Introduction to methods to improve the refresh rate of high-end LED displays

Introduction: To meet the needs of high-end display screens, in addition to having a high refresh rate so that the LED display screen can play images smoothly without flickering, it also needs to have the ability to output high color levels to achieve more colorful LED display screen images.

Grayscale of LED display

Grayscale is the resolution of the brightness of each LED on the display. For example, 4-bit grayscale means that the LED has 16 levels of brightness change. The grayscale control of the LED driver chip is implemented as shown in Figure 1. The grayscale of the LED brightness is controlled by the OE width and SDI on the driver chip. Taking the grayscale 5 to be displayed by the first LED in Figure 1 as an example, the SDI must open the output switch at OE widths of 1 and 4 to obtain an overall LED display grayscale of 5. The grayscales of 9, 4, and 11 are similar. Different combinations of SDI and OE widths can obtain different LED grayscales, which will also show different LED brightness changes. In addition, the shorter the unit width of OE, the shorter the cycle of completing a grayscale change, that is, the higher the refresh rate that can be obtained per unit time.

Relationship between the shortest OE pulse width and high refresh rate

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

Frefresh : refresh rate (Hz)

According to the above reference formula, if a single controller has 8 output ports and a monochrome screen with a cross-sectional area of ​​64×64, the number of serial chips required is NIC=32, and the output grayscale is set to 12 bits (4,096 levels). If a driver chip with 16 output channels, a data transmission speed of 20MHz and an OE minimum pulse width of 300ns is used, the refresh rate can be calculated to be 723Hz. However, if the output grayscale is to be increased to 14 bits (4,096 levels), the refresh rate will drop to 196Hz. If the output grayscale is to be increased to 16 bits (65,536 levels), the refresh rate will only be 50Hz. The screen update rate of the general system input is at least 60Hz, so such a low refresh rate can no longer meet the needs of general display systems.

In the above situation, if you want to increase the output grayscale and refresh rate at the same time, you can choose a driver chip with a smaller OE pulse width. If you use a chip with an OE minimum pulse width of 50ns, even if the data transmission speed is 10MHz and the output grayscale is increased to 16 bits (65,536 levels), the refresh rate can still output 287Hz. When the output grayscale is set to 14 bits (4,096 levels), the refresh rate can be increased to 1001Hz. When the output grayscale is set back to 12 bits (4,096 levels), the refresh rate can be greatly increased to 1,953Hz. Therefore, the smaller the OE pulse width, the higher the output color scale and the refresh rate of the picture. High output color scale provides a more colorful LED display image, and high refresh rate provides smooth and flicker-free picture playback on the LED display.

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

Figure 3 Output current waveform with larger 0E pulse width

The size of the OE pulse width is a 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 level and a faster screen refresh rate, you must reduce the OE pulse width. However, a smaller OE pulse width requires a faster rise/fall time (tr/tf) to maintain the integrity of the pulse width. However, a faster tr/tf will cause the output current of a general LED driver chip to produce a surge. 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. The surge of the output current may not only break through the output channel of the driver chip, causing damage to the chip, but also make the electromagnetic interference of the entire LED display screen more serious, and the display screen will jitter or even damage the system.

Figure 4 Small OE pulse width produces severe surge

Improvement of current surge

To improve the output current surge of the above LED driver chip, two design methods can be used: reducing the switching speed of the output channel and staggering the switching time between the output channels. The so-called switching speed of the output channel is to control the slew-rate of the output channel. The longer the rise/fall time (tr/tf) of the output current, the smoother the waveform of the output current rise/fall, and the more it can suppress the current surge phenomenon and reduce electromagnetic interference. However, if the tr/tf is too large, it will produce a distorted waveform and affect the response speed of the output current. Therefore, 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.

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In addition, staggering the switching time between output channels can also improve the output current surge of LED driver chips, that is, by not turning on and off the output channels at the same moment, the instantaneous current of the power supply line can be reduced. As shown in Figure 5, the four output channels OUT0~OUT3 on the left are turned on at the same moment, resulting in a large surge current. On the other hand, the four channels on the right are staggered, and the instantaneous current of the power supply line is evenly dispersed, reducing the peak current and improving the output current surge and electromagnetic interference problems. Figure 6 is an actual measurement of the staggered switching time waveform of the output channels of the Polymer LED driver chip. The output channels are turned on one after another, and there is a delay time of about 15ns between adjacent channels.

Figure 5: Staggering the switching times between output channels

Demand for high-end displays

To meet the requirements of high-end display screens, in addition to having a high refresh rate so that the LED display screen can play images smoothly and without flicker, it is also necessary to have the ability to output high color levels to achieve more colorful LED display screen images. The above two requirements can be achieved by selecting an LED driver with a shorter OE pulse width to increase the refresh rate and output color level, but the use of external grayscale control will still be affected by the system transmission speed and bandwidth limitations, thereby reducing the refresh rate and output color level. Another option is to use an LED driver chip with built-in PWM control, which can increase the transmission speed with a smaller data transmission volume, thereby achieving the effect of increasing the refresh rate and output color level. For LED drivers with shorter OE pulse widths, refer to Macroblock Technology's MBI5036, and for LED driver chips with built-in PWM control, refer to Macroblock Technology's MBI5042.

Figure 6: The actual measured switching time waveform of the staggered output channels of the clustered LED driver chip

References:

[1] Macroblock MBI5036 Preliminary Datasheet V2.00

[2] Macroblock MBI5042 Preliminary Datasheet V1.00

[3] Macroblock MBI5040 Preliminary Datasheet

[4] The secret of LED screen driving technology: AC response[OL].

[5] Characteristics and scanning drive scheme of LED dot matrix display screen[OL].