“The Next Step” in Precision 3D Location Sensing - LPS331AP

Accurate location of mobile devices will be the key enabler for many emerging Location-Based Services, which are widely expected to be the next wave of “killer applications” in the mobile world. The challenge is to provide the means of identifying the location of the mobile device in three dimensions in a way that meets a variety of conflicting constraints including spatial resolution, reliability, physical size, robustness, and cost.

For the horizontal part of the location (latitude and longitude), the universally adopted solution is Global Navigation Satellite System, which allows the horizontal position of the device to be calculated to within a meter in optimum conditions where the device can receive signals from four or more satellites. ST has already demonstrated a solution for indoor navigation developed in conjunction with CSR that can locate devices horizontally and vertically, even in the absence of any satellite signals.

For the third dimension (vertical elevation), atmospheric pressure can provide greater resolution than GNSS - especially when fewer than four satellite signals are visible - as pressure drops steadily with increased elevation. ST’s new pressure sensor can accurately measure air pressure from 260 millibars, which is the typical air pressure at a height of around 10 km (about 1,500 m higher than the summit of Mount Everest) to 1,260 millibars, which is the typical air pressure at 1,800 m below sea-level, about half the depth of the deepest mine ever dug. Housed in a tiny 3x3mm package and offering low voltage operation and ultra-low power consumption, the new device is ideal for use in smart phones, sports watches, and other portable equipment, as well as in weather stations and automotive and industrial applications. The LPS331AP has already been adopted for use in Samsung’s latest and most advanced smartphone.

“Since launching the Consumerization Wave of MEMS devices, we have developed and delivered devices able to detect gravity, acceleration, angular rate, and the Earth’s magnetic field,” said Benedetto Vigna, Executive Vice President, Analog, MEMS & Sensors Group, STMicroelectronics. “All these sensors help smart consumer devices detect and react to motion. Now we are introducing a device that senses the weight of the atmosphere and will help consumer products recognize if a person or an object is moving up and down.”

ST’s LPS331AP pressure-sensing device is fabricated using a proprietary MEMS technology, called “VENSENS”, that allows the pressure sensor to be fabricated on a monolithic silicon chip. Manufacturing the device in this way eliminates wafer-to-wafer bonding and maximizes reliability. The sensing element in the LPS331AP is based on a flexible silicon membrane formed above an air cavity with a controlled gap and defined internal pressure. The membrane is very small compared to traditional silicon micro-machined membranes and is protected from breakage by built-in mechanical stoppers. A piezoresistor, a tiny structure whose electrical resistance varies as the membrane flexes in response to changes in the external pressure, is embedded in the membrane and the change in resistance is monitored, thermally compensated, and converted to a digital pressure value that can be read by the equipment’s host processor using the industry-standard I2C or SPI interfaces.

Main technical features of the LPS331AP:

-High resolution, low-noise sensor able to detect differences within centimeters of altitude variation;
-ST’s proprietary VENSENS technology providing high burst resistance, up to 20 times full scale;
-Selectable ODR from 1 Hz to 25 Hz;
-Low power consumption: low res. mode 5.5µA, high res. mode: 30 µA;
-Supply voltage: 1.71 to 3.6 V;
-A temperature range extending from -40 °C to +85 °C;
-Integrated temperature sensor enabling temperature span compensation;
-Fab calibration of pressures and temperatures, eliminating the need for in-house calibration of shipped devices;
-Supplied in a 3x3x1mm plastic land-grid-array (HCLGA-16L) package. The package is holed to allow external pressure to reach the sensing element.

The LPS331AP is now in volume production at a typical price of $2.6 in quantities of 1,000 units. Samples and evaluation kits are available to support design-in and shorten time-to-market. ST’s pressure sensor evaluation kit consists of a motherboard (STEVAL-MKI109V2) and a plug-in module, the STEVALMKI120V1 adapter including the LPS331AP pressure sensor.