A Novel Single X-Wire AM Mini-LED Backlight Unit with Motion Blur Reduction for High-Image-Quality LCD TVs

Abstract

We have demonstrated a high-image-quality 85-inch 4 K high-dynamic-range (HDR) LCD display with 2304 dimming zones using a single X-wire AM mini-LED backlight unit (BLU) with motion blur and residual image reduction technology. Our latest work proposed the new single X-wire AM driving method, consisting of one matrix control IC/dimming controller (DCON) and X-wires of mini-LED driver ICs (dimmers), which eliminates extra scanning, sensing lines and pixel/source driver IC using an adaptive addressing dimming method. Additionally, a single X-wire controlled system can realize a flexible and expandable layout, which can achieve ultra thin and low cost in ultra-high dimming zones and large size display. Furthermore, the horizontal-scan (H-scan) driving method, which reduces motion blur and residual images using a scanning black frame insertion (BFI) technology, is utilized. The proposed mini-LED BLU can save about 9% power consumption compared to the conventional one in normal displays.

1. Introduction

Recently, mini-light-emitting diodes (mini-LEDs) have attracted significant attention owing to their high contrast, wide color gamut, environment friendliness, long lifetime, high reliability, high luminance and low power consumption, etc. [1,2]. Therefore, a mini-LED can serve as a local dimming backlight for high-dynamic-range (HDR) LCDs and emissive displays [3]. When serving as the local dimming backlight unit, thousands of mini-LEDs are arranged in one plane and controlled spatially and temporally depending on the input image; in this case, the backlight generates an approximate image to make a brighter image in the bright image area and a darker image in the dark image area to achieve HDR. By using sufficient local dimming zones [4,5], such an LCD can achieve a comparable contrast ratio and image quality with an OLED, while having higher brightness and longer lifetime with lower cost. Despite this, increasing dimming zones can suppress the halo effects [5].

However, increasing local dimming zones and mini-LEDs leads to higher cost, thickness, and more complex driving system, which slow down the transition to mini-LEDs. Several mini-LEDs are driven by a passive matrix (PM) [6,7,8] to achieve HDR images, which means more than one thousand dimming zones and driver ICs are required. Therefore, the number of layers in the BLU board and complexity of the PCB increase, increasing the cost and thickness of the panel. On the other hand, the active matrix (AM) using a TFT backplane or driver ICs, which are similar to AMOLED, is also reported [9,10,11,12]. Though these methods can eliminate the need for considerable ICs to drive mini-LEDs, keeping down the cost of the panel, the excessive scanning and sensing line cannot be eliminated either. Additionally, a conventional driving scheme is not suitable for a high-gray-level micro/mini-LED display because of its fast rising characteristic. What’s more, increasing size adds a higher requirement for uniformity and power consumption in the driving backplane. In addition, because of the sample-hold characteristic of LCDs, fast-moving scenes displayed on the LCD display panels are blurred, which is widely known as motion blur [13]. Like black data or frame insertion, backlight local dimming and high-frequency driving are used in LCD technologies to improve the motion picture response time (MPRT) characteristic [14,15,16,17]. However, traditional BFI technology without local dimming just inserts one-frame black data keeping the backlight unit on, causing high power consumption and bad image quality. On the other hand, the BLU with local dimming inserts one-frame black frame, turning off the backlight to improve the MPRT characteristics, causing a higher transport speed rate in the dimming data for BLU than for display data.

This paper presents a new single X-wire AM mini-LED driving method with a new technological solution for the fast MPRT enhancement driving method to improve the image quality and price competitiveness of a mini-LED backlight unit. The thickness, complexity and cost of the mini-LED backlight unit are improved by using the design approach of the proposed single X-wire AM driving method with the adjustable addressing dimming control method using one DCON and X-wire dimmers. What’s more, the 16-bit gray level is realized by 4-bit current and 12-bit PWM hybrid control. The motion blur issue is improved by using the design approach of the H-scan driving method, thus improving the transmission efficiency of the front-end driver IC.

