LIS331DLF datasheet(15/38 Pages) STMICROELECTRONICS

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LIS331DLF

Functionality

Doc ID 15101 Rev 4

15/38

Functionality

The LIS331DLF is a “nano”, low-power, digital output 3-axis linear accelerometer packaged

in a LGA package. The complete device includes a sensing element and an IC interface

able to take the information from the sensing element and to provide a signal to the external

world through an I2C/SPI serial interface.

3.1

Sensing element

A proprietary process is used to create a surface micro-machined accelerometer. The

technology allows to carry out suspended silicon structures which are attached to the

substrate in a few points called anchors and are free to move in the direction of the sensed

acceleration. To be compatible with the traditional packaging techniques a cap is placed on

top of the sensing element to avoid blocking the moving parts during the moulding phase of

the plastic encapsulation.

When an acceleration is applied to the sensor the proof mass displaces from its nominal

position, causing an imbalance in the capacitive half-bridge. This imbalance is measured

using charge integration in response to a voltage pulse applied to the capacitor.

At steady state the nominal value of the capacitors are few pF and when an acceleration is

applied the maximum variation of the capacitive load is in the fF range.

3.2

IC interface

The complete measurement chain is composed by a low-noise capacitive amplifier which

converts the capacitive unbalancing of the MEMS sensor into an analog voltage that is

finally available to the user by an analog-to-digital converter.

The acceleration data may be accessed through an I2C/SPI interface thus making the

device particularly suitable for direct interfacing with a microcontroller.

The LIS331DLF features a data-ready signal (RDY) which indicates when a new set of

measured acceleration data is available thus simplifying data synchronization in the digital

system that uses the device.

The LIS331DLF may also be configured to generate an inertial wake-up and free-fall

interrupt signal accordingly to a programmed acceleration event along the enabled axes.

Both Free-Fall and Wake-Up can be available simultaneously on two different pins.

3.3

Factory calibration

The IC interface is factory calibrated for sensitivity (So) and Zero-

g level (TyOff).

The trimming values are stored inside the device in a non volatile memory. Any time the

device is turned on, the trimming parameters are downloaded into the registers to be used

during the active operation. This allows to use the device without further calibration.

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