About the principle and development trend of LED display driver design

I. Overview The large screen has always been an important part of the development of the LED field. It is an important part of the large-scale entertainment, sports events and square theme display. Since the birth of the blue LED, the full-color screen has maintained a high-speed development trend. In the decorative display market LED will play a positive role, and the market expansion is obvious. China is a global LED display production country, completely dominated from LED chips , driver ICs, controllers, screen manufacturing and other links.

The origin of 16-bit shift constant current IC: The two-color screen is mainly based on display text, and the single-chip scanning is convenient. Due to the increase of the number of LEDs, in order to save O/I resources, 74HC595 shift scanning is adopted. In order to be more suitable for LED applications, the constant current setting function is integrated on this basis, the current driving capability is increased, and 16-bit devices are packaged in accordance with the requirements and cost requirements, and are widely used in current full-color screens.

The current drive capability is continuously reduced. The TLC5940 has a single drive current capability of 120mA. Later, the TB62726, ST2221, and MBI5026 current drive capability were reduced to 80-90mA. Currently, the current drive capability of 45mA is basically used, such as MBI5024 and CYT62726. The main reason for the decrease in capability is that LED devices have higher and higher luminous intensity. In order to improve image quality, static screen design is increasingly used, and the demand for driving current capability is reduced. From the perspective of IC cost, the chip size can be reduced to reduce the cost. For this purpose, a 25mA static screen driver chip CYT62727 is designed.

At present, 80% of the world's LED screens are purchased and produced in mainland China. The 16-channel constant current device design is widely used. It will continue in the short term and will not disappear in the next five years. The main control technology is mature and the products have been serialized. Unless the system control technology and chip driver design have a huge leap and the cost is further reduced, the status quo will not change. In recent years, many companies have continuously introduced new architectures, which will be recognized by the market in the future. The biggest problem is that through the control technology, if the color consistency of the LED is changed, the new technology has no big breakthrough before, and it is not attractive to the application, and the purchase intention is not strong.

In recent years, mainland chip design companies will definitely occupy the LED display market. For example, Silan Mingxin is the mainstream LED chip supplier. For reasons of excellent quality assurance, a good direct sales model is the market magic weapon. . Future driver ICs also require a direct sales model. Display companies are mostly local companies in mainland China, and Taiwanese ICs need further improvement in terms of distribution period, delivery speed and trust.

In the driving application technology, the color correction technology needs to solve the bottleneck, the display attenuation consistency problem is prominent, wavelength correction and brightness correction are the next key breakthrough targets, firstly to solve the brightness consistency problem, and then the wavelength consistency Correction. This is a worldwide problem and a technical problem that needs to be solved urgently.

The development of control technology has reached the forefront of the world, but it has stagnated in the new mode control concept. The reason is that controller manufacturers and IC design manufacturers are not well coordinated, and each is relatively independent. The driving technology and control technology cannot be well connected. The development of 16-channel compatibility design is high, and new control technologies are rarely introduced, let alone technological innovation. The 16-channel constant current device has been used on LED screens for more than a decade. Until now, it has only reduced current, and it has to deal with fiercely competitive prices. There is no technical breakthrough. A number of companies have launched a series of ICs, which have not been well applied and are caused by the lack of control technology. In today's rapid development of computer technology, it can replace the simplified controller scale, but the technology migration also needs the support of IC design manufacturers, the market lack of control technology and chip driver integration solution provider.

In 2009, the LED display industry has a domestic market of more than 30 billion, and there are more than 30 companies with an annual output value of over 100 million. There are more than 10 million companies, and thousands of companies are engaged in the display manufacturing industry. LED displays continue to grow at a rate of 15%, and technology is maturing. China's large-scale event projects continue to drive LED growth. For example: Olympics, World Expo, Asian Games, Shanghai Disney, subway, high-speed rail and other engineering events.

Second, the screen principle design

The text display, as long as the content is clear and has sufficient brightness, will basically meet the needs of customers. But for the evaluation of the display quality of the image display, the problem is much more complicated. It is generally a subjective way to evaluate the display quality of the display image. The so-called subjective evaluation is a man-made method of judging, and judging by the quality of the image display. such. The evaluation results are not only related to the quality of the image itself, but also related to the subjective factors of the observer. It is difficult to say that it is a standard of fairness and certainty. Despite this, there is still no good way to do so. Subjective methods are still the most effective and practical method before the emergence of objective measurement methods.

