Design of driving circuit for DMD projector based on STM32 microcontroller

DLP projection technology is a technology that uses the digital micromirror device (DMD) developed by Texas Instruments as the main key processing element to realize the digital optical processing process. The color displayed by DLP is high in clarity, bright, delicate and realistic, and it is a fully digital display with extremely high reliability. It can provide the best image effect in various products (such as large-screen digital TVs, corporate/family/professional conference projectors and digital cameras (DLP Cinema)). At present, most home or commercial DLP projectors use a single-chip structure, which makes them easy to move and carry, and thus they are increasingly widely used. On the basis of the current application development, higher requirements are put forward for the simplicity of its structure and the convenience of carrying. Traditional DLP projectors receive external signals through the DVI interface and transmit them to the DLP controller after signal conversion to control the DLP display. They occupy a large space, have a limited signal receiving mode, and are difficult to integrate into existing instruments and equipment. If the digital signals in existing instruments and equipment can be sent directly to the DLP without multiple data conversions, the size and cost can be reduced, and the DLP can be easily integrated into instruments and equipment.

DLP projectors use three-color LEDs as light sources, so the choice of LEDs is also crucial. In recent years, RGB three-color LEDs have surpassed other light-emitting devices in terms of heat dissipation, reliability, color saturation, and energy efficiency, and their use in lighting design has become increasingly common. Currently, many LED device manufacturers use independent red, green, and blue LED combinations to provide the required colors. There are some disadvantages in using discrete LED packages in applications, such as the waste of space caused by conforming to the package structure, and the extra effort required to achieve effective color mixing of light sources that are separated far apart. Therefore, an integrally packaged LED chip is needed to replace the traditional independent light source, that is, a product that integrates red, green, and blue LED chips in a single package, in which each LED chip can be independently controlled to provide a variety of different color outputs.

Taking DLP1700 as an example, this paper improves the traditional DLP projection system from the aspects of signal input control and display light source. In terms of display light source, high-power RGB three-color LED is used to replace the traditional multiple single-color LEDs. In terms of signal input control, the traditional DVI connector and MSP430 are eliminated, and the Stm32 microcontroller with I2C function is used to generate control signals and image signals to directly control the DLP1700 controller DLPC100, thereby controlling the display of DLP1700. This design can make the hardware circuit structure of the DLP display instrument more streamlined, circuit control is easier to implement, and can be easily integrated into various instruments.

1 Signal input control

The signal input end of the traditional digital micromirror display technology is provided by the DVI interface to provide image signals and horizontal field synchronization signals. The MSP430 microcontroller performs basic control on the DLPC100, and the control signal is transmitted through the I2C bus. The whole process involves many chips and the circuit is complex. In this design, we use the STM32 series microcontroller based on the Cortex_M3 core to improve the signal input part, and transmit the image signal and horizontal field synchronization signal sent by Stm32 to DLPC100 to control the display of DLP1700 and the drive of LED. Because the STM32 itself carries an I2C bus interface, it can replace the traditional MSP430 and I2C bus to perform a series of controls on the inside of DLPC100. The specific circuit connection block diagram is shown in Figure 1.

2 Display light source

Compared with discrete packaged devices, using an overall packaged RGB tri-color LED to replace the traditional multiple LEDs in an independent package can make the light source more compact and significantly reduce the distance between each independent light source. To achieve good color mixing, the maximum spacing between LED light sources cannot exceed 5 mm. The three-in-one approach can reduce this distance to only 1.5mm. After the LED spacing is reduced, the area required for effective color mixing can also be reduced at the same time. After research, it was found that this overall packaged LED also has good efficiency in heat dissipation.

The design in this article uses the Moonstone series 3W RGB tri-color LED of Anhua High-Tech Co., Ltd. This light source has the characteristics of higher light intensity and smaller volume. The performance parameters of the RGB tri-color LED light source are shown in Table 1. A single RGB tri-color LED light source can replace traditional halogen tungsten lamps and multiple single-color LED light sources. The space area used to achieve the same light effect is only a fraction of that of ordinary white light LEDs and a few tens of that of traditional halogen tungsten lamps. [page]

The lighting system is designed using optical design software, and the light source is modeled according to the parameters of the light source performance. The model of the RGB three-color LED light source is shown in Figure 2. The sizes of the three monochromatic LED light-emitting chips in the figure are 0.8x0.8 mm2 respectively, and the monochromatic light sources are 1 blue light-emitting chip, 1 green light-emitting chip and 1 red light-emitting chip, respectively. The distribution is shown in Figure 2. The light source has an emission angle of 120°, and the emitted light is set according to the parameters provided in Table 1. Using this light source to design the lighting system of projectors, rear-projection TVs, large-screen displays, etc. can simplify the structure, reduce the volume, and facilitate operation.

3 Circuit Principle and Overall Circuit Diagram

The traditional DLP1700 display system circuit diagram is shown in Figure 3. In the figure, the signal input uses the DVI interface, MSP430 microcontroller, and I2C bus, and the line control is relatively complex; the RGB EN and RGB PWM signals output from the DLPC100 on the right control the LED driver and then control multiple single-color LEDs. The improved system circuit is shown in Figure 4. The STM32 microcontroller controls the DLPC100 and inputs the data signal to the DLPC100, which then drives the LED driver. When the DLPC100 performs data format conversion, signal enhancement, DMD format conversion, etc. on the input signal data as shown in Figure 5, the signal is transmitted to the DMD chip DLP1700. The 480x320 micromirrors on the DMD chip deflect at a certain angle under the control of the input signal, and the LED driver drives the three-color LED to provide light source for the DLP display, and a perfect image display is obtained on the screen.

4 Conclusion

By improving the circuit and optical path system of DLP1700 display, a more concise and intuitive system is obtained. The STM32 series single-chip microcomputer replaces the complex input module design, which receives and processes digital signals and generates image signals. The input structure is simple and easy to program and control; the new overall packaged high-power RGB three-color LED replaces the traditional multiple single-color LEDs. Compared with the traditional discrete LED package, the three-in-one package solution has better performance in color mixing or space requirements, and has good heat dissipation capacity and reliability, which can bring higher flexibility to engineering application development. The improved DMD display instrument is easy to carry, simple in structure, small in size, energy-saving and environmentally friendly, and can be easily integrated into various devices, so that DLP instruments can better play their advantages, that is, light, high in reliability, easy to operate and control, etc., providing solid and powerful conditions for its larger-scale application.