STN-LCD color screen module design How to realize the high-performance design of the driving circuit of the LCD flat panel display is an important challenge currently faced by design engineers of handheld devices. This article analyzes the classification and performance characteristics of the LCD display panel, introduces the key device LDO in the LCD display design, the on time is only 150μs "target = _blank> LDO and white LED selection points, and the design method of the charge pump LED drive circuit . The internal structure of the STN-LCD color screen module is shown in Figure 1. Its upper part is an LCD screen composed of polarizers, glass, and liquid crystals, and below it are white LEDs and backlight panels. It also includes LCD driver ICs and LCD driver ICs. Provide a low-dropout voltage regulator (LDO) with a stable power supply, two to eight white LEDs and a boost regulator IC driven by LEDs. The circuit structure of the STN-LCD color screen module is shown in Figure 2. After the external power supply Vcc is stepped down and stabilized by the LDO, it provides an operating voltage to the LCD driver IC such as S6B33BOA to drive the LCD display graphics and text of the color STN-LCD; the external power supply Vcc The charge pump is boosted and stabilized to provide constant voltage and constant current power to white LEDs such as NACW215 / NSCW335. The white light of the LED is reflected by the backlight board, which fully displays the 65K color of the LCD liquid crystal. The brightness of the LED directly affects the beautiful LCD color degree. The LCD is a type of flat panel display, and can be classified into three types: static drive (StaTIc), simple matrix drive (Simple Matrix), and active matrix drive (AcTIve Matrix). Among them, the simple matrix type can be divided into twisted nematic (Twisted NemaTIc, TN), super twisted nematic (Super Twisted NemaTIc, STN), and other passive matrix driven liquid crystal displays. Active matrix type can be roughly divided into thin film transistor type (Thin Film Device selection 1. LDO selection. Since consumer products of mobile phones, PDAs, digital cameras and video game consoles all use batteries as the power source, as the use time increases, the power supply voltage gradually decreases, and the LCD driver IC requires a stable operating voltage. Therefore, an LDO is usually used when designing the circuit. Provide a stable 2.8V or 3.0V voltage. The LCM will be installed above the mobile phone, very close to the radio frequency of the mobile phone, in order to prevent interference, low-noise LDO must be used, such as LP2985, AAT3215. 2. White LED. According to the design requirements of the backlight, white LEDs with small front drop voltage (VF) and front drop current (IF) and high brightness (500-1800mcd) are required. Taking mobile phone LCM as an example, 3-4 white LEDs are currently used. As the brightness of LEDs increases and mobile phone manufacturers require lower costs and power consumption, it is expected that by mid-2004, LCM will use 2 high-brightness white LEDs (1200-2000mcd ), PDA and smart phones will use 4-8 white LEDs as needed due to the large LCD screen. NACW215 / NSCW335 and EL 99-21 / 215UCW / TR8 are white LEDs with built-in reflectors. The brightness of the EL series is divided into T, S, R three levels, T is 720-1000mcd, S is 500-720mcd Applicable to mobile phone LCD backlight. LED drive circuit design The driving of the white LED needs to supply a constant voltage or a constant current, and as soon as the power supply of the mobile phone starts to work, its voltage starts to drop, so a boost device is needed to boost and stabilize the voltage. In order to reduce the influence of the operating frequency of the booster device on the radio frequency of the mobile phone, a capacitive charge pump using a capacitor as an energy transfer intermediate is generally selected, and a booster using an inductor as an energy transfer intermediate can output a higher voltage. The efficiency of the capacitive charge pump is divided into frequency doubling and fractional doubling according to its boosting method. The efficiency of the former is about 90%, the efficiency of the latter is about 93-95%; the efficiency of the inductive booster is about 83-85%. The capacitive charge pump is divided into constant voltage output and constant current output according to its output, and is divided into parallel constant voltage drive, single constant current drive and series constant current drive according to its method of driving LED. Inductive boosters are all constant current output, the output voltage is higher, and the LED is driven in series. Frequency-doubled boosted capacitive charge pumps such as AAT3110, 5V constant voltage output, maximum current 120mA, parallel drive LED, as shown in Figure 3. A capacitive charge pump with a fractional doubling frequency boost, such as AAT3113, has 4-6 channels of constant current output, each channel can output 20mA current, and a single constant current drives LED, with 32-level dimming function, as shown in Figure 4. AAT3134 divides the output DAC module into two blocks, the output of which can drive the LCM module of dual screen display size. NCP5009 is a backlight LED drive booster with photosensitive sensor. It is suitable for high-end mobile phone LCM with automatic dimming. It drives the LED in series. The circuit is shown in Figure 5. LM2704 is a backlight LED drive booster that can drive 4 series LEDs with constant current and PWM dimming. The circuit is shown in Figure 6. The new charge pump and booster output have built-in MOSFETs, which can dynamically adjust the internal resistance of the load, eliminating the need for an additional constant current resistor to balance the internal resistance of the LED. Capacitive charge pumps with high switching operating frequencies require small filter capacitors and low RF interference. The capacitor is best to choose ceramic capacitors, because ceramic capacitors have no polarity and have a low equivalent series resistance (ESR), the typical value is less than 100mΩ. The equivalent series resistance value of the ceramic capacitor, the quality of the dielectric material, and the capacitance value have a significant impact on the output ripple. The X7R capacitor dielectric is the best and the cost is slightly higher; the X5R capacitor dielectric is centered on the top and can be selected; the Y5V dielectric is poor and not recommended. 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Transistor, TFT-LCD quickly divides the CSTN-LCD market "target = _blank> TFT) and two-terminal diode type (Metal / Insulator / Metal, MIM). TN, STN and TFT type liquid crystal displays use the principle of liquid crystal molecular twisting The difference in view angle, color, contrast, and animation display quality has advantages and disadvantages, which makes it obvious that the application range of products is classified. In terms of the current application range and level of liquid crystal display technology, active matrix The driving technology is the thin film transistor type as the mainstream, which is mostly used in notebook computers and animation and image processing products; the simple matrix driving technology is currently mainly reversed nematic and STN, STN liquid crystal display through color filter (color filter) It can display the three primary colors of red, green and blue separately, and then through the adjustment of the proportion of the three primary colors, it can display the true color of the full-color mode. At present, the application of color STN-LCD is mostly mobile phone, PDA, digital camera and video game consumer products and Word processor-based. 