(1) Maximum number of displayed colors

The color of each pixel of the display is composed of three primary colors of RGB (red, green, blue). Low-end liquid crystal display panels, each base color can only represent 6-bit color, that is, 26 = 64 colors. Through simple calculations, we can know that the maximum number of colors that each individual pixel can represent is 64 × 64 × 64 = 2.62K colors; the high-end liquid crystal display panel uses FRC technology to make each primary color can represent 8-bit color, that is, 28 = 256 colors, the maximum number of colors that a pixel can represent is 256 × 256 × 256 = 16 KK colors. The display panel displays a richer color and a layered look. At present, the liquid crystal display on the market has adopted both display panels, so you can pay attention to it.

The illuminance unit of the surface is lux; multiply this value by the effective display area of ​​the screen to obtain the illuminance intensity of the entire screen at the optimal viewing angle, assuming that the illuminating intensity of each pixel in the screen is constant in the corresponding space, then This value can be considered as the luminous flux of the entire screen. Generally, the outdoor LED display must have a brightness of 4000 cd/m2 or more to have an ideal display effect under daylight. Ordinary indoor LED, the maximum brightness is about 700~2000cd/m2.

The luminous intensity of a single LED is in cd, and is equipped with a viewing angle parameter. The luminous intensity has no relationship with the color of the LED. The luminous intensity of a single tube varies from a few mcd to five thousand mcd. The luminous intensity given by the LED manufacturer refers to the point at which the LED illuminates at a current of 20 mA, with the highest luminous intensity at the best viewing angle and at the center position. The shape of the top lens and the position of the LED chip from the top lens when packaging the LED determine the LED viewing angle and intensity distribution. Generally speaking, the larger the LED viewing angle is, the smaller the maximum luminous intensity is, but the luminous flux accumulated on the entire three-dimensional hemisphere is unchanged.

When a plurality of LEDs are relatively tightly discharged, the illuminating spheres are superimposed on each other, resulting in a relatively uniform distribution of luminous intensity of the entire illuminating plane. When calculating the luminous intensity of the display, it is necessary to multiply the maximum luminous intensity value provided by the manufacturer by 30% to 90% according to the LED viewing angle and the emission density of the LED as the average luminous intensity of the single tube.

Generally, the luminous life of LEDs is very long, and manufacturers generally indicate that they are more than 100,000 hours. Under the optimal conditions set, the brightness decay period of LEDs should be paid attention to, the brightness decay period and the material process of LED production. Manufacturers have a lot to do with, and generally, brands with slower brightness decay should be used when economic conditions permit.

In actual use, light intensity calculations often use data units that are easier to map or use. For the LED display screen, the active illuminant generally adopts cd/m2 as the unit of luminous intensity, and the observation angle is an auxiliary parameter, which is equivalent to the screen body.

Most of the screens use straight-line elliptical LEDs, and most of the domestic packaging technologies can meet the needs of screen design. LED chips are mostly made of CREE or domestic LED light-emitting chip packages such as Silan Mingxin and Xiamen Sanan.

(2) Driver chip timing

CYT62726 is a 16-bit shift register. Multiple CYT62726 serial data shifts. Each clock cycle CLK transfers 1-bit data SDI, and serial data input driver on/off control. Schmitt buffer input. When the data "1" is written to the switch of the SDI control shift register / the rising edge of CLK.

Gray control module simulation waveform

CLK serial data shift clock. Schmitt buffer input, all data/off control shift shifts are caused by the rising edge of CLK with the highest bit sync of 1 bit, and single data shift to SD at the same time. After the rising edge of CLK is asserted, it continues for a rising edge of 100ns.

The LE edge triggers the latch. Schmitt buffer input. Currently corresponding to the data in the shift register, the data is latched on this rising edge.

Shift latch simulation waveform

All OE outputs are blank. Schmitt buffer input. When OE is low, all constant current outputs (OUT0?15) are executed. When OE = 1, all constant current output controlled switches are in data control data / latched state. The OE decides to execute the data length time.

This timing transmission method is based on the 74HC595 general logic data transmission method, which has been used for more than ten years on the LED screen, which is old and outdated. LED screens are waiting for new data transmission formats, simplified and efficient transmission methods, thereby reducing design complexity, reducing design costs and improving screen reliability.

