Toshiba Constant Current LED Drivers

LED variable message signs have become an integral part of our daily routine. From time and temperature displays to convenience store specials, to large traffic control systems, we receive information from LED signs which impacts our daily lives.

As LED sign manufacturers compete for their share of any of the several growing markets for LED variable message signs, a critical factor is the actual appearance of their signs. A major factor in the appearance of an LED sign is consistency in the brightness of the LEDs across the face of the sign. The impact of variations in brightness is even more critical in multicolor LED signs where red and green are mixed to yield a variety of different colors, or where red, green and blue are mixed to produce full color displays. As the LEDs are energized at different ratios to produce a desired color, variations inherent in the sign hardware and the LEDs themselves can result in distortions of the desired hue.

What Causes The Apparent Variation In The LED Brightness?

There are several factors which contribute to an LED sign's appearance with respect to consistency across the face of the sign. These factors include:

• Power supply fluctuations
• Internal power distribution
• Environmental factorsIndividual
• LED characteristics

In general, LED brightness is proportional to the current flowing through the LED. Since current in most LED driving schemes is a function of the voltage applied, power supplies have a major impact on the appearance of an LED sign. In a sign that utilizes more than one power supply to power the LEDs, any variation between the power supply output voltages has the potential to impact the relative brightess of the LEDs. As more supplies are used, the chances increase of having a single power supply output drift from its intended value. Also impacting the voltage delivered to the LED is the voltage distribution network within the sign. Each socket, connector, cable and ground connection results in a voltage drop, with the LED furthest from the power supply potentially seeing less voltage than the closest LED. This results in different currents flowing through the LEDs at various locations on the sign, affecting the relative brightness of the LEDs across the face of the sign. These factors are further impacted by the effects that varying environmental conditions, such as the temperature within a sign, can have on the power supplies and the electronic components, all of which impact the brightness and consistency of the LEDs.

Further complicating efforts to deliver a consistent sign are the LEDs themselves. LEDs are manufactured in a "Batch" type and performance variations can be expected from lot to lot. This is due to differences in raw material sources, crystal growth, handling, storage conditions for the raw materials, and basically all of the other variables that go into a manufacturing process.

The LED binning process is able to address much of the LED impact on a sign's brightness consistency. As a part of the LED manufacturing process. LEDs are sorted for brightness and assigned to "bins" based upon the measured brightness. In some cases, the sorting process may also address variations in color (wavelength) as well.

LEDs are provided by the LED manufacturer in "bins" that group LEDs into homogeneous groups of like brightness. An LED brightness bin typically groups LEDs within a 2 to 1 brightness ratio, with some degree of overlap between adjacent bins. It is widely accepted throughout the industry that this 2 to 1 grouping of LEDs is consistent, in terms of brightness, to the human eye. These bins are typically assigned a letter or number code which identifies a 2 to 1 brightness range measured in millicandelas. Figure 1 shows a typical bin table.

A problem that sign manufactures encounter with this binning process, however is the variation in the forward voltage drop (Vf) of an LED. Although the LEDs are sorted for brightness, the sorting is performed at a consistent current, typically 10 or 20 mA (see figure 2). This compensates for the variations in Vf with respect to the binning process but does not address the fact that most of today's conventional LED driving circuits are not constant current sources. Vf may vary from lot to lot, or between manufactures and will vary from color to color. Although the Vf of a given LED part number generally remains fairly consistent, there are cases where Vf may vary from the typical value by as much as 0.5 volts or more as the specifications (see Figure 2) for an LED indicate. The consequences of this in a conventional sign circuit is that an LED with lower Vf will see more current, resulting in a difference in brightness between the LEDs.

Each of these factors has the potential to affect the appearance of an LED variable message sign but are not typically significant enough to materially affect the visible properties of the sign. However, their impact is cumulative and the combined effect can and does impact the appearance of an LED variable message sign.

What can be done to address the factors affecting a sign's appearance?

There are steps, which can be taken to ensure brightness consistency across the face of an LED sign. Better power supplies, remote voltage sensing, sophisticated power distribution schemes, and additional sorting of LEDs are all possibilities, but they come at a significant cost.

The most reliable and cost effective way to address the brightness consistency issue is through the use of today's new Constant Current LED drivers. These devices are designed specifically to address the variables that affect the brightness consistency of an LED sign.

Figure 3

What is a Constant Current LED Driver?

A Constant Current driver is an integrated circuit device designed specifically to work with LEDs in sign and display applications. Through the use of user programmable, constant current outputs, the Constant Current driver regulates the current through the LED independent of external factors, such as variations in LED Vf, power supply drift, etc. Once a current value is selected, the LED will continually see that current, regardless of the factors outlined earlier. A unique, integral feedback mechanism actually monitors the current flow in a given LED or string of LEDs and dynamically adjusts the output to maintain the programmed current level.

How Does A Constant Current LED Driver Work?

The constant current output stage is an open collector driver in the case of the "sink" driver or a darlington in the case of the "source" driver both of which employ an innovative, dynamic, current regulating scheme. The desired current is programmed simply by placing a single resistor from the device to ground. The value of the resistor determines the desired LED current, and is selected from a graph (see Figure 3) which correlates the desired current to a resistor value. The output will support a single LED or series/parallel combinations of LEDs, maintaining the desired current on each output string (see Figure 4), typically eliminating the requirement for a current limiting resistor in each string.

Figure 4

The Constant Current driver actually incorporates several of the functions necessary for the control of LEDs in a dynamic variable message sign application. In addition to the constant current output feature, the device family also addresses the decode and set up functions by incorporating integral shift register and latch stages. Serial data, sent directly to the driver, is decoded and latched to the constant current outputs, setting up the desired output sequence (see Figure 5).

Advantages Of The Constant Current LED Driver?

The Constant Currant driver offers several advantages over older, voltage switching technology devices. First and foremost, the Constant Current feature addresses many of the variables inherent in an LED message sign which can have an impact on the sign's overall appearance and performance. By maintaining a known constant current through each and every LED or LED pixel, we eliminate many of the prime contributors to uneven illumination of LEDs, resulting in a better looking sign product.

Figure 5

Also, since several functions are performed by the device, the actual component count within a given circuit actually decreases even as performance is enhanced. A series resistor per LED/Pixel may be eliminated with the addition of just one resistor per device. Even though the cost of the resistors is negligible, the installation cost and the possibility of introducing a fault at every manufacturing step should be considered. Also, if separate decoding logic is employed, this is replaced as well, again favorably impacting the cost and reliability of the product and freeing up valuable PC board real estate.

Finally, the Constant Current driver offers an ideal design platform for a family of LED sign products. Should the color requirement change from red to green (1.8 volt Vf to 2.1 volt Vf - typ.) no change is required within the driver stage of the design. The selected current will be maintained throughout the product. Should a current change be desired, a resistor change per driver is all that is required.

TOSHIBA TB627 Series Constant Current LED Drivers

As the world leader in LED technology and development, TOSHIBA has a unique insight into both the current and future needs of the LED display designer. The TB627 Series family of Constant Current drivers was developed to support virtually all LED display applications, ranging from monochromatic message boards to full color LED video displays with additional devices planned as technology evolves.

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