Abstract: This paper introduces the LX1970, an integrated visible light brightness sensor capable of simulating human eye brightness. The performance characteristics, working principle, and typical application circuits of the LX1970 chip are presented. The LX1970 is suitable for brightness monitoring systems of flat panel displays and can be used in laptops, flat-screen TVs, and new mobile phones. Keywords: brightness; human eye simulation; sensor; backlight; measurement; LX1970 1. Main Features Currently, laptops, personal digital assistants (PDAs), flat-screen TVs, and mobile phones all use liquid crystal displays (LCDs). However, LCDs themselves do not emit light; they only reflect or transmit ambient light. To facilitate screen viewing in low-light environments or at night, a backlight must be added to the LCD to enhance contrast. A visible light brightness sensor can automatically adjust the brightness of the backlight (usually a white light-emitting diode) according to the ambient brightness, thus achieving optimal display effects and reducing backlight power consumption. Microsemi, a US-based company, has introduced the LX1970, an integrated visible light brightness sensor that simulates the human eye. This device can be used to construct brightness monitoring systems for flat panel displays. Furthermore, it can be used as a controller for outdoor lighting (such as streetlights), enabling the lights to automatically turn on at dusk and off at dawn. The LX1970 visible light brightness sensor features the following characteristics: ● It incorporates a PIN photodiode, a high-gain amplifier, and two complementary current output terminals. The spectral characteristics and sensitivity of this photodiode array are very similar to those of the human eye, thus allowing it to sense the brightness of the environment and convert the received visible light into a current signal to control the brightness of the backlight. ● Peak emission wavelength is 520nm, current sensitivity is 0.38μA/lx, and dark current is 10nA. ● Low nonlinearity error and good repeatability. The current asymmetry between the two complementary output terminals is only ±0.5%, and either terminal can be selected as the output. ● Simple external circuitry, low price, and easy to use. Effectively attenuates ultraviolet and infrared light without the need for filters. ● Low power consumption and low-voltage power supply. Uses a 2-5.5V power supply with a typical current as low as 85μA. Operating temperature range: -40℃ to +85℃. Its dimensions are only 2.95mm × 3mm × 1mm. 2. Working Principle of LX1970 The LX1970 uses an MSOP-8 surface-mount plastic package. Its pin arrangement and internal block diagram are shown in Figure 1. The LX1970 chip has a 0.369 mm² light-receiving area on its front side. UDD and USS are connected to the positive and negative terminals of the power supply, respectively. SNK is the lead-out terminal for the current receiver, and SRC is the lead-out terminal for the output current source. The remaining NC pins are unused. When the chip is operating, the photocurrent generated by the photodiode is amplified by a high-gain amplifier and sent to two current output terminals: one is the current receiver pin SNK, and the other is the output current source pin SRC. The currents of these two pins are SNK and SRC, respectively. ISNK is the current injected into the chip, also known as the sink current. These two current signals can be converted into voltage signals USNK and USRC respectively through R1 and R2. Changing the resistance value of R1 (or R2) can adjust the voltage gain; the allowable resistance value range is 10kΩ to 50kΩ. C1 and C2 are filter capacitors, which can be used to determine the sensor's response time. The output USNK is inversely proportional to the ambient brightness, and USRC is directly proportional to the ambient brightness; the two have complementary output characteristics, and either signal can be selected as the output voltage UO. The relative sensitivity versus wavelength response curve of the LX1970 is shown by the thick line in Figure 2, while the thin line represents the response curve of the human eye (peak wavelength 550nm). As can be seen from the figure, the LX1970 receives light in a wavelength range very close to that of the human eye, and is also as sensitive as the human eye. Its peak wavelength λP is 520nm, and its wavelength range is approximately 350nm–800nm, covering the entire visible light spectrum (400nm–700nm), while the ultraviolet spectrum (<400nm) is also covered. Its narrow wavelengths in both the infrared and infrared bands (>700nm) indicate that it has the highest sensitivity to visible light. The LX1970 has a sensitivity K of 0.38 μA/lx at a peak wavelength of 520nm, meaning that for every 1 lx (lux) change in illuminance, the output current changes by 0.38 μA. Converting illuminance to luminance L (unit: cd/m²), we can assume the light is incident on an ideal plane that satisfies total internal reflection, and then, according to 1 lx = 0.314... The brightness value can be obtained by converting cd/m2. Typically, the ratio of brightness to illuminance can be determined experimentally. 