Table 1-2 describes two common LED Driver topologies. In the direct drive topology, each LED has own drive channel. In the time-multiplexing topology, a group of LEDs (in column) share one drive channel and each LED lights up in turn by time-multiplexing way. In detail, after the first line of LEDs are lighted up, the second line of LEDs are then lighted up, and so on, up until the last line of LEDs are lighted up, and then the scan returns to the first line and enters the next cycle. Time-multiplexing uses persistence of vision of human eye and makes human eye can feel continuous image even LEDs light up in turn.

In a mini- or micro-LED display application, each pixel corresponds to an RGB LED, direct drive is not practical when the screen resolution getting higher and higher. For instance, a 4K screen has 3840 × 2160 pixels and each pixel needs three channels (red, green and blue). Considering using a 48-channel direct drive LED driver, which needs 3840 × 2160 × 3 / 48 = 518,400 pcs drivers that is almost impossible from PCB layout and costs perspectives.

Table 1-2 LED Driver Topology Comparison

	Direct	Time-Mutliplexing
Structure
Componnet	N × LED Driver	(1 × LED Driver) + (1 × Controller) + N ×(Switch MOSFET)
Zone Current	Peak zone current – Average zone current = IZONE	Peak zone current = N × IZONE Average zone current = IZONE

The time-multiplexing design using the single driver IC to turn on more LEDs and then saves PCB layers and cost, is more practical and essential for narrow pixel pitch (NPP) LED display. In fact, the smaller the pixel pitch, the greater the number of time-multiplexing in the design.

Though the time-multiplexing design saves PCB layers, cost and also has great challenges on display performance. Because the high efficiency of time-multiplexing is at the sacrifice of the display refresh rate. Note that the definition of refresh rate in LED display is different from that in LCD or OLED display. Usually, the refresh rate of your LCD or OLED display refers to how many times per second the display is able to draw a new image. This is measured in Hertz (Hz). For example, if your display has a refresh rate of 144Hz, that means refreshing the image 144 times per second. However, the refresh rate defined in LED display is the reciprocal of the amount of time that all LEDs are lighted up one time in turn (same measured in Hertz). Normally, the refresh rate of LED display needs to be very high up to 2000Hz or higher, such as 1920Hz, 3840Hz and even 7680Hz, to avoid dark scan lines or brighter/dimmer lines when using the camera to capture the LED display image.

The refresh rate depends on the number of time-multiplexing (or number of scan lines). From the IC design perspective, supporting more scan lines can use less drivers and then further saves PCB layers and cost, that makes the driver product be more competitive in the market. However, when there is more scan lines, for example, doubled scan lines, the total time needed to light up all LEDs in one cycle doubles without changing the average brightness of the display, therefore, the refresh rate is halved.

Both high refresh rate and large quantities of scan lines are not easy to achieve. Fortunately, TI’s matrix LED display driver TLC6983 and TLC6984 have built-in SRAM to support more multiplexing and also increase the refresh rate by shortening the grayscale (GS) data transmission time. In addition, TLC6983 and TLC6984 have an internal frequency multiplier to generate the GCLK by SCLK, up to 160MHz, which makes higher refresh rate support be possible. The application note Example of LED Display Screen Design Requirements Based on TLC6983 shows a display example achieving 7680Hz refresh rate, 120Hz frame rate, 18 scan lines and 16-bit grayscale intensity.