introduction
Due to the cross effect of STN type liquid crystal display module, you may sometimes see such a phenomenon when testing: when displaying content, if there are more image disturbances displayed on a certain column, then this column The display of the remaining pixels will be lighter than the display of the pixels in other non-display areas, and the more pixels displayed in this column, the lighter the display of other pixels in this column will be, and the overall view will be one by one. The vertical bars are very obvious. We conducted an analysis and experiment for this kind of phenomenon.
1 Experimental process
First of all, we must remove the COG modules with serious vertical bars and the ICs of the same product modules without vertical bars. Test the electro-optical parameters, and you will find that the contrast curve range with vertical bars is obviously narrower. It is only about 1V. Left and right, and the contrast range without vertical bars is relatively wide, reaching above 3V. At the same time, one of their power consumption currents was tested, and it was found that the power consumption current with more vertical bars was relatively large, and the power consumption current without vertical bars was relatively small.
After discovering this rule, in order to reverse the verification, the same product was drawn out, and then divided into two groups according to the power consumption current. The contrast range that consumes a lot of current is indeed narrower.
We have conducted experiments from the perspective of IC again. The LCD screens of the same design and technology use different ICs, and we find that the use of different IC vertical bars is different. Some are obvious, and some are not obvious. No, the reason is: different ICs have different drive capabilities, and vertical bars with poor drive capabilities will be more serious.
2 cause analysis
Dot-matrix LCD screens usually use dynamic driving methods. Generally, the row electrodes are scanning electrodes, and the voltage of a series of scanning pulses is applied in the order of time. The column electrode is generally the address electrode, which will be synchronized with the scan voltage and input the gate voltage waveform and the non-gate voltage waveform respectively. At the moment when both sides input the driving voltage waveform at the same time, they will synthesize a driving waveform on 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 there will be a certain voltage on the non-display point, there will also be a faint display on the non-display point, that is, a certain black background will appear. The row scan electrodes are cyclically scanned and have nothing to do with the displayed content, but the column electrodes are strobed electrodes and are related to the displayed content. The signal in the column where the gated point is located is different from that of the column where the non-gated point is located. When the power consumption current of the LCD screen is too large or the driving capability of the IC is too small, vertical bars will appear.
Solution
Using an IC with a strong driving capability will undoubtedly directly reduce the chance of vertical bars, but this will cause a substantial increase in cost, and reducing the power consumption and current of the LCD screen is undoubtedly a feasible and very 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 most important factors. We also did a set of small experiments on this, replacing the 20 ohm glass with 15 ohm glass, and using the old liquid crystal and the new liquid crystal that have been mixed for many times to do the orthogonal experiment. The experimental results clearly show that the power consumption current of the small square resistance and the new liquid crystal is significantly smaller than that of the old liquid crystal and the large square resistance. In addition, when drawing, it is necessary to increase the line as much as possible, increase the proportion of conductive gold powder, improve the accuracy of alignment, and clean the electrode gap, etc., which are all effective methods to reduce the power consumption current. In addition, from the perspective of the process of the module, ensuring a precision of its electrode alignment, controlling the process and proportion of the ball crushing, etc. can also effectively reduce the power consumption of the circuit.
In summary, if you want to eliminate the phenomenon of vertical bars, the key is to choose an IC with stronger driving capability and reduce the power consumption current of an entire module, so that the two can be well matched. At the same time, we are also studying some Methods to improve the vertical bars, such as adjusting the polarizer, reducing the voltage to make the background lighter, etc., can not solve the problem fundamentally, but it will still reduce the phenomenon, which is also a better method.
