LED Drivers General Application Note

Technique for adjusting the brightness of LED display modules for brightness compatibility.

Proper brightness binning of LEDs is critical to the fabrication of an esthetically pleasing LED display. Be it a simple, monochromatic, scrolling message board or a full color video display, the LEDs must share consistent characteristics with respect to brightness to avoid the quilted, patchwork appearance often associated with LED displays.

Although most LED manufacturers are quite capable of supplying properly binned devices the nature of the process often dictates that the LED manufacturer must ship several brightness bins to satisfy a users requirements. Even if a given shipment may be satisfied with a single brightness bin it can be expected that the next shipment may be of a different brightness bin. This situation is typical of discrete LED devices, LED numeric displays and LED dot matrix blocks.

Figure 1 - Constant Current LED Driver

Although an inconvenience to the display manufacturer, such a system is manageable provided LEDs of the same bin are used to produce a single display. This, however, becomes more difficult to manage as modules are built to stock and then withdrawn to assemble the end product sign or display. Inventory control, in this case, must include a provision for identifying the bin of the module. It must also ensure that there is an ample quantity of compatible modules from which the end product can be assembled. This situation is further complicated when multiple colors are used.

There is, however, a viable solution to the brightness consistency issue. The new Toshiba Constant Current line of LED interface drivers provides a means to balance the brightness of LEDs of different bins to allow their use within a given display. Although the LEDs still must be properly binned, the new Toshiba devices permit the use of multiple bins of LEDs on a single display without compromising the appearance of the display.

Figure 2 - Monochromatic display module adjustment (Note: It is recommended that Radjust < Rext)

With respect to the issues discussed here, these devices also include a provision for allowing the user to adjust the current delivered to the LEDs of a given module. This feature effectively enables the user to adjust the brightness of an entire module to match the brightness of a reference module. Should the module be of the multicolor type (red/green or red/green/blue) each color may be adjusted to a reference brightness level independently. Such an adjustment, at the assembled module level, allows the user to compensate for differences in the brightness of modules manufactured with LEDs of different bins thus making the modules assembled for stock compatible for use in the end product display.

Figure 2 illustrates the LED driver circuitry necessary to implement the current (brightness) adjustment for a monochromatic display module. Figure 3 illustrates the circuitry for a bi-color (red/green) display module. In each case, the adjustment necessary is limited to a single potentiometer per color.

It must be noted that there are limits that should be observed when designing in the module adjustment feature. First and foremost, there are limits to the degree of adjustment which can be realized. Should the LED bins utilized be too far apart in terms of brightness the adjustment may not permit the dynamic range necessary to equalize the brightness between the modules. Also, should current be increased significantly to raise the brightness of a module the impact that the additional current may have on the life of the LEDs must be considered.

Figure 3 - Bi-color display module adjustment example. (Note: It is recommended that Radjust < Rext)

Ideally, a reference brightness level should be selected which is both acceptable to the application and effectively centered within the brightness bins expected of a given LED part number. Modules assembled from LED bins both above and below the reference brightness may then be adjusted to a compatible level during the electronic test phase of the manufacturing process.

TB627XX Constant Current LED Interface Driver

Selection of Rext (current setting resistor): The value of the resistor, Rext, determines the current that the outputs of a TB627XX constant current driver will maintain. Resistor Rext becomes part of a reference circuit which is "mirrored" by each output of the device.

Figure 4- Rext - Iout

Rext is selected from the graph Rext - Iout (see Figure 1) which is included in the device specification. To determine the value of Rext the desired current value (Iout) must first be located along the vertical axis of the graph. This point is then transferred horizontally to the intercept point on the curve. The corresponding point on the horizontal axis of the graph will yield the value of Rext required to program the device for the desired current.

Power Dissipation Considerations

A constant current driver regulates the current flow through the LED by effectively absorbing the excess voltage as required to maintain the programmed current level. Because of this, it is necessary to be aware of the power dissipation limits of the device during the design process.

Power dissipation may be minimized by selecting an LED power supply voltage by the following formula:

VLED (total supply voltage) = VCE + Vf LED + VOL
Where:

VCE = the saturation voltage of circuit multiplexing transistors
Vf LED = typical forward voltage drop of the LEDs
VOL = 0.7 V for sink drivers/2.0V for source drivers

The anticipated power dissipation of the device may be calculated by the following formula:

PD = # outputs x IOL(ave) x VOL + VDD x IDD
Where

# outputs = the number of outputs of the device
IOL(ave) = the average output current
VOL = voltage drop from the driver output to ground
VDD = logic supply voltage
IDD = device logic supply current

Should the VLED supply circuitry not permit the desired supply voltage level setting a series resistor(s) may be added the the LED circuit without affecting the constant current operation of the device.

Printed Circuit Board Layout Considerations

The constant current drivers utilize one pin for both logic ground and power ground. Care should be taken to minimize inductance and impedance within the ground pattern to avoid switching noise induced misoperation.

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