TN LCD Technical Support
- Theory of Operation
- Polarizers
- Multiplexing
- Direct Drive LCD Performance Specs
- LCD Part Numbering
- Light Valve Application Notes (pdf 19.6K)
- A simple way to light up an LCD
- Handling Precautions
Theory of Operation for TN Displays
The LC material inside TN displays is made up of long, rod-like or cigar-shaped organic molecules that have both liquid and crystalline properties. These LC molecules have the unique characteristic of re-aligning themselves when an electrical field is applied across a thin layer. When sealed between parallel glass plates having printed-image electrodes, a thin layer of LC material serves as part of a field-effect voltage-operated shutter to the passage of light. The nematic molecules used in TN displays try to keep their long axes parallel to each other. LC displays have specially treated glass surfaces which cause the molecules to align in a specific orientation relative to the glass. Within the LC glass sandwich, the surface of the top glass plate always keeps the LC molecules near it rotated 90° with respect to those near the lower plate. Thus, the LC molecules between the two plates form a "spiral staircase" so that light passing through the top plate twists 90° before exiting through the bottom one. The basic LCD cell has no readily discernible optical characteristics and looks transparent under any condition. But if a piece of properly oriented linear polarizer (such as those found in polarized sunglasses) is placed on each glass surface, the basis of the cell's display properties is provided. The top filter passes light, with a particular orientation, the LC material shifts this light 90°, and if the bottom filter is positioned 90° with respect to the top filter, this twisted light is in phase with the bottom filter and passes freely through the cell, which looks perfectly transparent. When voltage is applied to the patterned electrodes across such a cell, the LC material's molecules line up between the electrodes and disrupt the spiral staircase. Light passing through the sandwich is now improperly oriented with respect to the bottom polarizing filter and is absorbed in the regions between the electrodes. The resulting dark-on-light display provides legibility over wide angles in a wide range of ambient light conditions. In effect, energizing the electrodes with low voltage is equivalent to turning portions of the polarizing filters 90° with respect to each other.
Polarizers
LCD manufacturers combine basic cells, polarizers, and electrodes to implement two main display modes. They first position a display's top polarizer in phase with polarized sunglasses (which are always oriented in one standard direction) that viewers might wear; otherwise, viewing the display while wearing sunglasses would inadvertently extinguish it. The bottom polarizer can be either in phase or out of phase with the top one. In the first case, a transmissive display with light characters on a dark background results. In the regions between the energized overlapping electrodes, the 90° spiral staircase disappears, and light passes freely through the two in-phase polarizers. Users often backlight such "transmissive" displays with any convenient source - incandescent, fluorescent, or neon, for example - or with ambient light. One need not waste much power in such backlighting; watches require as little as 22mW, and instrument displays are effectively lit with 200mW. In the second case, which has already been described and where the bottom polarizer is 90° out of phase with respect to the top one, the 90° spiral staircase passes light except where overlapping energized electrodes break the spiral to produce dark characters on a clear background. Often, manufacturers attach mirrors or other reflective devices to the bottoms of such cells to bounce ambient light to viewers; hence the term "reflective" mode for this polarizer orientation. "Transflective" displays use both ambient light and backlighting. To color displays, LCD manufacturers provide polarizers dyed in a variety of colors; the most popular are red, blue and black.
Multiplex Displays
TN liquid crystals work very well in multiplex applications, however, it is important to understand the restraints that multiplexing applies to the display so that they can be properly implemented. The most important thing to remember in multiplexing displays is that the elements are all interconnected, and there is a cross-talk problem which must be addressed. This has the effect of limiting the magnitude of the instantaneous drive voltage to a selected element. If the instantaneous voltage is limited, this makes the Rms voltage for a given selected segment a function of "n," and it is the Rms voltage that liquid crystals respond to. Therefore, the higher number of lines multiplexed, the lower the Rms voltage on a selected dot for a whole scan cycle, and the worse the ratio between Rms for an on segment and off segment. In effect, this makes the effective drive voltage on the display a function of "n," or the number of lines it is multiplexing. So the higher the multiplex ration, the poorer the viewing angle and contrast ratio. The minimum acceptable aesthetic performance of the display is somewhat in the eye of the beholder. Generally speaking, We do not recommend that displays be multiplexed in excess of 32 levels, using standard LCD materials. Applications which require multiplexed displays to work over extended temperature ranges require consideration of two important factors. The first is that the LC material threshold does have a temperature coefficient which is approximately - 10mV per degree Celsius. This does not cause problems over limited temperature ranges, but does require compensation over broad temperature ranges. The second factor which should be considered is that, as discussed above, multiplexed displays have limited effective drive voltage. Therefore, particular attention should be paid to the low temperature performance based on the fact that low drive voltages have a detrimental effect on low temperature performance.
Direct Drive LCD Performance Specs
| Parameter | Units | Min | Typ. | Max. |
|---|---|---|---|---|
| Operating Voltage | V | 3 | 5 | 10 |
| Operating Frequency | Hz | 30 | 60 | 120 |
| Current* | µA | - | 3 | 10 |
| Capacitance* | pf | - | 3000 | - |
| DC Resistance* | MΩ | 20 | - | - |
| Allowable DC Offset | mV | - | - | 50 |
| Response Times (25°C) Rise to 90% ONN |
ms | - | 30 | 50 |
| Decay to 10% OFF | ms | - | 50 | 100 |
| Contrast Ratio | - | - | 20:1 | - |
| Viewing Angle (at 5V) | - | - | ± 75° | - |
| Storage Temp. Range | °C | -30 | - | 85 |
| Estimated Life | hrs. | - | 50,000 | - |
*All Segments ON.
Specifications are for 3½ digit 0.5" Digit Height LCDs
LCD Part Numbering Information
| 3 | 5 | 5 | 5 | - | R | P | H | - | 0 | . | 2 | 5 |
| 1 | 2 | 3 | 4 | 5 | 6 | |||||||
1.  ® Base Part Number2. Optical Modes:
|
4. Polarizers:
|
Light Valve Application Notes (pdf 19.6K)
A Simple Way to Light your LCDs
- Use an AC transformer (available at Radio Shack) to step down line voltage to 5 or 6 VAC.
- Apply one lead to the back-plane pin.
- Apply the other lead to all segments by metal clips or aluminum foil.
- To light only one segment, other segments not in use should be grounded with the back-plane.
Handling Precautions
Because LCD Panels are made of glass, it is important to handle them carefully.
Please use the following as a guideline for handling ® liquid crystal displays.
- If the LCD panel breaks, do not get the liquid crystal in your mouth. In case of contact with your skin or clothes, wash it off immediately with soap and water.
- Glass is easily chipped or cracked through rough handling. Excercise care to not apply unreasonable pressure to the display.
- When soldering displays with pins, avoid excessive heat. Keep the soldering temperature between 260°C and 300°C and apply heat for no more than 5 seconds. Never use wave or reflow soldering. Cover the surface of the LCD to avoid flux spatter. Remove flux residues afterward.
- Do not use DC voltage to drive the LCD and keep the voltage within the specified limit. Excess voltage will shorten the display's life. Within limit, the viewing angle can be fine tuned by varying the voltage (especially for STN). Response time will increase as the temperature decreases.