The data and clock need to be coordinated. However, in the circuit design, the data is transmitted serially, and the clock is transmitted in parallel. The data delay will cause the output to be misaligned. This is the biggest disadvantage of the 4-wire transmission format. The data and clock are not well synchronized, the cascading is poor, and the controller costs are high. The outdated data transmission format, the controller generates gray scale, the screen refresh rate is low, and the amount of transmitted data is large, which is the bottleneck of the current development of the LED screen.

(3) Driver chip block diagram

The internal circuit of CYT62726 is relatively simple. The resistor adjusts the output current value through the current mirror. The chip sets the 16-channel current value uniformly. Therefore, in the screen design, a single IC drives a single color, and three ICs form 16 pixels. In this way, brightness uniformity can be obtained by LED sorting, and three ICs set different driving current values ​​to form a suitable 16-pixel white balance. The chip constant current error is very important, and the current error parameter also affects the LED white balance level.

The shift register (D flip-flop) is responsible for the data (SDI) queue, moving the data queue according to the clock (CLK) time. The correct data is stored by the latched signal (LE/), which may be a binary 1 or 0. The grayscale length of the execution is determined by the enable (OE) signal. The grayscale performance is a multiple of the enable, enabling data. The width determines the minimum gray level.

Third, LED screen circuit design

(1) Driving peripheral device selection

In the PCB design with about 3-6 pieces of CYT62726 distributed, a capacitor of about 1000uF is set. When selecting a filter capacitor, a low ESR (Equivalent Series Resistance) capacitor is used to minimize the output ripple. These materials maintain their capacity over a wide range of voltages and temperatures compared to other dielectrics.

A decoupling capacitor is connected between the power source and the ground. It has three functions: one is as the storage capacitor of the integrated circuit; the other is to filter out the high frequency noise generated by the device and cut off the propagation through the power supply loop. The third is to prevent the noise carried by the power supply from interfering with the circuit.

For the design of LED dot color products, it is very important to add filter capacitors inside the lamp points. The main reason is that the more the color changes, the more the power supply fluctuations will increase. The filter capacitors are more important here than any design. For most high current designs, a 470 to 1000uF capacitance is recommended. The design here cannot be without this capacitor.

See the figure below. Usually when we design the circuit, we will design the decoupling capacitor at the IC input: on the one hand, the storage capacitor of the integrated circuit, and on the other hand, bypassing the high-frequency noise of the device. The typical decoupling capacitor value in a digital circuit is 0.1uF. The typical value of the distributed inductance of this capacitor is 5uH. The 0.1uF decoupling capacitor has a distributed inductance of 5uH. Its parallel resonant frequency is about 7MHz. For 10MHz or less. Noise has a good decoupling effect. The selection of decoupling capacitors is not strict, and can be taken as C=1/F, that is, 0.1uF at 10MHz.

For most high circuit designs, a 0.01 to 0.1uF capacitor is sufficient at the input. The design here cannot be without this capacitor.

In the VCC power supply, it is recommended to connect a 10Ω resistor in series (the general design does not have this resistor). When the LED screen works, the content fluctuates greatly and will exceed 10V. It is recommended that VCC still need resistance to reduce the impact, mainly to reduce the peak caused by voltage fluctuations, especially LED display, Vp-p will be several times higher. The IC power input terminal is also the most vulnerable to impact. The presence of the resistor also increases the capacitance filtering effect. It is also considered to increase the stability of the voltage by adding a 4.7uF capacitor. The 16-bit constant current device VCC has a high withstand voltage, and the output constant current port withstand voltage is not the same. It is very important to add a resistor here. Later, we will introduce the combination of PCB design and separate power supply to avoid peak shock.

In designing the product, it is necessary to determine the output current value. The 23rd pin of CYT62726 is designed to facilitate the setting of current. The peripheral resistor selection is calculated according to the previous chapter formula. It can also be selected according to the following table. The reference design 910Ω is about 20mA current value. The PCB board-level design resistor should be close to the IC pin 23 and 1. Reducing the PCB-level resistance of these two pins will increase the reference constant current accuracy.

CYT62726 is a compatible 16-bit constant current device. In serial 16-bit data design, multi-chip cascading mode is adopted. CLK, LE, and OE are parallel transmission structures. In the data transmission, 74HC245 needs to be added to improve the driving capability. 3-6 CYT62726 sets a piece of 74HC245.SD data is serial transmission mode, according to the design design can be used 74HC245, or not through 74HC245, because the data serial transmission has sufficient driving ability.

