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A Relatively Easy Way to Realize a Programmable LED Driver

LEDs are increasingly being used as energy saving light sources as a response to new energy regulations. They have decisive advantages over conventional lights: they consume less energy, have longer lifetimes, and are available in various colors. For example, thanks to LEDs, the largest church in the world, St. Peter’s Basilica in Rome, can now be seen in a new light. Via an intelligent control system, even the smallest details of its important treasures can be illuminated with preset lighting scenarios. These digitally controlled systems integrate programmable LED drivers, with which the LEDs can be activated as desired. Figure 1 shows an example of a 3-channel LED driver configuration.

Each of the three output voltages of the digital-to-analog converter (DAC), in this case the AD5686 from Analog Devices, Inc. (ADI), controls a voltage-to-current converter stage in whose load path the individual LED is situated for each LED channel. All three converter stages are realized by an operational amplifier (op amp), ADA4500-2, and a connected MOSFET, which controls the LED current. This LED current can theoretically be up to several amps, depending on the voltage source (VS) and the load resistance, which is 2 Ω in this circuit. Thus, the selection of a suitable MOSFET is very important.

The quality of the DAC output voltages is strongly dependent on the reference voltage source VREF. A high quality reference source should be used. The ADR4520 is one such example, as shown in Figure 1. It offers very low noise, extremely high and long-term accuracy, and high temperature stability.

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