2. Single X-Wire AM Mini-LED Backlight Unit with Motion Blur and Residual Image Reduction

2.1. Proposed Single X-Wire AM Mini-LED Backlight Unit

To achieve a high-uniformity and high-quality image display, compensation and AM control are required. Figure 1a presents the current technical AM backlight driving system, which uses three control lines, scanning line, data line and sensing line, to realize AM control and compensation. In this case, increasing dimming zones leads to more complex routing, a thicker backplane and higher cost. The proposed single X-wire AM mini-LED backlight unit for the ultra-high dimming zone HDR display is shown in Figure 1b. The single X-wire AM driving system can realize row scanning, data input and sensing, three functions combined in one single X-wire, which makes the backlight unit able to fabricate a single layer board with flexible and expandable layout with low cost. On the other hand, we emphasize the advantage of the proposed single-wire AM mini-LED backlight unit more clearly by eliminating the extra pixel/source driver or PCB, with one matrix controller DCON- and DCON-controlled dimmers. Figure 2 shows the concept of the proposed backlight unit driving and programming method with one X-wire line to reduce the scanning and sensing lines in the backlight driving system. One DCON can support 64 channels as a maximum, with 128 dimmers as a maximum in each X-wire. As shown in Figure 2, each dimmer controls four dimming zones. Therefore, there are 128 (64 channels × 2) dimming zones as a maximum vertically and 256 (128 dimmers × 2) dimming zones horizontally. Therefore, one DCON with 64 string dimmers can support 32,768 dimming zones as a maximum.

2.2. Adjustable Addressing Dimming Control Method

We propose a new AM control method, using the matrix controller DCON to send relevant address information or dimming data to mini-LED driver ICs, which are called dimmers. The DCON is a dimming control IC that uses a single X-wire to control dimmers. The dimmer is a mini-LED driver IC with four channels of current output, controlled by digital current and PWM data, where current data decide the amount of current paths and PWM data control the effective dimming time. In the addressing and data input stages, the data are transferred from DCON to dimmers, while data are read out from dimmers to DCON.

First, the initialization instruction is sent by DCON to the first dimmer, the dimmer work state is set to address the setting mode with the data transmission state being set to PASS. After initializing the first dimmer’s address information, the first dimmer sends the instruction to the second dimmer, until all dimmers’ address information is initialized. Then, the MASK instruction is sent to set every dimmer into non-transport state, which prepares it for accepting its corresponding address. After that, the address information is sent, until all the dimmers are set well. Before the dimming stage, all dimmers are set to work state and the data transmission state being set to PASS, meaning the X-wire signal can pass through all dimmers. The addressing stage is active only when the first debugging or dimmers need to change the addressing information. During the dimming data input stage, each dimmer picks up its dimming data from the corresponding address. The proposed driving scheme where the present driver IC sends instructions and other dimming data from other corresponding address to the next driver IC eliminates the gate and source lines between matrix controller ICs and each driver IC, thereby simplifying the structure and reducing excessive scan and data lines. On the contrary, during the readout stage, DCON reads feedback information from the first dimmer, which is transported from the last dimmer in each X-wire.

The X-wire data format is shown in Figure 3, which consists of the command orders, dimming data (current and PWM value) and readback orders, where a command in X-wire is used for controlling the dimmers, dimming data consist of a 4-bit current and 12-bit PWM data can be used directly by an X-wire-controlled dimmer; readback command following readback data is the last part of the X-wire signal. The 4-output dimmer is controlled by the X-wire, command in dimming data is used for identify the X-wire command, address information is set for address matching and the four-channel dimming data, which include current and PWM information, decide the effective driving current. Furthermore, the bi-directional communication system that consists of one DCON and DCON-controlled dimmers eliminate extra source/pixel IC or PCB, lowering the cost of the panel. What’s more, without extra scan and sense line, the layout of dimmers becomes more flexible and easier to expand. Therefore, the dimmers can be set in one string, N type or S type, as shown in Figure 4. These differential two-dimensional layouts of dimmers can minimize the number of output channels in DCON IC or expand dimming zones for an ultra-large display.

2.3. MPRT Enhancement Driving Method (H-Scan)

Figure 5 shows the concept of the proposed backlight unit scanning black frame insertion method with the H-scan by inserting a black frame into each frame to reduce the motion blur and residual image in the LCD display. The display area is divided into multiple horizontal sections. BLU turns off horizontal sections sequentially. The H-scan is a black insertion scanning method that synchronizes with input display data.

