Proximity sensing for smarter mobile power management

Proximity sensing for smarter mobile power management

Improvements in optical proximity sensors are enabling designers of smart mobile devices to enhance power management and add new features

BY BOB KIRK
Strategic Business Development, Image Sensor Unit
ON Semiconductor
www.onsemi.com

Smart mobile devices are evolving quickly, offering an ever-increasing range of networking and media features that allow users to stay connected and entertained almost continuously. As a result, more effective power management is needed to achieve acceptable battery life.

Turning off circuitry such as the display backlight, when unused, is an effective way to conserve power. Designers use various techniques to keep the backlight off as much as possible without annoying the user, such as detecting when a flip-cover is closed or if the user has not touched the screen for some amount of time.

A more recent development is to detect when a device is being used in a manner where the display does not need to be lit, such as when held next to the ear during a phone call. In modes that require the display to be lit, power can be saved by reducing backlighting to the minimum acceptable level. In darkened surroundings, the display can be dimmed significantly with considerable power savings. Proximity sensors can help improve both of these approaches.

Proximity and power management

Proximity sensors have been used for years in industrial automation. The most basic sensors can determine the presence of a production unit interrupting a light beam shone onto the sensor from the opposite side of a path or conveyor. More sophisticated sensors reflect light off the target back to a photo sensor located near the light source. This approach can be categorised as a near-far detector as it can detect when an item is close to the sensor.

This same near-far sensing can be applied to smart mobile devices. In its most basic form, it can determine when a flip-lid or keyboard tray is closed. It can also be used to detect when a phone is held close to the face.

Infrared LEDs operating in the 875nm near-infrared (NIR) range just outside the visible band have relatively low energy consumption suitable for use in battery-powered devices. Silicon photo-diodes, which have a broad spectral response, are highly effective for detecting emissions in this region.

To make a useful proximity sensor, visible and infrared light from sources such as the sun and incandescent and fluorescent lights must be filtered out. In addition to applying an optical filter, the LED can be pulsed at a relatively high frequency and a high-pass filter applied to the photo-diode signal, as illustrated in Fig. 1 .

Fig. 1: Near-far proximity sensing in a smart mobile device.

The received signal is sampled and integrated to determine if the target object is near or far away. With proper calibration, and if the reflectivity of the target (such as the user’s head or ear) is known, the distance can be computed. Reflectivity depends on unpredictable factors such as the presence of jewellery or hair, which can prevent accurate distance calculation. However, assumptions can be made to create a near-far sensor enabling the display backlight to be turned off when the phone is held near the ear, as well as allowing automatic speaker mode and volume control as the phone is moved closer or further away.

Several types of proximity sensors are available. Analog sensors typically use an external resistor to set the detection threshold and a second external resistor sets the LED drive current. Digital sensors are generally more sophisticated with an analog-to-digital converter and digital signal processing to filter the signal and provide various threshold detection options controlled via an I2 C interface. The sensor readings can be accessed over the I2 C interface and typically an interrupt pin is provided to provide a simple near/far output signal.

The power levels used to drive the IR LED are extremely small and the amount of light received at the sensor is greatly attenuated by distance. Therefore the sensitivity of the receiver is an important characteristic. For advanced applications requiring accurate position detection, the resolution of the proximity sensor also becomes important.

Smarter dimming

The broad spectral response of silicon photo-diodes allows use in Ambient Light Sensors (ALS) to dim the display backlight. To avoid false-positive readings preventing dimming under low-light conditions, the ALS can be optimised to mimic the “photopic” response of the human eye, which is more responsive to wavelengths toward the centre of the visible spectrum. This can be achieved satisfactorily using a photopic optical filter.

ALS devices are already used in smart mobiles, and can reduce overall backlight consumption by over 75%. Typically the ALS is placed behind the dark glass region of the screen, which attenuates incident light by up to 90%. This calls for low-lux devices having sensitivity in the 0.1 to 100 lux range and resolution of around 0.1 lux. At such low light levels, the small receiver current, or dark current, generated by thermal noise sources in the silicon can cause significant measurement errors. Hence dark-current compensation is a key consideration when selecting low-lux ALS devices.

An analog ALS typically features a photodiode, a transimpedance amplifier and dark-current compensation circuitry. The output is a current source and may be converted to a voltage with an external resistor. Some provide multiple gain ranges to optimise performance in overlapping light intensity ranges.

Digital ALS devices integrate an analog-to-digital converter (ADC) and typically communicate results via an I2 C interface. Most also provide a linear binary output of the ADC called the count, and provide some means to adjust the count to be equal to lux. Alternatively the adjustment can be performed with a multiply operation in the I2 C host processor.

Some ALS devices produce a logarithmic response to light intensity, which closely mimics the non-linear response of the human eye. Square-root response has also been shown to be useful. Frequently the linear-to-log or linear-to-square root conversion is performed in the I2 C host processor.

The ALS and proximity sensor can be integrated, yielding a valuable bill-of-materials (BOM) reduction and associated cost savings by sharing optical packaging and the I2 C connection. No additional pins are required. Integrating the LED, however, is less advantageous, since LEDs require much less complex silicon processes yet take considerable die area compared to the sensors.

Gesture detection

The illuminance equation for the optical system of Fig. 1 can be expressed as follows:

where ρ is the reflectivity of the target

Mv is the luminance of the IR LED

DIR and DPS are the distances from LED to target, and from target to proximity sensor.

Since the reflectivity of the target is subject to variations, this equation alone does not provide an accurate assessment of the distance to the target. However, adding a second light path provides two equations that can be solved to reveal both distance and reflectivity.

Fig. 2: Distance measurement with four light paths.

Figure 2 shows how four light paths are used to determine the position of the target in x, y, and z axes. An arrangement such as this allows optical proximity sensing to support features such as simple gesture detection with no need for the user to touch the glass. Using triangulation it is possible to determine if one’s finger is moving left or right or from bottom to top, creating a pan gesture. Similarly, zooming can be achieved by moving a finger closer to the screen or further away. Other gesture modes, including 3D gestures, are also possible.

New generations of optical sensors, such as integrated ambient light and proximity sensors, can help improve power management, enabling significantly reduced power consumption and longer battery life. Improvements in proximity sensing are also opening the door to novel contactless 3D gesture detection. ■