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Digital Ambient-Light Sensors Offer Improved Response to Tinted Glass

Analog and digital ambient light sensors (ALS) play a key role in controlling backlighting in LCD displays used in portable electronics like tablets, smartphones, notebooks and TVs. Besides continuing to lower the power consumption of these devices with wider dynamic range, suppliers are also improving the spectral response to closely match the human eye for accurate light measurements, especially when the top display glass is tinted or dark in color. Black glass shifts the response to various light sources, all of which have different light-emission spectra, resulting in measurement errors.

To address this issue in the latest round of tinted displays, some leading manufacturers have improved the black-glass operation and tailored their respective digital ALSs to replicate the response of the human eye. These companies include Maxim Integrated, OSRAM Opto Semiconductor and Texas Advanced Optoelectronic Solutions (TAOS).

Maxim has unwrapped a new version of its low-power digital ALS. The MAX44007 factors in the black-glass response to improve the accuracy and sensitivity of the light sensor. By combining dual-photodiode architecture with a unique adaptive-gain block, Maxim’s MAX44007 allows the user to adjust internal gain settings to compensate for the dark glass profile, thereby offering improved response to low-light operation.

To eliminate the time-consuming process of manually programming the device, MAX44007’s integrated adaptive-gain block automatically selects the optimum gain range, providing an extremely wide dynamic range while offloading work from the application processor or microcontroller. As a result, the MAX44007 can measure light levels from 0.025 to 104,444 lux, yielding a dynamic range of more than 4 million to 1. In short, Maxim’s new digital ALS is designed to provide the most precise lux measurements over the widest range of ambient-light conditions.

Fabricated in proprietary BiCMOS technology, the MAX44007 integrates two optical sensors, a 16-bit analog-to-digital converter (ADC), and an I²C digital interface into a tiny 2 x 2 x 0.6mm package. This integration saves valuable board space while delivering the highest performance. To measure ambient light, the die is placed inside an optically transparent (UTDFN-Opto) package. A package-level optical filter prevents ultraviolet (UV) and infrared (IR) light from reaching the photodiode. Its optical response is also designed to match the spectral response of the human eye. A second photodiode array, sensitive primarily to the IR spectrum, is then used to match flourescent and incandescent light response from the part.

Aside from its extremely wide dynamic range, the power consumption of MAX44007 is extremely low to meet the tightest energy budgets. The typical operating current is 0.65μA from a supply voltage of 1.7 to 3.6V. According to Maxim, the MAX44007 consumes 100x less power than competing products in the market.

Since every clock and data bit transmitted on I²C interface requires power consumption, minimizing the number of I²C transactions results in energy savings (see graph below). To facilitate ultra-low power consumption of the sensor chip, an interrupt pin eliminates the need for the system to poll the device continuously. In addition to freeing up processing resources for the master, this scheme also improves overall system performance.

By using an interrupt pin on the digital ambient light sensor, the system requires minimum I²C transactions to make accurate light measurements, resulting in significant reduction in overall power consumption.

Accuracy is another salient feature of this solution. Typically, in light-sensing systems, any variance along the light path results in different measurements. Factors contributing to this variance include: placement of the sensor on the board, the distance tolerance between the top of the sensor and the glass surface and the transmission characteristics of the glass covering the sensor. Additionally, if the part-to-part variation of the sensor is also taken into consideration, the reading error can be as much as 50 percent. This causes false triggers and eventually poor yield at production.

With the all-in-one integrated solution provided by the MAX44007, the maximum total gain error is as low as 15 percent. Furthermore, the digital communication of the light sensor is immune to parasitics. Since the ADC integration time of the sensor can be adjusted from 6.25 to 800 ms, a default integration time of 100 ms ensures excellent 50/60-Hz rejection. Both these features provide robust and reliable light measurement to obtain high yield at production.

Human-eye-like sensitivity

For its ability to adapt to changing light conditions at the blink of an eye, OSRAM Opto Semiconductors’ SFH 5712 received a product of the year award for 2010 from Electronic Products Magazine this year. Aside from human-eye-like performance in detecting light levels, OSRAM’s low-voltage digital ALS offers the highest data rates on the I²C interface at 3.4MHz.

The SFH 5712’s high accuracy light level measurement ranges from 3 to 65,000 lux. The sensor is compact, requiring only four connections and usingno external analog components to make measurements (see image below). Housed in a surface-mount Chipled package, the sensor’s operating current is 50μA, but maintains a current of 1.5μA in standby mode, consuming little power. Moreover, the sensor is insensitive to mains-power-related flickering at 50/60 Hz, which is primarily produced by fluorescent lamps.

The sensor is housed in a miniature surface mount Chipled package that requires only four connections and uses no external analog components.

TAOS is also competing to deliver digital ALSs with human-eye-like performance. The company is readying its fourth generation ALS family, which will be announced shortly, according to Jerry Koontz, director of marketing. To produce accurate ALS while operating behind spectrally distorting glass and wide range of lighting conditions, the forthcoming ALS from TAOS implements dual-diode architecture. Additionally, it will incorporate circuits that provide reduced power consumption to meet requirements of battery-operated mobile handsets, and will block UV light for improved lux accuracy, Koontz said.