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LED Signage and LED Matrix Displays

Part 1 of this three-part series introduced the technical and engineering steps necessary to design an LED display system from individual LED lamps. Part 2  provided the remaining steps needed to implement a basic LED display. In the third and concluding installment of this tutorial, we'll explore some of the issues which affect the image quality and reliability of LED displays. We'll also become acquainted with the technologies and design techniques commonly used to deal with them.

Anti-ghosting / ghost-canceling / pre-charge FETs

Ghosting, spike noise, or phantom noise are unwanted lighting effects caused by Anode gate "float" which can occur in time-multiplexed LED driver. Since LED lamps (PN junction of diodes) have relatively high levels of capacitance, their residual charge can keep triggering capacitive charge transfers between the floating nodes. And every time there's forward electron flow through a PN junction 

The situation where this phenomenon is most is a diagonal line image. Figure 1b shows an example of so-called "ghosting" caused by anode float. Modern LED driver ICs, such as theTLC59283, employ so-called "pre-charge FET" circuits which eliminate these ghosting effects (Figure 1a). As explained earlier, the root cause of ghosting is stray charges on the LED's anode which forward-bias its PN junction and cause it to light at unwanted times. These pre-charge FETs are designed to insure the LED lamps remain reverse-biased and unlit except when the driver circuit is actually on.

How-to Design LED Signage and LED Matrix Displays (Part 3)

Figure 1. Pre-charge FETs (right) and the ghosting (left).

Blank bands, black bands and Enhanced spectrum PWM 

LED display designers face several other challenges as they strive to produce ever-larger products which deliver the pest-possible image quality. One of the biggest issues is eliminating the blank bands which can occur when capturing the image of an LED display on a camera. As we discussed in Part 1 of this series, this is caused by "slow-synching" between the display and the camera. This can be avoided by using a faster frame refresh rate (FRR). Unfortunately, larger displays require faster FRRs. As a result, it becomes increasingly difficult to achieve an FRR that's sufficiently high to avoid slow-synching effects as display size increases.

Another issue is black bands which appear when a camera captures a display image image at the moment some of its LEDs are OFF. This can be avoided by keeping the LED lamps ON during a camera scan period but, as the following example will show, that's not always possible. 

Black bands become a more significant problem as PWM control LED ICs grow to control larger, higher-quality displays where the length of their PWM operation cycle time grows longer. For example, the latest 16-bit PWM control with a 25 MHz reference clock requires 2.6 ms = 216 bit / 25 MHz, which is a frame refresh rate of 381 Hz. Here, a gray scale code of 128 for a total of 216 clock cycles generates 5.1 us (= 128 / 25 MHz) of ON time, and 2.6 ms minus 5.1 us of OFF time. The camera captures LED lamps in the OFF state during this 2.6 ms period.

Black-banding can be mitigated using a technique called enhanced-spectrum PWM (ES-PWM), a method for PWM generation which divides one long PWM cycle into shorter sub-PWM cycles. In the above example, if 128 clocks of the ON period are divided into 16 periods of 8 clocks each, creating an effective FRR of 6 kHz (= 381 Hz x 16). At 6 kHz, the refresh rate is high enough to avoid black bands with most cameras.

An original PWM code cannot always be equally divided. In this situation, the ES PWM function splits one ON period into rounded integers. For instance, to divide a gray scale code of 100 into 16 pieces, the ES-PWM circuit generates twelve of 6 clocks and four of 7 clocks to maintain a total gray scale of 100 (= 6 clock x 12 + 7 clock x 4).

Detecting LED open, LED short, & output leakage conditions

Many LED display systems are controlled remotely, making it difficult for an operator to detect any failures. Because the human eye is sensitive to a faulty lamp that remains constantly ON or OFF, the failure of even a few lamps can degrade the quality of a viewer's video experience. As a result, many displays implement ways to detect open and shorted LEDs, as well as output leakage conditions which can cause LEDs to malfunction.

An LED open detector (LOD) function monitors LED lamps for open-circuit failures. Under normal circumstances, a driver IC's constant-current output terminal stays at the head room voltage required by the constant-current circuit. When the constant-current circuit's LED fails and becomes an open circuit, the constant-current circuit drives its output terminal to almost zero voltage. The LOD function detects these telltale voltage changes and generates an error signal.

Similarly, an LED short detection (LSD) monitors the LED lamp for conditions which indicate the LED, and/or its driver are short-circuited to its anode's supply voltage. When the LED fails in a shorted mode, its output terminal reverts from its normal bias state to the full voltage applied to the anode. The LSD function distinguishes this voltage difference and generates an alarm signal.