(2) Static LED display design reference

The static driving method is designed to benefit the life of the LED. As the cost of the driving IC is continuously reduced, more and more static design methods are adopted. The static is for the scanning screen design. The CYT62726 output port is only connected to one or one. String LED, data transfer is driven by a single pixel, the IC uses the most number of designs. The static design can play LED performance, and the driving current value is the normal working value of the LED, which is beneficial to the optimal life of the LED.

In the static screen design, multiple CYT62726 cascading modes, CLK, LE, OE are parallel transmission structures. In the data transmission, 74HC245 needs to be added to improve the driving capability. Generally, 3-6 CYT62726 is required to set one piece of 74HC245.SD data. The serial transmission mode can be adopted as 74HC245 or not through the 74HC245 according to the design design, because the serial transmission of data has sufficient driving capability.

(3) 1/4 scan application reference design

The 1/4 scan design method is also one of the more design methods for LED screens. Mainly products that do not require too high LED brightness, such as indoor screens; screens that do not require too high grayscale levels, such as those used for digital graphic displays.

1/4 scan design data, clock, latch, and enable transfer are the same as static design. In order to increase the refresh rate, the amount of data transferred will increase several times.

The 1/4 scan needs to increase the B0-B3 scan strobe line. In each frame time, B0-B3 will be strobed in order, and B0-B3 occupy (1/4) S time in 1S clock per unit time.

Whenever B0-B3 is gated, the LED data that is gated is shifted to that pixel and latched and enabled (execution display).

The 16 ports of CYT62726 drive 4 LEDs in the B0-B3 strobe line, corresponding to 4 single color pixel points, IC drive current is the sum of 4 LED currents.

The 16 port currents of the CYT62726 are uniformly set. In order to maintain the white balance, R'G'B is designed with three CYT62726s. The selected communication number is sent by the controller to drive the PMOS to turn on and off the B0-B3 strobe line. The PMOS drive capability is related to the number of LEDs connected to the strobe line B0-B3, which is the sum of the LED currents on the entire strobe line. 4953, but pay attention to MOS drive capability.

In the 1/4 scan design, CYT62726 is a multi-chip cascade mode. CLK, LE, and OE are parallel transfer structures. In the data transfer, 74HC245 needs to be added to improve the drive capability. Generally, 3-6 CYT62726 is required to set one 74HC245. The SD data is a serial transmission method. It can be designed to pass the 74HC245 or not through the 74HC245 because the data serial transmission has sufficient driving capability.

In the 1/4 scan design, the general drive current is large. When a single red LED is used, a series resistor breakdown is required to share the heat of the chip. The best way is to connect the two red LEDs in series to increase the brightness and reduce the drive current, but it needs to be combined with the design panel display structure.

(4) 1/8 scanning single and double color screen reference design

The 1/8 scan design is mainly designed for single- and dual-color screens that do not require too high grayscale levels, such as digital display, station, dock, bank exchange rate card, and bar character screen. The 1/8 scan is 8 times less than the static display CYT62726, and the design cost is the main reason for adopting the scan design.

The 1/8 scan design data, clock, latch, and enable transfer modes are the same as the static design. In order to increase the refresh rate, the amount of transferred data is increased by several times. Displaying grayscale reduction can reduce the amount of data transfer, which can be one of the reasons for the scanning design.

1/8 scanning needs to increase the A0-A2 scanning strobe line, A0-A2 decoding signal is sent by the controller, 74HC138 decoding is added in 1/8 scanning, 74HV138 is 3/8 decoder, in each frame unit During the time, B0-B7 will be strobed once in order, and B0-B7 will occupy (1/8) S time in 1S clock per unit time.

Whenever B0-B7 is gated, the LED data that is gated is shifted to that pixel and latched and enabled (execution display).

The 16 ports of CYT62726 drive 8 LEDs in the B0-B7 strobe line, corresponding to 8 single color pixel points, and the IC drive current is the sum of 8 LED currents.

The 74HC138 selects the communication number to drive the PMOS to turn on and off the B0-B7 strobe line. The PMOS drive capability is related to the number of LEDs connected to the strobe line B0-B7. It is the sum of the LED currents on the entire strobe line, usually 4953, but pay attention to the actual Drive capability. The schematic diagram is omitted after the space is extended, and the B0-B7 gate line can also be extended to the subsequent stage.