3. Typical Applications of LX1970 3.1 White Light Brightness Measurement Circuit The circuit for measuring white light brightness is shown in Figure 3. In operation, the light source, composed of an RCC, a current source, and a white LED, emits visible light. The LX1970 receives this visible light and converts it into a current signal. Next, a microammeter is connected in series at the SNK and SRC terminals to measure the photocurrents ISNK and ISRC, respectively. The microammeter readings reflect the brightness level. 3.2 When the ambient light dims significantly, the LX1970 automatically turns on the LCD backlight to illuminate the white LED. The automatic brightness control circuit is shown in Figure 4. In the figure, resistors R1 and R2 are used to set the minimum and maximum brightness values. Changing the capacitance of capacitor C adjusts the response time and filters out 50Hz mains interference. The LX1970 uses a +3.3V to +5V power supply. If only the SRC terminal is used, the SNK terminal should be left floating. Assuming an output voltage of 0.25V to 1.25V is needed to drive the white LED, where 0.25V represents the minimum brightness and 1.25V represents the maximum brightness, the ratio of R1 to R2 can be determined by the following formula: R1 = [(3/0.25) - 1]R2 = 1/1R2 The value of R2 can be calculated based on the maximum output current (ISRCmax) of LX1970 at a given brightness. In fact, when ISRCmax is 50μA, R2 is 25kΩ. Thus, substituting into the above formula, we can obtain the value of R1, which is 275kΩ. 3.3 Design of LX1970 Evaluation Board The LX1970 evaluation board can not only check the quality of LX1970, but also conduct various experiments on the LCD backlight brightness control circuit to provide a basis for the development of new products. In addition, the component layout and printed circuit design on the evaluation board are also of reference value. The LX1970 evaluation board circuit is shown in Figure 5. It has the following characteristics: First, a turntable (with 7 small holes of different diameters) can be used to change the size of the LX1970 incident light window. The turntable and the sensor are mounted together in the housing. Second, the amplifier gain is adjusted via potentiometers RP1 to RP3, and then the LX1970 drives two white LEDs (LED1 and LED2) to emit light, thereby achieving brightness adjustment to adapt to different ambient brightness conditions; Third, the wiring methods of jumpers J1 to J4 are changed to isolate or bias different circuits. Fourth, the circuit has four selectable control ports, including the SRC voltage divider diode output port (A), the SRC voltage adjustment port (B), the SRC fixed voltage port (C), and the SNK voltage adjustment port (D). In addition, there are two output terminals (SRC and SNK). Port A is a pull-down terminal (grounded via an external potentiometer, which can replace the LX1970 for manual brightness adjustment). Ports B, C, and D are pull-up terminals (connected to a positive power supply or other positive voltage via an external circuit). Ports B and C are used to set the minimum output voltage (adjusting RP1 to the minimum) or adjust the output of the SRC terminal. Port D is used to set the maximum output voltage (adjusting RP2 to the maximum) or adjust the output of the SNK terminal. Jumper J1 is used to select the power supply. When J1 is connected to the "LX1970" position, the system only supplies power to the LX1970; when J1 is connected to the "LED Driver" position, it can simultaneously supply power to the LX1990, which is the LED driver controller. When J2 is in the "SRC Open" position, it disconnects the external circuitry of the SRC pin; when J2 is in the "Voltage Divider" position, it connects the external circuitry of the SRC pin. When J3 is in the "VD2 Input" position, it short-circuits the isolation diode VD2; when J3 is in the "VD2 Output" position, VD2 is not short-circuited. When J4 is in the "Drive" position, the output of the SNK terminal is connected to the input terminal ISET of the LX1990; when J4 is in the "SNK Pull-Up" position, the output of the SNK terminal is connected to a high level through port D. It should be noted that: First, when using port B, an external PWM signal can be applied to the high end of the voltage divider consisting of (RP1+R1) and (RP2+R3), and the voltage division ratio can be controlled by adjusting the values ​​of RP1 and RP2; Second, when using port C, DC voltage can be used to control the brightness; Third, using C1 and C3 can reduce the response speed of the LX1970 to avoid the sensor being affected by the external 50Hz light source frequency.