static lcd display in stock

A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals. Liquid crystals do not emit light directly, instead using a backlight or reflector to produce images in color or monochrome. LCDs are available to display arbitrary images (as in a general-purpose computer display) or fixed images with low information content, which can be displayed or hidden, such as preset words, digits, and seven-segment displays, as in a digital clock. They use the same basic technology, except that arbitrary images are made up of a large number of small pixels, while other displays have larger elements. LCDs can either be normally on (positive) or off (negative), depending on the polarizer arrangement. For example, a character positive LCD with a backlight will have black lettering on a background that is the color of the backlight, and a character negative LCD will have a black background with the letters being of the same color as the backlight. Optical filters are added to white on blue LCDs to give them their characteristic appearance.

LCDs are used in a wide range of applications, including LCD televisions, computer monitors, instrument panels, aircraft cockpit displays, and indoor and outdoor signage. Small LCD screens are common in portable consumer devices such as digital cameras, watches, calculators, and mobile telephones, including smartphones. LCD screens are also used on consumer electronics products such as DVD players, video game devices and clocks. LCD screens have replaced heavy, bulky cathode ray tube (CRT) displays in nearly all applications. LCD screens are available in a wider range of screen sizes than CRT and plasma displays, with LCD screens available in sizes ranging from tiny digital watches to very large television receivers. LCDs are slowly being replaced by OLEDs, which can be easily made into different shapes, and have a lower response time, wider color gamut, virtually infinite color contrast and viewing angles, lower weight for a given display size and a slimmer profile (because OLEDs use a single glass or plastic panel whereas LCDs use two glass panels; the thickness of the panels increases with size but the increase is more noticeable on LCDs) and potentially lower power consumption (as the display is only "on" where needed and there is no backlight). OLEDs, however, are more expensive for a given display size due to the very expensive electroluminescent materials or phosphors that they use. Also due to the use of phosphors, OLEDs suffer from screen burn-in and there is currently no way to recycle OLED displays, whereas LCD panels can be recycled, although the technology required to recycle LCDs is not yet widespread. Attempts to increase the lifespan of LCDs are quantum dot displays, which offer similar performance as an OLED display, but the Quantum dot sheet that gives these displays their characteristics can not yet be recycled.

Since LCD screens do not use phosphors, they rarely suffer image burn-in when a static image is displayed on a screen for a long time, e.g., the table frame for an airline flight schedule on an indoor sign. LCDs are, however, susceptible to image persistence. The LCD screen is more energy-efficient and can be disposed of more safely than a CRT can. Its low electrical power consumption enables it to be used in battery-powered electronic equipment more efficiently than CRTs can be. By 2008, annual sales of televisions with LCD screens exceeded sales of CRT units worldwide, and the CRT became obsolete for most purposes.

Liquid Crystal Displays (LCD) is a technology that does not emit light, it only controls how an external light source passes through. This external light source could be the available ambient light, in the reflective display type, or the light from a backlight led or lamp, in transmissive display type. LCDs are constructed with two plates of glass (upper and bottom), a thin layer of liquid crystal (LC) between them and two light polarizers (Application Note AN-001 – Basics of LCD Technology, Hitachi, Application Note AN-005 – Display Modes, Hitachi).The polarizer is a light filter for the light electromagnetic field. Only the light components in the right electromagnetic field direction pass through the polarizer, while the other components are blocked.

The liquid crystal is an organic material that rotates the electromagnetic field of the light 90 degrees or more. However, when an electrical field is applied to the LC it does not rotate the light anymore. With the addition of transparent electrodes in the upper and bottom display glass, its possible to control when the light passes through, and when not, with an external source of the electrical field. Figure 1 (see Application Note AN-001 – Basics of LCD Technology, Hitachi) above illustrates this operation control. In Figure 1, the display is dark when there isn’t an electrical field. This is because both polarizers filter the light in the same direction. If the polarizers are orthogonal, then the display will be dark when the electrical field is present. This is the most common situation for reflective displays.

The minimum electrical field, or voltage, to control the LCD is called the ON threshold. The LC is only affected by the voltage, and there is hardly any current in LC material. The electrodes in LCD forms a small capacitance and this is the only load for a driver. This is the reason for an LCD being a low power device to show visual information.

However, its important to note that the LCD can’t operate with a direct current (DC) voltage source for too long. The application of a DC voltage will cause chemical reactions in LC material, permanently damaging it (Application Note AN-001 – Basics of LCD Technology, Hitachi). The solution is to apply an alternate voltage (AC) in LCDs electrodes.

In static LCDs, a backplane electrode is built in one glass and individual LCD’s segments, or pixels, are put in the other glass. This is one of the simplest LCD types and the one with the best contrast ratio. However, this type of display usually requires too many pins to control each individual segment.

In general, a driver controller sources a square wave clock signal for the backplane and a clock signal for the segments in the front plane together. When the backplane clock is in-phase with the segment clock, the root-mean-square (RMS) voltage between both planes is zero, and the segment is transparent. Otherwise, if the RMS voltage is higher than LCD ON threshold, the segment becomes dark. The waveforms for the backplane, on and off segment are shown in Figure 2. As can be seen in the figure, the ON segment is out-of-phase in relation to the backplane signal. The off segment is in-phase in relation to the backplane signal. The applied voltage could be between 3 and 5 volts for low cost, low power displays.

The clock signal for LCD’s backplane and segments usually are in the range of 30 to 100 Hz, the minimum frequency to avoid a visual flicker effect on LCD. Higher frequencies are avoided to reduce the power consumption of the overall system. The system composed of LCD and drivers would consume little current, in the order of microamperes. This makes them perfectly suitable for low power and battery power supply source applications.

In the following sections, the design of an LCD static driver with GreenPAK device that can generate the backplane clock signal and the individual segment clock signal for a commercial LCD is presented in detail.

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