In the 1/8 scan design, CYT62726 is a multi-chip cascade mode. CLK, LE, and OE are parallel transfer structures. In the data transfer, 74HC245 needs to be added to improve the drive capability. Generally, 3-6 CYT62726 is required to set one 74HC245. The schematic is limited to the back of the space, and the number of CYT62726 cascades can be extended to the latter. SD data is a serial transmission method, and has sufficient driving capability. The existing screen design is mostly after 74HC245, or it can not pass 74HC245, which can reduce the SD data delay time.

Because the total driving current of 1/8 scan is large, in the case of 5V power supply design, the red LED has a low forward voltage, and it is necessary to increase the resistance to share the power consumption of CYT62726. If two red LEDs are connected in series at the same time, there is no need to set a resistor. Blue and green do not need to increase the resistance. Please pay attention to the power consumption problem for other design methods, depending on the actual situation.

(6) Constant current output accuracy and calculation

16-channel constant current is one of the important parameters of the chip. It has several key indicators: constant current minimum differential voltage; constant current error between chips; VCC voltage regulation; load regulation; temperature drift.

1) The output constant current differential pressure is desirably as low as possible, usually between 0.6-1V, preferably less than 0.6V. It can be seen from the above test curve that as the output current increases, the differential pressure will also increase. It is critical in a design, such as a 2R screen design:

A lot of screens are designed by 2R'''''''''''''''''''''''''''''''''''''''' Normal lighting, assuming that the driver IC needs 1V differential voltage, the lowest established constant current voltage is 5V. In the case of 5V power supply, it is possible to reach the 5V minimum voltage value away from the power terminal. The screen has a color cast and the output current is not adjusted to the ideal value. The problem is here.

It can be seen that it is most reasonable to maintain the constant current differential voltage to 0.6V. In order to reduce the cost, some ICs greatly reduce the size, which is the main reason for the high differential pressure. The 16-channel constant current IC is a linear constant current mode. The formation of the differential pressure is one of the most important heat sources of the IC. The lower differential pressure is beneficial to the heat dissipation of the chip.

2) Inter-chip constant current error ± 3%; inter-chip error is one of the important parameters of constant current output. We usually see ±1.5% of the intra-channel channel, ±3% constant current error between the channels, and the actual on-chip error. Can not be considered, because we will not use a single chip, the LED screen design mainly considers the inter-chip error.

3) Influenced by constant current accuracy of VCC voltage variation ±0.07%/V; VCC voltage change will affect the accuracy of output current. In PCB design traces, LED power supply and IC power supply should be considered separately to improve the filtering effect and achieve good results. Effect.

4) Load regulation rate, the current output characteristic affected by the load terminal voltage is maintained at ±0.01%/V; the load voltage is different or fluctuates, which will affect the constant current accuracy, although it is small. The solution is to widen the PCB power supply wiring as much as possible.

Select the appropriate width and copper thickness according to the table below:

5) Temperature constant current drift 0.0005% / °C. The ambient temperature and chip heating also affect the output constant current accuracy. It is also very important to have a 2% error from 30 °C to 70 °C.

(7) Output current calculation

Usually, a table is given to mark the resistance setting curve with the current output. Inside the IC, a current mirror is proportional to the actual output current. The design varies greatly depending on the design. The design is mostly similar. The table only indicates the output current curve, and can not accurately determine the specific resistance value. It is best to use the following formula to calculate the output current value:

Vref is the output voltage of 23 pins, and Vref is maintained at about 1.2V. For example, the output current is set at 20mA and the resistance value is around 910, namely:

The current is about 20mA, and the formula calculates the error current to be within ±5%. Rigorous circuit design requires accurate measurement. The current after confirmation is kept ±1% of the resistance value error, and the current accuracy in the batch is maintained within ±3%.

Fourth, chip test

(1) Drive IC withstand voltage requirements

The screen driver chip withstand voltage is very important. Although the screen power supply is only 5V, the display content changes drastically, so the voltage peak-to-peak value (Vp-p) generated is much higher than we think. The switching power supply itself has a peak voltage, which has a great relationship with the selection of the power supply, but the price allowed by the customer limits our power supply selection level. Here, the switching power supply is one of the factors. To reduce ripple, the screen PCB layout is also very critical. This is a very difficult problem and requires a dedicated PCB engineer. The distribution of power to the screen power supply is also an important factor.

