How to design LED signs and LED matrix display

LED-based signage and matrix displays bring more functionality and stunning visual effects to growing indoor and outdoor applications. The latest developments in LED technology make it difficult to tell whether you are seeing a still picture of a high-quality display or a traditional printing or drawing billboard. This tutorial will detail the basic technical principles of LED display systems and the design issues they need to consider when designing discrete LED bulb arrays.

LED driver base

First, we have to compare different LED driver circuits to determine the best solution.

Connecting voltage source

It is well known that an LED lamp (or diode) begins to conduct when it has a sufficient forward voltage (vf). The forward current usually glows when turned on. Based on this basic knowledge, the first option in Figure 1a can be derived, but this does not work. Because the LED current is an exponential function of its voltage bias (Equation 1), the light intensity of the LED is very sensitive to this voltage. In most cases, high current conditions typically turn an otherwise long-life LED into an expensive flash bulb.

The following is the reason why Figure 1a does not work. In Equation 1, is and rs are constants, depending on the LED product itself, regardless of whether vt is a thermal voltage. Assuming that the series resistance rs is an ideal value of zero, then a vf change of only 0.1v would result in a 47-fold difference in iLED.

For example, a target LED current value of 20ma will jump to 1a when its bias current exhibits a difference of only 0.1v. Even considering the actual rs value, real LED devices still have 10 to 20 times the difference when they have a 0.1v offset difference.

Figure 1. Comparing three LED driver circuits

Voltage source supporting current limit resistor

Now let's take a look at Figure 1b. Add a current limiting resistor rlimit to protect the LEDs. Due to the finite current resistor, the lamp will not be burned out. In the field of video display applications, this method is still not good enough to control the intensity of LED light. The LED curve and the load curve generated by rlimit determine its LED current value. As indicated by the red or blue mark, the LED and the resistor respectively have a forward voltage change and a resistance change due to manufacturing errors. These error factors can cause a non-negligible change in the LED current (green).

A constant current source

Figure 1c uses a constant current circuit instead of a resistor. The constant current driver circuit directly adjusts the LED current to a target value. Regardless of how many vf changes the LED lamp produces during the manufacturing process, the LED conducts a specific current value. The light intensity of the LED lamp is closely related to the charge through the pn junction, so the constant current driver is the ideal way to obtain a uniform light output from the LED lamp.

In addition, we all know that integrated circuits (ic) can provide good matching circuit pairs. This is another advantage of choosing the constant current method. Figure 2 shows the basic output stage structure of an LED driver. Many LED drivers ic on the market have a reference current setting terminal iref, which is a constant current mirrored to its output.

Figure 2. Basic Output Configuration for LED Driver ic

Figure 2 is the result of this discussion, the basic output circuit configuration of the LED driver.

Color drive

So far, we have been able to determine how to drive a single LED light. The next step is to achieve full color light output for the video display system. Any color can be generated by combining different shades of light, red, green and blue (rgb). A more familiar example is the use of a color selection tool on a personal computer (PC).

Digital or analog grayscale control

The pc operating system mixes three colors into 256 levels (8 bits per order) or more to display full color pixels. For LED display systems, the same concept of color-scale color intensity control is also required to achieve tone-level control or gray-scale control in the LED driver design.

First decide whether to use digital control or analog control. As mentioned earlier, the total number of charges passing through the pn junction determines the light intensity, so both digital and analog methods can control the light intensity. Figure 3 is a 50% gray scale control in digital and analog methods. In the overall 256 gradation example, the 50% indicates a target with 128 gray levels.

Figure 3. Digital and analog 50% intensity control

LED current and color change

At this time, it is necessary to consider the influence of the current change on the wavelength value of the LED light output. Changing the wavelength means changing the color seen by the human eye. Figure 4 is an example of a green LED lamp. Usually in the industry, 510nm is widely representative of green. Therefore, most LED lamp manufacturers design LED lamp products with a wavelength of 510nm at the maximum rated current. In Figure 4, as the LED current increases, the wavelength can reach 510 nm. The best way to get green is to try to get the lamp's drive current close to the maximum rating. This also explains why using digital control is better than using analog control.

Another advantage of choosing digital control is the ease of controlling the LED driver ic in the form of a digital circuit module. For 256-step grayscale control, the cost of digital control is lower than analog control.

Figure 4. Green LED current and wavelength example

This on/off digital control is called pulse width modulation (pwm) control, or pwm dimming. Now add the pwm control switch to Figure 2.

How to form a matrix or 2d image

The rgb LED lights can be tiled to form a 2-dimensional (2d) image.

Display system structure

Rgb LED lights can be used to form a square infrastructure or module. It usually consists of a pcb and a 16x16 to 64x64 pixel array, which varies from application to application. Multiple modules can be combined to form a mechanical system structure or panel. LED display system manufacturers typically offer a variety of panels. Each panel has a mechanical frame for multiple modules. It contains one or more control units to provide power distribution, data interfaces, and processors. In the construction of a display system such as a stadium large screen or a roadside billboard, a plurality of panels can be installed to form a final display. At the construction site, all data lines and power lines for each panel are concentrated in the central control unit.

