A direct-type RGB LED backlight driving technology for cockpit displays

Abstract: Direct-lit RGB LED is used as the backlight source of the LCD display in the cockpit, and the backlight module partitioning technology is adopted. Through the design of the backlight driving circuit and the use of CAN bus communication, the light intensity of the display backlight module can be automatically or manually adjusted according to the three primary colors of the surrounding light or the user's preference. This design can better meet the needs of users in the cockpit.

introduction

In order to enable users in the cockpit to obtain accurate and reliable information at any time, the display used in the cockpit must have high reliability; at the same time, in order to improve its readability and relieve visual fatigue, the light intensity of the display should also be able to be adjusted according to the surrounding environment and user needs, so that users can avoid not being able to see the displayed content clearly when the ambient light is strong, or feeling glare when the ambient light is dark. At present, liquid crystal (LCD) displays have been widely used in commercial, industrial and military fields. LED backlight is a key research topic of the National 863 Program. It has obvious advantages in stability, visibility, environmental protection, luminous efficiency, color saturation, volume, dynamic control, etc., and is rapidly becoming popular.

This paper designs a driving circuit that applies LED backlight to cockpit displays. It uses the world's leading direct-type RGB LED backlight driving technology, partitions the backlight module, and uses CAN bus technology to ensure effective communication. The circuit design enables the light intensity of the display backlight module to be automatically or manually adjusted according to the three primary colors of ambient light or user preferences. This design can better meet the viewing needs of users in the cockpit.

1 Backlight module

1. 1 RGB LED backlight module

Traditional LCD backlight sources use cold cathode fluorescent lamps (CCFLs), which have poor color reproduction and can only provide 72% of the color gamut in the NTSC (National Television System Committee) standard. LED backlight sources have good color reproduction, reaching 105% of the color gamut in the NTSC standard, or even exceeding 120%, and have a long life and do not contain mercury.

LED backlight modules are generally divided into edge-lit and direct-lit types according to the incident position; they can be divided into RGB LED and white LED according to the type of LED. Compared with edge-lit backlight sources, direct-lit backlight sources have more uniform brightness distribution because the LEDs are distributed behind the panel. RGB LED backlight uses RGB three primary colors of LED as independent light-emitting elements, which has better brightness, contrast and color reproduction than white LED.

Using RGB LED as the backlight module, the color gamut of the display can reach up to about 150% of the NTSC color gamut. At the same time, the RGB LED backlight source supports backlight area adjustment technology, and the dynamic contrast ratio can reach 10 million:1 level [5]. Through analysis, direct-type RGB LED is more suitable as the backlight source of LCD displays in cockpit environments. Figure 1 is a schematic diagram of the structure of an LCD display using a direct-type LED backlight source.

Figure 1 Top view of an LCD display with direct-lit LED backlight

Since LED light sources are point light sources, we need to perform optical design to form a surface light source with uniform brightness. In addition, we divide the backlight module screen into blocks, and use three LED driver integrated circuits in each area to control the luminous intensity of the three primary color LEDs respectively.

1.2 Automatically adjust light intensity

Since human eyes are accustomed to seeing objects in nature that are affected by ambient light, the brightness of the backlight module designed in this paper simulates the brightness characteristics of objects in nature. When the brightness of the R, G, and B three-color LEDs reaches the maximum value, the white balance of the display is best.

The circuit detects the intensity of ambient light, and decomposes the ambient light into red, green, and blue signals through the three-primary-color sensor. The sizes are represented by xR, xG, and xB respectively. Then, the intensity of the three primary colors of the backlight source is adjusted according to the intensity of the ambient light. The intensity of the three primary colors that the backlight module needs to emit is represented by yR, yG, and yB respectively. The relationship between the intensity of the three primary colors that the backlight module needs to emit and the intensity of the three primary colors of the ambient light is:

In the formula, i can be R, G, or B; bi is the light intensity of the i-th color of the backlight source when the ambient light is 0; ki is set with reference to the light intensity characteristics of ordinary objects in nature under natural light. Both ki and bi are greater than 0, and their values ​​are stored in the microcontroller and can be changed according to user needs.

The control module continuously measures the intensity of ambient light and the actual intensity of the three primary colors of the backlight module through the three primary color sensor, and controls the LED driver integrated circuit to output the corresponding PWM signal, so that the intensity of the backlight module can be automatically adjusted according to the illumination of the surrounding ambient light.

1.3 Manually adjust the light intensity

If the user is not satisfied with the automatically adjusted light intensity, the light intensity of the backlight module can be manually adjusted according to personal preference. The three primary color light intensity before the backlight module is adjusted is yi, and the three primary color light intensity after manual adjustment is y'i. b'i and k'i are calculated by y'i:

The b'i and k'i calculated by the two formulas may be very small. In view of this situation, bmini and kmini are set in the circuit. Compare b'i and bmini, and store the larger value as the adjusted bi value in the circuit; Compare k'i and kmini, and store the larger value as the adjusted ki value in the microcontroller. In this way, the relationship between the backlight module light intensity and the ambient light intensity is modified.

When the system is initialized, the initial values ​​of bi and ki are set to bsi and ksi and stored in the microcontroller. The user can restore bi and ki to bsi and ksi by pressing a button.

2 Driving and detection circuits

Cockpit display devices are mainly concentrated in medium size. This paper designs the backlight drive circuit of a 7-inch LCD display. The circuit schematic diagram of the detection and drive part is shown in Figure 2.

Figure 2 Schematic diagram of the detection and drive part

The circuit uses STC12C5624AD microcontroller as the control module; LM 2733 boost power supply produced by National Semiconductor is used to drive red, green and blue LEDs respectively; LM20 is used as a temperature sensor to detect the temperature of the LED at any time to ensure the normal operation of the LED; photodiode MCS3AS is selected as the three-primary color sensor to convert the light signal into electrical signals of the three primary colors of red, green and blue; buttons are used to control whether to reset to the initial value and manual dimming.

To improve the reliability of the system, the backlight module of the LCD is divided into 9 areas. At the same time, in order to filter out 50Hz and 60Hz interference signals, this circuit samples the ambient light and the light intensity signal of the backlight module by sampling 600 points in 1s and calculating the average value.

3 Communication module

To ensure the reliability of communication, this design uses CAN bus communication. CAN network is a serial communication network that effectively supports distributed control or real-time control. Its bus specification has been established as an international standard by the ISO International Organization for Standardization and is recognized as one of the most promising field buses. Due to its low design cost and high communication reliability, it is widely used in data communication between various detection and actuators in control systems.

The single-chip microcomputer, three primary color sensors, temperature sensors, and LED driver integrated circuits are used as a node, and SJA1000 and TJA1040 are used as communication devices of the node. The node single-chip microcomputer sends backlight information and ambient light information to the CAN bus, and can also receive information on the CAN bus. The transmission rate used in this design is 500kbps.

4 Conclusion

Through the above research on the backlight source of the LCD display, combined with the requirements of the cockpit environment for the display, a direct-type RGB LED backlight driving circuit for the cockpit is designed, which can automatically or manually adjust the brightness of the display backlight source. After testing and use, the LCD display using this design technology is still readable when the surrounding light is bright, ensuring the safety and reliability of the information displayed on the display.

Since this design adopts a partition design, its backlight circuit design scheme can also be used in large-size LCD displays.