Affected by the above factors, the voltage peak-to-peak (Vp-p) calculation is complex. From the application point of view, the static screen is usually between 7-9V. The scanning screen is about 10-13V. The peak-to-peak value of the scanning screen voltage is closely related to the increase of the scanning screen switching speed. The transmission rate is increased, the LED switching speed is increased, and the voltage fluctuation rate is also increased, and the voltage peak-to-peak value is also increased. .

In the beginning, I used the 74HC595 design screen. Later, I found that the 74HC595 with a voltage of only 9V (5V low-voltage process) was damaged. The analysis is considered to be caused by the voltage peak-to-peak value. The output port is continuously improved during the development of the screen. Withstand pressure, this problem has been completely solved. In these years, some companies have designed 16-channel ICs with a 9V design. There are no problems with some screens (mostly static screens), but some companies have high damage rates on LED screens (mostly scan screens or PCBs). Design differences), the problem stems from this.

In recent years, some companies have designed grid-mounted screens. This long-distance thin-line power supply method, voltage peak-to-peak (Vp-p) is particularly prominent, and some peak voltages are as high as 15V or more, resulting in a variety of screen driver chips in the grid. On the screen, the undervoltage condition is revealed. The 16-channel constant current driver also uses LED decoration fields outside the screen, such as guardrail tubes. The design supply voltage is above 12V, and the peak-to-peak voltage (Vp-p) of these design voltages is also as high as 15V or even higher. Therefore, driving the output voltage of the chip is a very important parameter.

(2) Pressure resistance test

Since it is an important parameter, it gives a simple and feasible test method. Randomly extract CYT62726 a total of 3pcs chip, test one of the ports of the chip, the chip gives normal power supply, is off state. The CYT62726 specification standard is a withstand voltage of 17V, and 17V is a pressure that can be guaranteed to the customer.

According to certain steps, the test is continuously improved from 5V to 17V, and the leakage is within 6nA. The LEDs connected in series are intuitively judged, and the LED does not see brightness. Continue to increase the voltage to 24V, the IC can still withstand, as the voltage increases, the leakage of the port also increases. When the voltage exceeds 0.5uA, the LED has the opportunity to see the brightness, and the IC above 25V will be damaged.

Result: The voltage of 17V loaded through the test port was within 10nA, and the result was satisfactory. It can be observed that the LED brightness lighting current is 1uA, and the current less than 0.5uA can be regarded as no leakage. The red data indicates that the chip has reached the limit and the work is no longer working.

(3) IC reliability test

The reliability test of the driver chip is complex and important. It not only analyzes the chip design, but also evaluates the reliability of the package carrier. For the screen driver chip, it belongs to the power device, which generates a large amount of heat and reliability test. It is vital.

The following is a general test result, mainly to evaluate the reliability of the package.

Five, LED screen control

(1) System

The development of domestic control systems is relatively mature, mainly in two ways. One is based on the Ethernet network mode, the Gigabit NIC distributes the data, and the controller distributes the data to the LED display unit board, which should be the mainstream application method in the future. The second is to read the video data from the graphics card DVI, and then send it out through the data card. It is also a network mode. The difference from the network is that it is a serial data format. The data segmentation of the terminal unit board is basically the same.

It is most common to read DVI data. It uses signal packet multiplexing technology to synchronously transmit display data and control data. The high-efficiency gray-scale segmentation algorithm can simultaneously read frames and line-frequency synchronization. It mainly consists of two parts: a sample data sending card and a field controller. Through large-scale logic and other components, real-time synchronous acquisition of the display data output by the computer, through the cache, format conversion, transmission from the large-capacity transmission channel to the LED display site, and finally converted into LED scanning control signal, on the LED display Realize the display of high-definition video, pictures, text and other program content.

Collect high-definition display data from the DVI interface of the computer and output differential signals. The display signal of the high-speed output of the DVI interface is serial grayscale data, 24-bit color data, and the weight data of each color is 8 bits, and the gray level is 256 levels. The gray scale implementation on the LED display is realized by controlling the lighting time PWM of each LED. In order to realize different gray levels more efficiently, each weight of the screen is displayed independently, that is, the entire screen is displayed separately. The brightness of 1~8 weights. The entire data format conversion process is completed by a series of processes such as weight separation-cache-partition extraction-data reforming, and finally the scan data of the LED display screen is obtained.

