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Designing LED signage and matrix display (Part 1)

Posted: 15 Sep 2014     Print Version  Bookmark and Share

Keywords:LED  signage  matrix displays  diode  resistor 

In outdoor display systems, very strong LED output is required to overcome the brightness of the sun in order to deliver the image to the human eye. In such outdoor systems, the static anode drive is preferable. On the other hand, in indoor systems, the time-multiplexing anode drive is a good method to reduce system building cost.

Since time-multiplexing has become the most commonly-used technique in today's applications, we'll use it for the applications we discuss in the remainder of this document.

How to create movie / video images
Earlier we discussed how to display a still image. If we keep changing that still image, we can turn it into a movie or video.

Frame rate / frame refresh rate
Old analogue TV systems used to show 24 different still images in one second, for a frame rate of 24. When an analogue TV camera views another analogue TV screen, it creates a zebra mix comprising video images and black bands (figure 9). This is caused by the synchronised TV camera and TV screen scanning rate. The same problem occurs when a camera taking a shot of an LED screen uses the time-multiplexing anode drive. Examples include a TV camera capturing an image of a concert stage with an LED display enlarging a performer on the back wall, or a TV camera viewing a stadium score/display panel at a sport event. To avoid this issue, LED displays today need to operate faster than camera systems, especially in a professional use LED display market.

Figure 8: Static and time-multiplexing anode drive.

Figure 9: TV camera viewing another TV screen causing black bands.

To meet this faster operation requirement, many LED display systems repeatedly show the same image within one frame period, known as the frame refresh rate. Figure 10 shows the relationship of the frame rate and refresh rate. There are only two frame images: A and B. Each frame repeats image x twice. Thus, this example is "Frame Refresh Rate" = 2 "Frame Rate".

Figure 10: Frame rate and frame refresh rate.

In a common LED display system, a frame rate is in the range of 50Hz to 120Hz, and a frame refresh rate is in the range of 50Hz to 2kHz.

Figure 11: LED display with ON/OFF control IC and with PWM control IC.

ON/OFF control driver or PWM control driver
To meet system requirements of frame rate and refresh rates, a decision needs to be made between two ways to implement the logic circuit. First is the ON/OFF control driver, and the second is the PWM control driver.

Figure 11a shows a system with an ON/OFF control IC, which has an ON/OFF register that corresponds with each bit to its output. A logic high of the register bit turns ON the corresponding output; a logic low turns it OFF.

Figure 11b shows a system with a PWM control IC, which has a grey scale reference clock input terminal that references the clock counter. Plus the IC has a set of registers that hold grey scale logic code. PWM comparators compare and generate PWM output patterns from the counter and grey scale (GS) register.

For both types of driver ICs, two operations are performed in parallel:
 • The constant current driver block drives its LED lamp array based on inputs from the current display cycle data.
 • Meanwhile, the data for the next display cycle is received into the shift register.

Summary
Beginning with a driver circuit for a single LED lamp, a complete LED driver IC structure is derived by reviewing details of the LED lamp physical characteristics; physical layout and structure of display system; and static and time-multiplexing control.

About the author
Masashi Nogawa is Product Marketing Engineer for DC Solutions at Texas Instruments.

To download the PDF version of this article, click here.


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