Figure 6 shows a comparison of the proposed scanning black frame insertion and conventional BFI and BLU blinking. First, the proposed H-scan driving method does not need high-frequency backlight switching data transfer compared with the conventional BFI method. Secondly, the proposed method supports local dimming for an HDR display. Last but not least, the proposed method matching the backlight unit turn-off stage with LC slow response time not only reduces power consumption but also improves the display quality in tailing and blurring reduction.

Figure 7 shows a driving waveform, which explains black frame insertion in scanning. The red one is the proposed H-scan waveform and the blue one is the conventional PWM driving waveform. Obviously, at the end of each frame, there is a black insertion period, which turns off the BLU. The H-scan operates while maintaining local dimming, to ensure luminance without loss is maintained. What’s more, it brings a higher contrast ratio, eliminates blinking issue and realizes motion blur reduction. The black insertion time is concluded as SPI delay, horizon input delay and refresh delay of LC, where the H-scan time should be less or equal to the total time of H start delay and refresh delay. H start delay depends on the number of dimming zones, which is caused by data input time. The refresh delay depends on the LC characteristics, which is different from each panel. After these delay times is the PWM driving period, which can realize different gray levels.

3. Results

Figure 8 shows the measurement results of the X-wire driving waveform. The yellow one is the X-wire signal that DCON sends to the first dimmer, and the blue one is the X-wire signal that the first dimmer sends to the second dimmer. It is obvious that the first dimmer picks its data for the corresponding address and sends to the next dimmer. Similarly, the red one is the X-wire signal that the second dimmer sends to the third dimmer. The X-wire waveform is clearly divided into three blocks. The first block is reused command instructions, which are used for status recognition. The second block is address and dimming data, where address and dimming data are one-to-one correspondence. The third block is the readback block, which is designed for checking and compensation for the operating point. The X-wire waveform has 3.3 V full output swing, and the transport speed rate is 330 kbps, which can satisfy the 4 K LCD display. The maximum transmission rate is 2 Mbps, so the proposed BLU is suitable for higher-resolution displays.

Figure 9 shows the measurement results of the different channels with the same PWM values in one dimmer. The four different waveforms in each picture are driving signals measured from channel 0 to channel 3. As shown at the top of Figure 9, the output waveforms reach the same PWM driving frequency. The dimmer is active at a low voltage level, which is independent of the rise and fall time; with the same current, four channels can achieve the same gray level and achieve uniformity in one dimmer. The mini-LED driver is active at a low level. Finally, we demonstrated an 85-inch ultra-high-definition (UHD) TV using the proposed single X-wire AM mini-LED backlight unit with motion blur reduction in a large-sized display with the H-scan driving method for the first time, which is shown in Figure 10. Figure 11 shows a display quality comparison of the TV with or without H-scan. When the H-scan function is turned on, it is confirmed that the motion blur and residual image are surely reduced. What’s more, the power consumption comparison is measured by the same TV under the same image with the proposed and conventional mini-LED BLU. The whole power consumption is reduced from 330 W to 300 W when using the proposed single X-wire AM mini-LED BLU. A performance summary between the proposed single X-wire AM mini-LED BLU and conventional mini-LED BLU is shown in

Table 1. Performance summary between proposed single X-wire AM mini-LED BLU and conventional mini-LED BLU.

Item	This Work	[6]	[9]
Backlight driving type	AM	PM	LTPS TFT
Panel size	85-inch	15.6-inch	2.89-inch
Number of dimming zones	2304 zones (H36 × V64)	2048 zones (H64 × V32)	2304 zones
Number of LEDs in a single dimming zone	4 LEDs	9 LEDs	1 LED
Thickness of the backlight unit	0.8 mm	1.56 mm	None

.

4. Conclusions

The low-cost, ultra-large-dimming-zone AM mini-LED backlight unit was fabricated utilizing the single X-wire communication of DCON and dimmers, using the H-scan method for MPRT enhancement for large-sized 85-inch LCD TVs with high image quality. The cost and thickness of the backlight board were significantly reduced to single layer by using the single X-wire AM control method. The proposed BLU achieves 0.8 mm in thickness and reduces about 9% of the power consumption compared to the conventional one in normal display. The H-scan driving method ensures that the black frame insertion data are synchronized with display data, reducing the complexity of the adaptive black data time algorithm and the cost of front-end IC because a higher transport data rate is not required. The proposed single X-wire AM driving method has great advantages of low cost and power consumption in ultra-high dimming zones, high image quality and large-sized HDR TVs.