The LED screen grayscale luminaire is calculated according to 256-level gray scale (8-bit), and the 8-bit weight data is D7 (128 weight), D6 (64 weight) &h el lip;...DO (1 weight) from high to low. ). Set the serial clock of the appropriate output display. Improve the parallel output of the RGB data signal group, you can increase the display area and meet the actual high-definition display. For different program sources and different display bodies, different values ​​of gamma correction are needed to obtain a more eye-visual display (visual sensitivity function) for a clearer image.

The synchronous video signal sent by the DVI interface has a large amount of data, and the data sending card compresses the data and sends it through the mature chip of the network. Considering the actual distance between the controller and the LED screen, it is transmitted by cable differential or optical fiber long distance. The LED display screen is composed of a plurality of display modules, and the display interface generally consists of the following signals: serial data; multiple sets of red, green, and blue signals; parallel clock; parallel latching; and exercise energy; (scanning signal), etc., usually up to 16 lines of scanning.

LED display screen for large-area display, the screen area is generally large, and the control data of the display screen is generally serial transmission, the control line is very long and easy to receive interference, and the signal can be stably transmitted under a large area. The frequency is limited. If you increase the control area of ​​the system, the general methods are: 1) increase the clock frequency of the display control signal, but this increase is limited; 2) reduce the refresh frequency, the refresh frequency reduction will affect the display stability, the effect is very poor; 3) Multiple controllers are processed at the same time, and increasing the scan controller inevitably increases the cost.

The control system design is complex and will not be elaborated here. For LED screen manufacturers, it is mostly hardware connection. The control system is provided by professional companies such as Jesse, Lingxing, Depuda and other famous LED control systems companies.

(ii) System components

System components are here for reference and not all of them. Usually meet customer needs, customize project content, and more. For the type of screen, the line-of-sight pixel density has a great relationship, which affects the cost, the material selection is the main factor of the price cost, and the warranty period.

(3) Duration of construction

For reference only: According to the specific engineering quantity of the display, it is estimated that the entire construction period of the outdoor display project will take about 45 days, and the time includes the procurement processing time of about 32 days and overlap.

Survey at the installation site (3 days): The construction technicians arrive at the project installation site to conduct on-site site surveys. Determine the installation position and environment of the display body; determine the location and environment of the equipment room where the control panel is installed; measure the actual transmission distance of control signals and data signals; design the wiring position of the transmission signal lines.

System wiring (7 days): The construction personnel shall lay out the communication cables according to the design plan after the technicians surveyed: clean the layout route; (including necessary excavation, drilling, etc.) to embed the line pipe, threading; install the wire connector.

Production of screen frame (15 days): The construction personnel shall carry out the construction of the frame according to the design drawings provided by the structural designer, the manufacture of the steel frame; the installation of the steel frame; the construction of the external decorative material of the screen; and the lead wire of the internal power supply system of the screen.

Screen installation (5 days): After installation of the frame, installation of the screen body, installation and fixing of the box body; connection of the box body; connection of signal lines; installation of the distribution box; connection of the power line; control network Erection.

System debugging (5 days): power supply system detection; display system detection and debugging; network system debugging; multimedia system debugging.

Sixth, the future of LED screen

This article focuses on the current status of LED screen design, for the "LED screen design and development trends (on)" section, the development trend design will be highlighted in (below). In order to cater to the new technology, the (below) article will be published a few months later. Here we will explain the main directions of the LED screen in the future.

(1) Control system development

The control system is an important part of the LED screen and the core of the technology. Over the years, China has occupied a dominant position, and many excellent control system enterprises have emerged, laying a solid foundation for China's LED industry. The control system occupies a dominant position in the international market and is also related to China's ideology. For example, the domestic industry has not used software as a cost, profit calculation, sales only calculate controller hardware costs, low prices, won the market, international companies do not Willing to work so hard, hindering investment interest and market access.

There are also problems, and profits directly affect the enthusiasm for investment. For example, innovation is not enough, and the expansion of new technologies is slow. Also in the development of control systems, innovation did not work well with IC design companies, and it was also the main reason for slow technology updates.

LED screen companies try new product design, not very active, want to design a mature step in place, but are not willing to mature? Specially prepared for large companies to actively try new technologies, and innovation is the main driving force for the LED industry in the future. Now screen production seems to be similar to everyone, there is no innovation, and profits are falling.