An output leakage detection (OLD) differs slightly from the first two safety functions. It's designed to detect conditions which arise when an LED is forced into its ON state due to debris forming a conduction path from an output terminal to the ground. When this occurs, the LED is turned ON – no matter what the output of its constant current-circuit driver happens to be. The OLD element produces a small amount of current at its output terminal node which it uses detect any leakage path by monitoring the terminal voltage. 

Low gray scale enhancement

The human eye has more sensitivity to darker light sources than brighter lights. In other words, it recognizes which of two dark light sources emits more photons. However, when the human eye is saturated with bright light from two different sources, it cannot distinguish the difference.

For handling video image, low gray scale data requires more attention. Here a technique like gamma correction is widely used. As for LED display systems, software programming can implement a gamma correction function with both ON/OFF and PWM control drivers.

Recent LED drivers, like the TLC5958, integrates more proactive improvements on low gray scale handling. A common problem is that red LED lamps are stronger than green and blue with dark white image output, even though red, green and blue all have the same low gray scale data. This occurs because red LED lamps can turn ON longer than green and blue lamps due to its lower forward voltage. A low gray scale enhancement (LGSE) function can correct this difference inside the IC. Figure 2a has no correction while 2b has been corrected.

How-to Design LED Signage and LED Matrix Displays (Part 3)_1

Fig.2. Two examples of low gray scale enhancement with both showing dark white image data.

Regarding this low gray scale concern, LED current PWM pulses need very sharp turn-ON and turn-OFF times, or rise and fall times, TR and TF. If TR and TF are slow, low gray scale problems can get worse.

"First Line" issues and integrated SRAM

As mentioned earlier, ES-PWM control speeds up FFR. By using ES-PWM with the time-multiplexing anode control, the first line of time-multiplexing gets darker. Figure 6a has two lines that appear to be more reddish than the others (very top and middle). All other lines look to be more white. This first line issue is caused when the green and blue lamps are not fully turned ON.

A solution to the root cause of the first line issue can be found by integrating static RAM (SRAM) bits to store gray scale PWM codes for the entire frame, thus avoiding data transfer time lag. For example, the TLC5958 integrates 48 k bits of SRAM on-chip for up to 32 times of multiplexing.

Design tips for display systems and driver ICs

Inrush current control

In general, an LED display system handles huge amounts of current. For example, eight pieces of 48-output LED driver ICs controls 25 mA each. The total current is 9.6A. The biggest problem with an LED display system is that this 9.6A of current keeps turning ON and OFF at very high frequency with fast TR and TF.

Many LED driver ICs come with noise reduction features such as delay between each output. Because a system handles 10 MHz order of digital signal on its PCB, noise management is an important design factor early into the project.

Thermal error flag / pre-thermal warning 

As stated, an LED display system handles huge amounts of current – which translates into huge amounts of heat. This excessive heat can cause thermal shutdown and unexpectedly stop LEDs from working. It is a major issue when the entire display stops working, but viewers might think that the system is simply turned OFF. However, in most cases, only a partial module stops working and viewers can see that something is wrong (Figure 3). Because of this, many LED driver ICs do not come with a thermal shutdown function. Instead, they come with a thermal error flag (TEF) or pre-thermal warning flag (PWF) function.

How-to Design LED Signage and LED Matrix Displays (Part 3)_2

Figure 3. LED display with some modules inoperative.

These flags are generated by a circuit similar to thermal shutdown detectors. Instead of stopping an IC when temperatures get hot, hot temperature condition flags are sent to an image processing controller. Upon receipt of a flag, the controller cools down the system by reducing screen brightness, showing darker images, or simply stops the system for a moment.

48-output driver

PCB layouts can be nightmarish on a typical LED display module design. We compare system concept sketches utilizing one 48-output driver (Figure 4a) and three 16-output drivers (Figure 4b). Both diagrams are a 16 x 16 RGB matrix, which equals 768 LED lamps. It is clear that a 48-output driver like the TLC5958 can simplify your PCB design.

(a) How-to Design LED Signage and LED Matrix Displays (Part 3)_3

 

(b) How-to Design LED Signage and LED Matrix Displays (Part 3)_4

Figure 4. PCB layout comparison between 48- and 16-output drivers.

Conclusion

With the numerical example specification, key points in IC data transfer calculations are reviewed as a final step of the LED display system building, which is a continuation of the discussion started in Part 1.

Source: http://www.led-search.com/news-view-3421.html
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How-to Design LED Signage and LED Matrix Displays (Part 3)
Topics: Lighting