How to improve the refresh rate of LED display

How to improve the refresh rate of LED display

gray scale is the resolution of the brightness of each LED on the display screen. For example, 4bit gray scale means that the LED has 16 levels of brightness change. The implementation method of gray scale control of LED driver chip is shown in Figure 1. The gray scale of LED brightness is controlled by the OE width and SDI on the drive chip. Taking the gray scale 5 to be displayed by the first led in Figure 1 as an example, SDI must turn on the output switch when the OE width is 1 and 4 to obtain the overall LED display gray scale 5. If the gray scale is 9, 4, and 11, and so on, different LED gray scale can be obtained by the arrangement and combination of different SDI and OE widths, which will also show different LED brightness changes. In addition, the shorter the unit width of OE, the shorter the cycle of completing a gray-scale change, that is, the higher the refresh rate can be obtained in unit time

the relationship between the shortest OE pulse width and high refresh rate

the shortest pulse width and reaction time (tr/tf) of OE in the drive chip determine the level of gray scale. The so-called shortest OE pulse width is the effective width that OE can be turned on under the condition of maintaining the linearity of output current of all channels. The smaller the OE pulse width, the higher the output color scale can be produced, that is, the faster the output current response, the higher the refresh rate and output gray scale. The refresh rate and output gray scale are related to the shortest pulse width of OE, the system data transmission speed, the number of serial chips and the number of chip output channels, as shown in Figure 2. The reference formulas are listed as follows:

according to the above reference formulas, if a single controller has 8 output ports, the belt cross-sectional area is 64 × 64 monochrome screen, the number of serial chips required is nic=32, and the output gray scale is set to 12 bits (4096 levels). If a drive chip with 16 output channels, a data transmission speed of 20MHz and an OE minimum pulse width of 300ns is used, the replacement calculation can get a refresh rate of 723hz, but if the output gray scale wants to be increased to 14 bits (4096 levels), the refresh rate will drop to 196hz, and if the output gray scale wants to be increased to 16 bits (65536 levels), The refresh rate only has a close relationship between the tensile value and the gauge distance in the 50Hz tensile sample, while the picture update rate input by the general system is at least 60Hz, so such a low refresh rate can no longer meet the needs of the general display system

in the above case, if you want to improve the output gray scale and the refresh rate, you can choose a driver chip with a smaller OE pulse width. If the chip with the shortest pulse width of 50ns of OE is used, even if the data transmission speed is 10MHz and the output gray scale is increased to 16 bits (65536 levels), the refresh rate can still output 287hz. When the output gray scale is set to 14 bits (4096 levels), the refresh rate can be increased to 1001hz, and when the output gray scale is set to 12 bits (4096 levels), the refresh rate can be significantly increased to 1953hz. Therefore, the smaller the OE pulse width, the higher the output color level and the refresh rate of which channel of the picture can be selected through ADDA, a DDS, a ddc3 address decoding. The higher the output color level provides a more colorful LED display image, and the higher the refresh rate provides a smooth and flickerless picture playback of the LED display

influence of the shortest OE pulse width on the output current surge

oe pulse width is the key factor affecting the output current surge. As shown in Figure 3, when the OE pulse width is greater than 500ns, the rise time of the output current is 37.99ns, and no surge is generated. However, if you want to get a higher output color scale and a faster picture refresh rate, you must reduce the OE pulse width, but a smaller OE pulse width requires a faster rise/fall time (tr/tf) to maintain the integrity of the pulse width, but a faster tr/tf will cause a surge in the output current of general LED drive chips, as shown in Figure 4. When the OE pulse width is less than 100ns, the rise time of the output current is 8.2ns, According to Faraday's law, vl=l (di/dt), it can be clearly seen that the output current produces a serious surge when it is turned off, and the surge of the output current may not only puncture the output channel of the driving chip, causing damage to the chip, but also make the electromagnetic wave interference of the entire LED display screen become serious, and the display screen will shake or even damage the system

improvement of current surge

to improve the surge of the output current of the above LED drive chip, we can reduce the switching speed of the output channel and stagger the switching time between the output channels. The switching speed of the output channel, that is, the slow rate that controls the output channel. The longer the rise/fall time (tr/tf) of the output current, the smoother the rise/fall waveform of the output current, and the more able it is to suppress the phenomenon of current surges and reduce electromagnetic interference. However, too much tr/tf will produce distorted waveforms and affect the response speed of the output current, so the LED driver chip must be able to achieve an optimal balance between the switching speed tr/tf of the output channel and the current surge

in addition, staggering the switching time between the output channels can also improve the output current surge of the LED drive chip, that is, reduce the instantaneous current of the power supply by not opening and closing the output channels at the same moment. As shown in Figure 5, the four output channels out0-out3 on the left are opened at the same time, resulting in a large surge current. In contrast, the four channels on the right are staggered, and the instantaneous current of the power supply is evenly dispersed, which reduces the peak current and improves the problem of surge and electromagnetic wave interference of the output current. Figure 6 shows the actual measurement of the LED drive chip, and the switching time waveform between the staggered output channels, The output channels are opened successively, and the adjacent two channels have a delay time of about 15ns

demand for high-end display screen

in order to meet the demand of high-end display screen, in addition to having a high refresh rate so that the LED display screen can play pictures smoothly and without flicker, it also needs to have the ability of high output color scale to achieve more colorful LED display image. For the above two requirements, the refresh rate and output color scale can be improved by selecting an LED Driver with a shorter OE pulse width, but the use of external gray scale control will still be affected by the limitations of system transmission speed and bandwidth, and the refresh rate and output color scale will be reduced. The other option is to build a PWM controlled LED driver chip, which can improve the transmission speed with a small amount of data transmission, and formulate a correct and reasonable solution to the problem, so as to improve the refresh rate and output color level. For LED Driver with short OE pulse width, please refer to mbi5036 of Jiji technology, while for LED driver chip with built-in PWM control, please refer to mbi5042 of Jiji technology