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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 

LED-based signage and matrix displays are paving the way for new dimensions of versatility and eye-pleasing visual effects to a growing number of outdoor and indoor applications. Recent advances in LED technology have even made it difficult to distinguish still images on their high-quality displays from traditional printed or painted billboards.

LED driving basics
First we will compare the various LED driving circuitries to determine the best method.

Connecting a voltage source
It is well known that an LED lamp (or diode) starts turning ON with enough forward voltage (VF). When ON its forward current emits light. From this basic knowledge, one can come up with the first option in figure 1a but it will not work. Because an LED current is an exponential function of its voltage bias (equation 1), light intensity from the LED lamp is very sensitive to the voltage. In most cases the high current condition turns the normally long-lived LED into a very expensive flash bulb.

Equation 1

Here's why figure 1a will not work. In equation 1, IS, RS is a constant, depending on the LED product, and whether VT is the thermal voltage. Assuming a series resistance RS is ideal and zero, only 0.1V of VF change makes 47 times difference in ILED.

Equation 1

For example, a target LED current value 20 mA jumps up to 1A with only 0.1V difference of its bias current. Even taking into account a realistic RS value, a real LED device still shows 10 to 20 times difference with a 0.1V bias difference.

Figure 1: Comparing three LED driver circuits.

Voltage source with current limit resistor
Now lets examine figure 1b. A current limit resistor RLIMIT is added to protect an LED lamp. Because of the limit resistor, the lamp does not blow up. Still, it is not great at controlling LED light intensity in video display applications. An LED curve and a load curve by RLIMIT determine its LED current value. As shown in red or blue annotations, this LED and resistor has variations of forward voltage and resistance from manufacturing errors. These error factors change the LED current (green) at non-negligible levels.

Constant current source
Figure 1c employs a constant current circuitry instead of resistors. This constant current driver circuit regulates an LED current directly at the target value. The LED conducts a certain value, no matter how much VF variation the LED lamp has from its manufacturing process. Because the light intensity of an LED lamp is strongly tied to charges crossing its PN junction, this constant current driver method is ideal to get uniform light output from LED lamps.

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