Si1120

6

Rev. 1.0

2. Application Information

2.1. Theory of Operation

The Si1120 is an active optical-reflectance proximity detector and ambient-light sensor with a pulse-width

modulated output. Depending on the mode selected, the duration of the PRX active (low) state is proportional to

the amount of reflected light, the amount of zero-reflectance offset, or the amount of ambient light. The detection

rate can be set by adjusting the frequency of the STX signal.

The dual-port, active reflection proximity detector has significant advantages over single-port, motion-based

infrared systems, which are good only for triggered events. Motion detection only identifies proximate moving

objects and is ambiguous about stationary objects. The Si1120 allows in- or out-of-proximity detection, reliably

determining if an object has left the proximity field or is still in the field even when it is not moving.

An example of a proximity detection application is controlling the display and speaker of a cellular telephone. In this

type of application, the cell phone turns off the power-consuming display and disables the loudspeaker when the

device is next to the ear, then reenables the display (and, optionally, the loudspeaker) when the phone moves more

than a few inches away from the ear.

For small objects, the drop in reflectance is as much as the fourth power of the distance; this means that there is

less range ambiguity than with passive motion-based devices. For example, a sixteen-fold change in an object's

reflectance means only a fifty-percent drop in detection range. The Si1120 periodically measures proximity at a rate

that can be set by an external controller.

The Si1120 modes are:

Proximity, 400 mA LED current

Proximity, 50 mA LED current

Proximity, 50 mA LED current, high reflectance range

Offset calibration (proximity mode, no LED current)

Visible ambient (10 klux sunlight)

Visible and infrared ambient light, low range (500 lux sunlight)

Visible and infrared ambient light, high range (128 klux sunlight)