A Preliminary Study on the Display of Vertical Bars in Dot-matrix COG Liquid Crystal Display Module

by:Kimeery     2021-08-24
Introduction Due to the cross effect of STN LCD screenmodules, this phenomenon may sometimes be seen during testing: When displaying content, if there are more pixels displayed on a certain column, then the remaining pixels on this column will display more than other non-display areas. The display of pixels is shallow, and the more the number of pixels displayed in this column, the lighter the display of other pixels in this column. On the whole, it is a series of vertical bars, which is very obvious. In response to this phenomenon, we conducted analysis and experiments. 1 In the experiment process, first we removed the COG module with serious vertical bars and the IC of the same product module without vertical bars, and tested its electro-optical parameters, and found that the contrast curve range with vertical bars was significantly narrower, only about 1 V, but no vertical bars. The contrast range is relatively wide, reaching more than 3 V (see Figure 1). At the same time, they tested their power consumption current, and found that the power consumption current of more vertical bars is large, and the power consumption current of no vertical bars is small. After discovering this rule, in order to reverse the verification, the same products were divided into two groups according to the power consumption current. A group of power consumption current is about 9 LA, and a group of about 13 LA. The contrast range of the test power consumption current is really narrow. We conducted experiments from the perspective of IC. The LCD screens of the same design and technology used different ICs, and found that different ICs have different vertical bar phenomena. Some are obvious, and some are not obvious or even absent. The reason is different ICs. The driving ability is different, and the vertical bar with poor driving ability is serious. 2   Reason analysis Dot matrix liquid crystal display usually adopts dynamic driving method. Generally, the row electrodes are scan electrodes, and a series of scan pulse voltages are applied in chronological order. The column electrode is generally the address electrode, which will input the gate voltage waveform and the non-gate voltage waveform in synchronization with the scan voltage. At the moment when both parties input the driving voltage waveforms synchronously, they will synthesize a driving waveform on the pixel at the intersection of the row and the column. Pixels synthesized into a gated waveform are displayed, and pixels synthesized into a non-gated waveform are not displayed. However, since the non-display point also has a certain voltage, the non-display point also has a faint display, that is, a certain black background. The row scan electrode is a cyclic scan and has nothing to do with the display content, but the column electrode is a strobe electrode, which is related to the display content. The column where the gated point is located and the column where the non-gated point is located have different signals. When the power consumption current of the liquid crystal display is too large or the IC driving capability is too small, vertical bars will appear. 3  Solution Using an IC with strong driving capability will undoubtedly reduce the chance of vertical bars, but this will increase the cost, and reducing the power consumption current of the LCD screen is undoubtedly a feasible and economical method. So what are the factors related to the power consumption current? From the perspective of the LCD screen, the sheet resistance of the ITO glass and the conductivity of the liquid crystal are the two main factors. We have done a set of experiments on this, replacing 20 ohms with 15 ohm glass, and doing orthogonal experiments with the old and new liquid crystals that have been mixed multiple times. The experimental results clearly show that the current consumption current of the small square resistance and the new liquid crystal is significantly lower than that of the old liquid crystal and the large square resistance. In addition, when drawing, try to enlarge the line, improve the alignment accuracy, increase the proportion of conductive gold powder, and clean the electrode gaps are all effective methods to reduce the power consumption current. In addition, from the perspective of module technology, the accuracy of electrode alignment is ensured, and the degree and proportion of broken balls can also be controlled to reduce circuit power consumption. In summary, in order to eliminate the vertical bar phenomenon, the key is to choose an IC with strong driving capability and reduce the power consumption of the entire module, so that the two can be well matched. At the same time, we have also studied some methods to improve the vertical bar, such as adjustment Polarizers, lowering the voltage to make the background lighter, etc., although it can't solve the problem fundamentally, it will reduce the phenomenon, which is also a good method.
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