Ambient Light and Color Sensing Solutions for Lighting Control and Optical Feedback from STM

The battery life of portable display products such as DVD players, camcorders, and digital cameras, as well as mobile phones and laptop PCs, is highly dependent on the power used by the LCD and on its brightness setting. These sensors, with a standard I²C/SMBus (System Management Bus) interface, are easily integrated into any system to enable the LCD to be adjusted automatically to match ambient lighting conditions. In addition, the VM6101 can be used to provide color balancing adjustment for projection systems and RGB backlighting.

“Ambient light sensing can benefit numerous applications,” commented Jean-Yves Gomez, General Manager of ST’s Imaging Division. “Backlight control for LCDs, whether in mobile phones and other portable consumer devices, or in flat-panel TVs can dramatically enhance the consumer experience, while reducing power consumption and preserving battery life. Current discrete solutions are expensive and cumbersome to integrate. With this new device, ST is offering a level of integration and versatility never seen before in the light sensing market.”

The light sensor is the first in a new series of imaging products. It has been developed using the technical expertise of ST’s Imaging Division in the fields of CMOS imager-on-chip devices and color processing for camera applications, in which ST is a market leader*, to satisfy the need for an integrated, low-cost light sensor for a variety of automatic backlight control applications. In contrast to most imaging markets, where the general trend is towards more pixels for higher resolution, the VM6101 has four (red, green, and blue, and one with no filter), to measure total ambient light level and the separate RGB color components.

Various discrete solutions based on monochrome light-to-voltage sensors have been available in the market, but most require additional processing or conversion circuitry to operate as part of a complete application, thus resulting in a significantly higher bill of materials; the few solutions that can perform color light measurement carry a heavy premium, which precludes many high-volume consumer applications. In contrast, the VM6101 measures Yellow, Red, Green, and Blue components separately on a single chip, which also includes all system components and uses an industry-standard communication bus for data readout and device setup – all at an attractively low price.

Part-to-part matching methods in manufacture ensure repeatable light measurements, while pixel-to-pixel matching in the VM6101 is better than plus or minus 1% (over the full illumination range, at constant temperature). The sensor’s spectral response is matched to that of the human eye through novel frequency-response shaping techniques that reject the IR spectrum, and a very wide dynamic range from 30mLux to 30KLux is achieved by using a light-to-frequency pixel architecture. The device features very low power consumption of 1mA in active mode, and just 1µA in standby mode.

In addition to features that reduce system cost and complexity when used with MCU-based controllers, the VM6101 also offers autonomous operating modes to further reduce or eliminate the CPU overhead. A PWM output can be programmed to give direct control of an LED driver IC, using the data from any of the four sensor pixels. Also, a highly flexible automatic light threshold detector can generate an interrupt signal to alert the system to a change in ambient light.

The VM6101 is based on ST’s proprietary CMOS sensor technology and will be manufactured at the company’s leading-edge facilities at Crolles, France. The device is packaged in a tiny 3 x 3 x 0.7mm 8-lead optical MLPD package. Pricing for the VM6101 is $0.77 for quantities of 10,000 units per year.