A High Accuracy Digitial Ambient Light Sensor XD1407

With the emergence of a large number of portable electronic products, such as smart phones, laptops, digital cameras and so on, the demand for visual experience and energy saving increasingly high. And, as microelectronics technology matures, highly integrated ambient light sensors widen their use in portable electronic products. Portable devices are widely equipped with high-definition display, whose backlight system is the most dominant power consumer. However, short battery lifetime in portable electronics was always the hard nut to crack to manufacturers and criticized by consumers. For battery technology has no breakthrough, ambient light sensors give an effective solution. Ambient light sensors do photoelectric conversion and analog-to-digital conversion to generate ambient light illuminance data, which can be used to determine whether an interrupt outputs. Systems equipped with ambient light sensors make adjustments corresponding to ambient light illuminance. For example, with ambient light sensors, the display backlight is adjusted in real time to suit ambient light illuminance, avoiding the glare in low light or lack of brightness in high light. This not only save system power consumption, but also meet the human visual requirements. Besides electronic products, ambient light sensors have been applied to areas of industry, medicine and military. It is of great significance to conduct research on ambient light sensors.This paper firstly introduces the background and development of ambient light sensors, then gives a detailed description of functional requirements and design specifications. Design of key modules of the ambient light sensor is prominently studied. According to noise suppression requirements of ambient light sensors, some techniques are proposed as following. In the photodetector module, the subtraction circuit and error correction circuit are designed to accurately filter out dark current of photodiode. And photodiode is covered with optical coating to filter out the infrared from ambient light, making the spectral response close to the human eye’s photopic vision. The solutions of range switching are put forward in analog-to-digital conversion module. That is to change PD pairs, integration time and the on duty cycle of reference current. The range switching takes into account the requirements of wide dynamic range and high resolution. In order to meet the requirements of low-power portable applications, a sleep circuit is designed to reduce the oscillator frequency. In sleep time, the circuit works at a lower clock frequency and saves power consumption. Due to these designs, the ambient light sensor features excellent noise rejection characteristics, high sensitivity, wide dynamic range and low power consumption.Based on the theoretical and technical analysis, ambient light sensor XD1407 is designed in 3.3V 0.35μm CMOS technology. The functional verification is preceding using simulator Hsim and Spectre, and simulation results are within the error ranges of design specifications. The ambient light sensor has good performance and meets design requirements.