Figure 5. LED display system consists of module/panel/display

Pixel pitch

A set of LED display systems consists of a large number of LED lights and a large power supply. When designing the system, it is necessary to focus on the density optimization of the LED lamp. This density of LED lights is called the distance or pixel pitch of each pixel. If the pixel pitch is too dense, it will not improve the image output quality and increase the cost once it exceeds the accuracy recognized by the human eye. The two single light sources recognizable by the human eye are when 1/60 (=1 arc minutes) of one arc is formed at these two points.

Figure 6. Human-readable resolution

Fig. 6 is how to calculate the human eye distinguishable pixel pitch dpp1. As shown in Equation 3, where l is the line of sight.

In best practice, dpp1 can be considered too large, and three times dpp1 is good enough for high quality video systems. In Equation 4, dpp is the guiding standard.

The simple memory method of Equation 4 is:

Required pixel pitch (mm: mm) = "line of sight" (meter: m)

For example, a 5m line of sight system requires 5mm pixel pitch to achieve good resolution. Another visual example is shown in Figure 7, which shows how the low pixel pitch reduces the output image quality. Images with a pixel pitch of 12.5mm (top) look rough and cannot be recognized at close range. However, the image begins to become clear when viewed at a certain distance, similar to viewing an image with a pixel pitch of 5 mm (bottom). This example clearly illustrates the relationship between line of sight and pixel pitch.

Figure 7. Comparison of different pixel spacings and line of sight

Static drive and time division multiplex drive

It can be seen from Fig. 2 that the cathode of the LED lamp is driven by the LED driver ic which is common in the current market. Here we will discuss the anode driver circuit of the LED lamp. The use of a constant current driver for the cathode is advantageous, and the anode is desirably provided with only sufficient voltage. But still need to make important decisions on how to drive the anode!

Figure 8 compares the static anode driver system to the time division multiplexed anode driver system. The static anode driver configuration is very clear: one LED driver ic drives one LED. Static anode drivers require a large number of LED drivers ic when designing systems with a large number of pixel points. In contrast, time division multiplexed anode driver systems allow multiple LED lamps to share an ic, thus using fewer LED drivers. The trade-off of time-division multiplexed drives is that the output LED light intensity is reduced by time sharing.

In outdoor display systems, a strong LED output is needed to overcome the brightness of the sun so that the human eye can see the image clearly. In this outdoor system, a static anode driver is more suitable. On the other hand, in indoor systems, time-division multiplexed anode drivers are a good way to reduce system construction costs.

Time-division multiplexing has become the most commonly used technology for current applications, so we will use it in the applications discussed in the rest of this article.

Figure 8. Static anode driver and time division multiplexed anode driver

How to create a movie/video image

Before we discussed how to display still images. If we change the still image constantly, we can turn it into a movie or video.

Frame rate/frame refresh rate

Old-fashioned analog TVs typically display 24 different still images in one second at a frame rate of 24.

When an analog TV camera captures another analog TV screen, a zebra pattern blend of video images and black strips can be produced (Figure 9). This phenomenon is caused by the synchronization of the TV camera and the TV screen. The same problem occurs when the camera that shoots the LED screen uses a time division multiplexed anode driver. Examples of applications include shooting a background image on a background wall with an LED display to magnify an actor's stage image or using a television camera to capture a sporting event scoreboard or signage in a stadium. To avoid this problem, LED displays now need to run faster than camera systems, especially in the dedicated LED display market.

Figure 9. The black strip caused by the TV camera taking another TV screen

To meet the requirements of faster operation, many LED display systems repeatedly display the same image in one frame period, called the frame refresh rate. Figure 10 is a graph of frame rate versus refresh rate. There are only two frame images: a and b. Repeat "Image x" twice for each frame. Thus the example "frame refresh rate" = 2 x "frame rate".

Figure 10. Frame rate and frame refresh rate

In a typical LED display system, the frame rate is in the range of 50hz to 120hz, and the frame refresh rate is between 50hz and 2khz.

On/off control drive or pwm control drive

In order to meet the system frame rate and refresh rate requirements, it is necessary to choose between two methods of implementing logic circuits. The first is the on/off control driver and the second is the pwm control driver.

Figure 11a is a system that uses on/off control ic with an on/off register for each bit corresponding to an output. The logic high of the register bit turns on the corresponding output, while the logic low turns it off.

Figure 11b is a system that uses pwm to control ic with a grayscale reference clock input that can reference the clock counter. In addition, the ic also has a set of registers that hold grayscale logic code. The pwm comparator compares and generates the pwm output mode through the counter and grayscale (gs) registers.

For both types of drives ic, both jobs are performed side by side:

- the constant current driver module drives its LED lamp array according to the input of the current display period data;

- Receive data from the next display cycle in the shift register.

Figure 11. LED display system with on/off control ic and pwm control ic

to sum up

Firstly, the driver circuit of a single LED lamp is introduced, and then the detailed physical characteristics of the LED lamp, the physical layout and structure of the display system, and the static and timely division multiplexing control are discussed, and then the complete LED driver ic structure is obtained.

Intelligent chargers step forward to achieve intelligent, flexible and energy-efficient charging performance.



Intelligent chargers combine all the existing technology benefits and new features that assure better communications with the battery and more flexibility of use. The innovative modular design provides highly efficient charging, reliability and ease of service.

AGV Intelligent Charger

Intelligent Battery Charger,Intelligent Lipo Battery Charger,Guided Vehicle Intelligent Battery Charger,Automation Intelligent Rapid Charger

Xinxiang Taihang Jiaxin Electric Tech Co., Ltd , https://www.chargers.be