. “So an intense knowledge about process, parameters, and circuits is mandatory to achieve such a challenging goal,” he explains.

All the devices in the AS1904/5/6 family operate down to a voltage of 1 V and feature 20-percent lower current consumption than similar competing products, Mr. Kogler notes. Furthermore, a hysteresis in the threshold is said to reduce the potential of fast transients and noise on the supply line that lead to unwanted resets.

 

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