LED indoor scanning screen application problems and solutions

The indoor display is mainly used for small pitch, fine texture, color contrast, green power consumption, etc. Therefore, the best choice for indoor display is the most suitable for dynamic scanning, with small space, high density and low. Power consumption, with high pixel, high refresh, high density screen and so on.

The challenge of the indoor scanning screen is that the phenomenon of dark sweeping, smearing, unevenness of the flower screen and block during low-gray operation and dynamic display may cause the screen to be easily observed by the human eye when the display is applied indoors. Missing, this article will propose solutions to the above problems, so that the display screen is more perfect, more comfortable to watch.

Display for indoor use

The display screen is divided into a static screen and a dynamic scanning screen. If the static screen is applied indoors, the advantage is that the application is simple and the controller is easy to implement, but the number of driving ICs used when making the ultra-dense screen is very considerable, so Applying it to indoor display screens will encounter shortcomings such as limited space, excessive energy consumption and insufficient pixels. Therefore, most of the applications in indoor display screens are dynamic scanning screens, and dynamic scanning screens can be made into ultra-dense screens. It also has the advantages of low energy consumption and high space tolerance, but the dynamic scanning screen is designed to take advantage of human visual persistence, so it will encounter some shortcomings in application, such as dark sweeping, smear problem, flower screen Inconsistent with the block, in view of the above problems, the corresponding technology is proposed in this paper to solve and improve the shortcomings of the dynamic scanning screen.

technical problem

As the number of scans increases and the LED dot pitch shrinks, problems arise, such as up/down smear problems due to capacitive effects of the lamp and LED, first sweep darkness, or potential Inconsistent problems caused by unevenness of the block.

In addition to the above-mentioned problems, due to the wide application of LED indoor display, the dot pitch is getting smaller and smaller, and the current demand for LED is not as big as before. Now the current demand may reach 3 mA, 2 mA, or even 1 mA. effect. As the dot pitch and current demand become smaller, the chip skew/bit skew required by the LED driver IC is becoming more and more strict. Therefore, the brightness unevenness (flower screen) that was previously covered by a large current is also It will then emerge.

Indoor scanning screen related problems and solutions

1. Sweeping dark lines

Because the scanning screen operates in a line by line, as shown in Figure 1, if a frame ( Frame The LED conduction time is much shorter than the off time, so that the parasitic capacitance effect on the PCB lamp board causes the column voltage to increase, so the voltage of this column will be higher than that of the other row sweeps when the first row is turned on, so that The LED current decreases when the line is turned on, causing the LED brightness to decrease as shown in Fig. 2. When the actual dot screen is displayed, the first line sweep is observed to be dark.

Figure 1 scan screen operation mode Figure 2 line sweep operation PCB parasitic capacitance effect

solution:

The line sweep compensation technology is used to compensate the current line during the line sweep to supplement the current lost due to the capacitance effect on the PCB light board to eliminate the dark line problem generated by the scan screen in the first line. Figure 3 is an ideal LED. The current waveform, Figure 4 is the waveform affected by the capacitive effect on the PCB panel, and Figure 5 is the result of the compensation technique proposed in this paper.

Figure 3 ideal LED current Figure 4 actual LED current Figure 5 compensation LED current technology

As a result of the upper screen observation, FIG. 6 shows the problem of the line sweeping dark line, and as shown in FIG. 7, the line sweeping dark line problem has been eliminated.

Figure 6 line of dark line problem Figure 7 current compensation technology, dark line elimination

2. Smear phenomenon

Another problem with the indoor scanning screen is the smear phenomenon. Mainly due to the charging and discharging factors of the parasitic capacitance on the PCB due to the charging and discharging of the parasitic capacitance on the PCB, the LED that should not be lit is lit, especially when used for oblique scanning. More obvious, and the smear problem has the upper smear and the lower smear effect on the LED scanning screen.

Upper smear effect:

When the scan operation is as shown in Fig. 8, when the first column is turned on, the LED1 is lit, and the first row Cpar1 is also charged, and when the second column is turned on, the LED4 should only be illuminated. However, LED3 also illuminates, because the line parasitic capacitance Cpar1 on the PCB is charged and held by VLED1, and a bleed path is formed via LED3 to the Driver-IC to make its LED illuminate when the column is changed. The smear effect.

