Nanopower magnetoresistive sensor ICs cut costs

Graham Prophet -May 23, 2014

Honeywell has introduced a nanopower anisotropic magnetoresistive sensor IC; the technology provides position detection based on magnetic field sensing, using a very small and low-cost package, with on-chip integration of level detection and switching to give a simple on/off output and to replace reed switches where they are still the only option.

The device's high sensitivity, Honeywell says, allows you to cut costs, and to use the sensors in battery operated equipment with extremely low power requirements, where the solid state, non-contact design provides a reliable, durable alternative to reed switches.

The Nanopower Anisotropic Magnetoresistive Sensor ICs provide the highest level of magnetic sensitivity (as low as 7 Gauss typical) while requiring nanopower (360 nA average, in a typical reed-switch-replacement application). Smaller and more durable and reliable than reed switches, at the same sensitivity and essentially the same cost, the new sensor ICs can be deployed where previously only reed switches could be used for reasons of low power requirements and large air gap needs. The 360 nA figure is based on a 0.015% duty cycle of typically 15 µsec wake/100 msec sleep cycle.

Response time is not specified in the initial data sheet; the 100 msec cycle appears chosen to match the requirements of a typical reed-switch application, detecting whether or not (say) a door is open or closed, within 1/10th of a second. The 15 µsec on-time implies an actual detection in single-figures-µsecs, which could support detection of events at up to (perhaps) around 100 kHz – at the cost of running the device at its on-state current, which is 1 mA typical – sleep current is 160 to 260 nA over the supply voltage range.

These ICs' higher sensitivity can function over air gaps twice the distance that Hall-effect sensors can accomodate. The higher sensitivity improves design flexibility and can offer significant application cost savings by utilising smaller or lower strength magnets.

The Nanopower Series Magnetoresistive Sensor ICs are designed for use, Honeywell says, in a wide range of battery-operated applications including water and gas meters, electricity meters, industrial smoke detectors, exercise equipment, security systems, handheld computers, scanners, as well as white goods such as dishwashers, microwaves, washing machines, refrigerators and coffee machines, and medical equipment such as hospital beds, medication dispensing cabinets, infusion pumps, and consumer electronics such as notebook computers, tablets, and cordless speakers.

"Due to the significant price increases for rare earth magnets, design engineers using Hall-effect sensors have been looking for ways to decrease the total cost of design by using less magnetic material, or moving to a more common magnet in their applications," said Josh Edberg, senior product marketing manager for Honeywell Sensing and Control. "Design engineers are also looking for an alternative to reed switches to reduce size and increase quality and durability, while maximising battery life."

The Nanopower Series is available in two magnetic sensitivities:

  • Ultra-high sensitivity SM351LT: 7 Gauss typical operate, 11 Gauss maximum operate, very low current draw (360 nA typical);
  • Very high sensitivity SM353LT: 14 Gauss typical operate, 20 Gauss maximum operate, very low current draw (310 nA typical).

The sensors operate from 1.65 to 5.5V; omnipolarity allows the sensor to be activated by either a north or south pole, eliminating the need for the magnet polarity to be identified, simplifying installation and potentially reducing system cost. The push-pull (CMOS) output does not require external resistors, making it easier and more cost-effective to operate.

The non-chopper-stabilised design eliminates electrical noise generated by the sensor. The SOT-23 surface mount package is smaller than most reed switches, allows for use in automated pick-and-place component installation and can reduce production costs.

Nanopower Series Magnetoresistive Sensor ICs information page
Nanopower Series Magnetoresistive Sensor ICs application note (PDF)

Honeywell Sensing and Control; http://sensing.honeywell.com

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This article originally appeared on EE Times Europe.

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