By Bob Kirk, Strategic Business Development, Image Sensor Business Unit, ON Semiconductor
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 figure 1.
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. Analogue 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 analogue-to-digital converter and digital signal processing to filter the signal and provide various threshold detection options controlled via an I2C interface. The sensor readings can be accessed over the I2C 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.
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 analogue ALS typically features a photodiode, a trans-impedance 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 Analogue-to-Digital Converter (ADC) and typically communicate results via an I2C 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 I2C 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 I2C 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 I2C 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.
The illuminance equation for the optical system of figure 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.
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.