The solution is to use the external bleeder circuit as the drain path of the parasitic capacitance of the row PCB. As shown in FIG. 9, the charge of the parasitic capacitor is discharged in advance when the line is changed, so that the problem of the discharge path via the LED can be solved. And then the upper smear phenomenon is eliminated.

Figure 8 on the smear phenomenon Figure 9 on the smear bleeder circuit architecture

Lower smear effect:

When the scan operation is as shown in FIG. 10, when the first row of the first row is turned on, the LED 1 is turned on, and at this time, the first column Cpar1 is also discharged, and the second row and the second column are turned on. At the same time, LED4 should only be illuminated, but LED2 also illuminates. Because the PCB parasitic capacitance Cpar1 on the column is discharged to the low potential by the first line, the charging path is formed via LED2 to the Driver-IC during the line feed. To make its LED bright, this is called the lower smear effect, and the operating waveform is shown in Figure 11.

Figure 10 below the smear phenomenon Figure 11 smear effect waveform

The solution proposed in this paper

Using the built-in pre-charging circuit of the display driver IC as shown in Figure 12, the application performs the charging action in advance during the line-feed scanning operation. The PCB parasitic capacitance on the column can be charged first, and the voltage on the column is increased. When the line is changed, the charging path of the column capacitor can be blocked to eliminate the smear effect, and the operation principle of the lower smear is eliminated as shown in FIG.

Figure 12 Driver IC built-in pre-charging circuit architecture Figure 13 Adding pre-charging circuit to eliminate the smear principle

The results of the upper screen observation, Figure 14 shows that the phenomenon of smear is obvious, and Figure 15 shows the use of pre-charging technology to eliminate the phenomenon of lower smear.

Figure 14 shows the phenomenon of smear. Figure 15 Pre-charging technology, the phenomenon of smear is eliminated

3. Block unevenness problem:

In order to solve the problem of the next smear, we will do the pre-charging action to increase the potential of the column, but this way will lead to the problem of uneven block. The influence of block unevenness can be clearly observed especially in low ash. If it is used in medium-high gray scale, this phenomenon does not occur and the block is uneven. This is because the difference in PCB layout and the difference between the driver ICs make the potential inconsistent. The result produced. As shown in Figure 16, the brightness of the left and right sides will be different, and is in units of IC blocks.

Figure 16 block unevenness causes brightness difference

This paper proposes to use the function of improving block unevenness in low ash. As shown in Fig. 17, it can be observed that there is no significant difference in brightness between the left and right blocks.

Figure 17 block unevenness has been resolved

4. Flower screen phenomenon:

The electrical characteristics of the LED particles themselves are related to the process. The offset of each batch of process will cause the LED characteristics of different batches to be different, and there will be an error between the LED particles of the same batch. Therefore, the error of these characteristics will cause the phenomenon of the screen. As shown in Figure 18, the same gray scale is displayed on the entire screen. The left side shows that the whole screen is very messy, even if the brightness of the whole screen is raised, it is very messy. Usually, the processing of such a screen phenomenon requires the use of brightness correction to improve the flower screen phenomenon. However, the cost of using the brightness correction is very high, and the LED needs to be recalibrated after aging, that is, it needs to be corrected once every time, which is inconvenient. Therefore, this paper also proposes to provide a brightness equalization method inside the IC, so that the whole screen is more even and smooth, as shown in Figure 19.

Figure 18 is the same gray scale rendering picture messy Figure 19 the same gray level rendering picture is quite uniform

in conclusion

With the increasing demand for LED display screens , display products are evolving toward a clear, refined and smooth development direction. The LED dot pitch is also gradually reduced from the P10 of the outdoor screen to the P3 of the indoor screen. P2, even P1.2, etc., also poses more stringent challenges in LED packaging, drive control, and system thermal design. In order to save the layout area and heat dissipation of the lamp panel, the application of multi-row scanning has become a trend adopted by LED displays, especially for ultra-dense screen applications. At present, the scanning screen has been gradually expanded from the original 8 scanning screen to 16 sweeping, 32 sweeping and other applications. The problems and challenges arising from the display system, including the system PCB board, LED electrical characteristics, etc., through the improvement of functions within the chip IC, to overcome these problems, so that customers can use it simply and conveniently. For more information and detailed instructions, please refer to the accumulation of product and documentation information.