Grayscale is the resolution of the brightness of each LED on the display. For example, a 4bit grayscale indicates that the LED has a 16th order brightness change. The gray scale control of the LED driver chip is shown in Figure 1. The gray scale of the LED brightness is controlled by the OE width and SDI on the driver chip. For example, the gray level 5 to be displayed by the first LED in Fig. 1 is required. The SDI must be turned on at the OE width of 1 and 4 to open the output switch. To get the overall LED display gray scale is 5. The gray scales of 9, 4, and 11 are similar, and different LED gray scales are obtained by combining different SDI and OE widths, and different LED brightness changes are also displayed. In addition, the shorter the unit width of OE, the shorter the period for completing a gray scale change, that is, the higher the refresh rate that can be obtained per unit time.
The shortest pulse width and reaction time (tr/tf) of OE in the driver chip determine the gray level. The so-called shortest OE pulse width is the effective width that OE can open under the condition that the linearity of all channel output currents can be maintained. The smaller the OE pulse width, the higher the output color gradation, that is, the faster the output current response, the higher the refresh rate and output gray scale. The refresh rate and the output gray scale are related to the OE shortest pulse width, the system data transmission speed, the number of serially connected chips and the number of chip output channels. As shown in FIG. 2, the reference formulas are listed as follows:
According to the above reference formula, if a single controller has 8 output ports with a 64×64 monochrome screen, the number of serial chips required is NIC=32, and the output gray scale is set to 12 bits (4,096 levels). If a driver chip with 16 output channels, a data transmission speed of 20 MHz, and an OE minimum pulse width of 300 ns is used, the refresh rate is 723 Hz, but if the output gray scale is increased to 14 bits (4,096) The refresh rate is reduced to 196 Hz. If the output gray scale is to be increased to 16 bits (65,536 levels), the refresh rate is only 50 Hz, and the screen update rate of the general system input is at least 60 Hz, so such a low refresh rate cannot be achieved. Supply the needs of a general display system.
In the above case, if you want to increase the output gray scale and want to increase the refresh rate, you can select a driver chip with a smaller OE pulse width. If the OE has a chip with a minimum pulse width of 50 ns, even if the data transmission speed is 10 MHz, the output gray scale is increased to 16 bits (65,536 levels), the refresh rate can still be outputted at 287 Hz, and the output gray scale is set to 14 bits (4,096 levels). When the refresh rate is increased to 1001 Hz, the refresh rate can be greatly increased to 1,953 Hz when the output gray scale is set back to 12 bits (4,096 levels). Therefore, the smaller the OE pulse width, the higher the output level and the refresh rate of the picture, the higher output level provides a more colorful LED display image, and the high refresh rate provides a smooth and flicker-free display of the LED display. Play.
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 500 ns, the rise time of the output current is 37.99 ns, and no glitch is generated. However, if you want to get a higher output level and faster picture refresh rate 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 Sex, but the faster tr/ tf will cause the output current of the general LED driver chip to generate a glitch. As shown in Fig. 4, when the OE pulse width is less than 100 ns, the rise time of the output current is 8.2 ns, which is known by Faraday's law. L(dI/dt), it can be clearly seen that the output current generates a severe surge phenomenon when it is turned off, and the output current surge can not only break through the output channel of the driver chip, causing damage to the chip, but also making the entire LED The phenomenon of electromagnetic interference on the display becomes severe, and the display screen may cause jitter or even damage to the system.
The spur that wants to improve the output current of the LED driver chip can be performed by reducing the switching speed of the output channel and shifting the switching time between the output channels. The switching speed of the output channel, that is, the Slew-rate of the control output channel, the longer the rise/fall time (tr/ tf) of the output current, the smoother the waveform of the rising/falling output current, and the more the current burst can be suppressed. Wave phenomenon, reducing electromagnetic interference. However, the tr/tf excessively produces a distorted waveform that affects the response speed of the output current, so the LED driver chip must have the ability to achieve an optimal balance between the switching speed tr/tf of the output channel and the current glitch.