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Abstract : This article introduces the internal structure, main components, and design selection requirements of the color STN-LCD module.
Keywords : Color STN-LCD module, LCM, LED, charge pump, booster, LDO, backlight panel
More and more mobile phones, PDAs, digital cameras and video game consoles use STN-LCD full-color displays. Therefore, the demand for STN-LCD color screen modules (LCM—LCD Module) has increased sharply. The quality of domestic LCD liquid crystal displays is comparable to that of foreign ones. Many mobile phone designers and manufacturers have begun to require the use of LCD color screen modules produced by domestic LCD manufacturers. In 2001, 380 million mobile phones were produced worldwide, which will increase to 430 million in 2003 and 520 million in 2005. In 2004, LCM color screen mobile phones will account for 35-40% of the total mobile phone production, so the annual demand for STN-LCD color screen modules (LCM) will increase to several hundred million pieces.
LCM internal structure
The internal structure of the STN-LCD color screen module is shown in Figure 1. Its upper part is an LCD screen composed of polarizer, glass, and liquid crystal, and below it are white light LEDs and a backlight panel. It also includes an LCD driver IC, a low-dropout regulator (LDO) that provides a stable power supply to the LCD driver IC, two to eight white light LEDs, and a boost regulator IC for LED driving.
LCM circuit structure
The circuit structure of the STN-LCD color screen module is shown in Figure 2. The external power supply Vcc is stepped down and stabilized by LDO to provide working voltage to LCD driver IC such as Samsung's S6B33BOA to drive the color STN-LCD to display graphics and text; the external power supply Vcc is stepped up and stabilized by the charge pump to provide a constant constant voltage and constant current power supply to the white light LED such as 99-21UWC. The white light of the LED is reflected by the backlight panel to fully display the 65K color of the LCD. The brightness of the LED directly affects the beauty of the LCD color.
LCM main optoelectronic devices
l Color STN-LCD
l LCD Driver: S6B33BOA
l LCD Driver LDO: AAT3221-2.8V AAT3221-3.0V
l White LED: 99-21UWC/TR8 99-215UWC/TR8
l LED Driver: AAT3110 AAT3113 AAT3123 AAT3134 NCP5007 NCP5008/9
l Backlight Board
LCD
LCD is the abbreviation of Liquid Crystal Display. LCD is a type of flat panel display. It can be classified into three types according to the driving method: static drive (Static), simple matrix drive (Simple Matrix) and active matrix drive (Active Matrix). Among them, the passive matrix type can be divided into twisted nematic type (Twisted Nematic; TN), super twisted nematic type (Super Twisted Nematic; STN) and other passive matrix driven liquid crystal displays; and the active matrix type can be roughly divided into thin film transistor type (Thin Film Transistor; TFT) and two-terminal diode type (Metal/Insulator/Metal; MIM) two types. TN, STN and TFT liquid crystal displays have different levels of viewing angle, color, contrast and animation display quality due to their different use of liquid crystal molecular twisting principles, which also makes them clearly separated in the application scope of the product classification. In terms of the scope and level of current liquid crystal display technology, active matrix drive technology is dominated by thin film transistor type (TFT), which is mostly used in notebook computers and animation and image processing products. The current pure matrix drive technology is mainly based on twisted nematic (TN) and super twisted nematic (STN). STN liquid crystal displays can display the three primary colors of red, green and blue respectively through color filters, and then through the proportion of the three primary colors, they can display true colors in full color mode. Currently, the application of color STN-LCD is mainly in mobile phones, PDAs, digital cameras, video game consoles and word processors.
LCD Driver IC
LCD driver ICs are mostly products from Hitachi and Samsung. For example, Samsung's S6B33BOA is a 65K color saturation STN-LCD driver IC with a very good performance/price ratio.
Since consumer products such as mobile phones, PDAs, digital cameras and video game consoles are all powered by batteries, the power supply voltage fluctuates greatly as the usage time increases. LCD driver ICs require a stable operating voltage, so when designing circuits, a low dropout regulator (LDO) is often used to provide a stable 2.8V or 3.0V voltage, such as AAT3221.
White LED
According to the design requirements of the backlight source, a white LED with small front-end voltage (VF) and front-end current (IF) and high brightness (500-1800mcd) is required. Taking mobile phone LCM as an example, 3-4 white LEDs are currently used. With the increase in LED brightness and the requirements of mobile phone manufacturers to reduce costs and power consumption, it is expected that by the middle of 2004, LCM will use 2 high-brightness white LEDs (1200-2000mcd). PDAs and Smartphones will use 4-8 white LEDs as needed due to their larger LCD screens.
EL 99-21/215UCW/TR8 is a white light SMDLED with a good performance/price ratio and built-in reflector. Its brightness is divided into three levels: T, S, and R. T is 720-1000 mcd and S is 500-720 mcd. Both are suitable for mobile phone LCD backlight. Its quality is equivalent to NACW215 / NSCW335.
LED Driver
The driver of white light LED needs to supply constant voltage or constant current, but the working voltage of mobile phone power drops at the beginning, so a boost device is needed to boost and stabilize the voltage. In order to reduce the impact of the working frequency of the boost device on the radio frequency (RF) of the mobile phone, a capacitive charge pump with a capacitor as the intermediate of power transmission is generally selected; a booster with an inductor as the intermediate of power transmission can output a higher voltage.
The efficiency of capacitive charge pumps is divided into two types according to their boosting method: frequency doubling and fractional frequency doubling. The former has an efficiency of about 90%, and the latter has an efficiency of about 93-95%; the efficiency of inductive boosters is about 83-85%; capacitive charge pumps are divided into constant voltage output and constant current output according to their output; they are divided into parallel constant voltage drive, single constant current drive, and series constant current drive according to their LED driving methods; inductive boosters all have constant current output, higher output voltage, and drive LEDs in series.
The capacitive charge pump with frequency doubling and voltage boosting, such as AAT3110, has a 5V constant voltage output and a maximum current of 120mA, and is connected in parallel to drive LEDs, as shown in Figure 3.
The fractional frequency boost capacitor charge pump, such as AAT3113, has 4-6 constant current outputs, each of which can output 20mA current. A single constant current drives the LED with 32-level dimming function, as shown in Figure 4. AAT3134 divides the output DAC module into two blocks, and its output can drive the large and small LCM modules of the dual-screen display respectively.
NCP5009 is a backlight LED driver booster with a photosensor, suitable for high-end mobile phone LCM with automatic dimming, and drives LEDs in series, as shown in Figure 5. NCP5007 is a backlight LED driver booster that can drive 5 LEDs in series with constant current and PWM dimming, as shown in Figure 6.
The new charge pump and booster output terminals have built-in MOSFETs, which can dynamically adjust the load internal resistance, eliminating the need for external current equalization resistors to balance the different internal resistances of LEDs.
A capacitive charge pump with a high switching frequency requires a small filter capacitor and has less interference to RF.
Ceramic capacitors are the best choice for capacitors because they are non-polar and have a low equivalent series resistance (ESR), with a typical value of less than 100mΩ. The equivalent series resistance (ESR), dielectric material quality, and capacitance value of ceramic capacitors have a significant impact on output ripple. X7R capacitor dielectrics are the best, but the cost is slightly higher; X5R capacitor dielectrics are in the middle and can be used; Y5V dielectrics are poor and are not recommended.
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