In terms of control system development, software will replace hardware, improve the level of intelligence, and reduce hardware design costs. The rapid development of computers should have greatly improved the level of LED screen control, but our stubborn consumers have not changed him for many years. This is also related to the unwillingness of controller designers to reduce the price of control systems, and the speed of software advancement is slow. System software is a big trend. Who can improve the speed of system software, who can occupy the core area of ​​LED screen control. But occupying the core of the screen control does not mean that it will bring you core benefits. In the future, there must be a system provider to change the pattern completely.

The system provider means that the IC design company provides the control system directly to the market, and the control system cannot be used as a profit point. For example, our LED screen IC leader accumulates companies that have introduced screen driver ICs every year, but can always sell them or old products. Is it that this multi-chip design is not good? The actual decision-making layer should mean that this is the lack of control technology. Only IC design innovation has no control technology combination and cannot be successfully adopted by customers. Although control technology and IC design cooperation is a way, it is obvious that control system designers are not willing to go more hardware. The idea of ​​IC design creativity and control technology is not under the command of a team, and the development of LED screens is greatly limited.

Qualcomm has published a very detailed case of mobile phone design on the website, complaining that mainland companies cannot be as flexible as European and American companies.高通可曾想山寨厂家根本就没有安排能读懂案例的人员。假设MTK只提供芯片，不提供软件，山寨手机市场会有吗？我说的不一定对，大家可理解LED控制系统未来的走向。

㈡ 驱动芯片发展

前面讲过目前普遍采用的16通道驱动芯片，是在74HC595基础上发展而来，几十年来都没有创新，这是不正常的。落伍的传输方式，阻碍LED屏幕创新，屏幕市场中国有绝对的话语权，必将诞生新的数据传输方式，革命性的改变现有数据传输格局。

随着世界经济发展，需求量提升，要求也会越来越高，特别是清晰度要大幅提升。大尺寸显示壮观的视屏播放，已经失去了它的魅力，什么时候在广场上也有电视机那样清晰？LED有理由设计的比LCD（液晶）更清晰，可以看看有LED衍生品OLED .

LED与传统的显示最大的区别是没有余辉。CRT（显像管）电子枪扫描， 荧光粉有余辉。LCD液晶屏内部形成电容，能保持一定的显示时间。R'G'B颜色的LED是完全没有的，因此驱动IC内置PWM是必须的。

集成电路设计要与LED屏幕市场共存，意识形态也很重要，有好的技术，还要了解大陆公司思想，未来IC设计一定是大陆的企业取胜。供货渠道、交货时间、账期、支持力度，外企还有不少的差距。从历史发展来看，东芝公司本来市占率很高，但是渠道和系统配合上面，自我为中心，导致市场份额不断减小。点晶公司一度也有很高的占有率，当初引脚兼容问题，和移师大陆决策没有把握好，失去了市场竞争力。

㈢ 屏幕制造发展

屏幕制造在我国已经有了完整的产业链，分工明确。但是整合度还是不够，质量保证不是很完善，人工制造环节太多。未来还需要大幅度整合，特别是高质量的单元板企业，在高质量环境下批量单元模组，规模化生产，然后再交给粗放式企业组装，整体降低设计成本，提升LED屏幕质量。

损坏率最高是LED,LED损坏主要是封装、组装过程环境得不到保证，未来控制芯片与LED芯片整合是方向。这不是一件小事，可能很难理解！

Reference materials:

CYT62725 16通道80（mA）LED屏幕恒流驱动器；主要针对1/8扫描，更多的驱动LED数量，单双色屏幕、护栏管等最佳的选择。

CYT62726 16通道45（mA）LED屏幕恒流驱动器；静态屏和1/4扫描兼容性型号。

CYT62727 16通道45（mA）LED屏幕恒流驱动器；主要针对静态LED屏幕应用，提高恒流精度。

CYT62728 16通道25（mA）LED屏幕恒流驱动器；针对高刷新率，超大屏幕静态屏幕应用规格，OE宽度150nS宽度，有快速响应灰度等级的能力。

CYT62729 8通道60（mA）LED屏幕、 灯饰恒流驱动器；针对高刷新率，兼容性8通道恒流，屏幕灯饰应用规格，OE宽度150nS宽度，有快速响应灰度等级的能力。