In response to the requirements of new energy regulations, LEDs are increasingly being used as energy-saving light sources. Compared to conventional lamps, they offer decisive advantages: lower energy consumption, longer lifetime and are available in a wide range of colours. For example, with the help of LEDs, the largest church in the world, St. Peter's Basilica in Rome, is now illuminated in a new light. Through the intelligent control system, even the smallest details of its important collections can be presented through preset lighting scenes. These digital control systems integrate programmable LED drivers so that the LEDs can be activated on demand. Figure 1 shows an example of a 3-channel LED driver configuration.

Figure 1. Simplified schematic of an LED driver for controlling three independent LEDs

Each of the three output voltages of a digital-to-analog converter (DAC) (in this case AD5686 from Analog Devices) controls a voltage-to-current converter stage, placing individual LEDs in the load path of each stage, for each LED channel. All three converter stages are implemented with an operational amplifier (op amp) ADA4500-2 connected to a MOSFET to control the LED current. Theoretically, this LED current can be as high as several amps, depending on the voltage source (VS) and load resistance, which is 2 Ω in this circuit. Therefore, choosing the right MOSFET is very important.

The quality of the DAC output voltage is largely determined by the reference voltage source, VREF. A high-quality voltage reference should be used. One such example is the ADR4520, shown in Figure 1. It features extremely low noise, ultra-high long-term accuracy and excellent temperature stability.

A typical rail-to-rail amplifier has some nonlinearity and crossover distortion due to the internal design of the ADA4500-2. Their input stage consists of two parallel differential transistors: a PNP stage (Q1 and Q2) and an NPN stage (Q3 and Q4), as shown in Figure 2.

Figure 2. Simplified version of rail-to-rail bipolar transistor input stage in an op amp

Depending on the applied common-mode voltage, the two input pairs produce different offset voltages and bias currents. If a common-mode voltage is applied to the amplifier inputs within 0.7 V of the positive or negative supply voltage (VS), only one of the two input stages will be activated. Then, only the errors (offset voltage and bias current) corresponding to the effective stage will appear. If the voltage rises to 0.8 V, both input stages will activate. In this case, the offset voltage can change suddenly, causing so-called crossover distortion and nonlinearity.

In contrast, the ADA4500-2 has an integrated input-side charge pump to cover the rail-to-rail input range without the need for a second differential pair, thus avoiding crossover distortion. Other advantages of the ADA4500-2 include low offset, low bias current, and low noise components.

In such circuits, attention must be paid to the inductance of the LED wiring in the load/current path. Wires are often meters long and can cause abnormal oscillations if proper compensation is not provided. Compensation in this circuit is achieved through a feedback path that returns the current measured by the shunt resistor to the input of the op amp. The existing resistor and capacitor circuits on the ADA4500-2 should be adjusted for the resulting inductance.

Using the circuit shown in Figure 1, it is easier to implement a multi-channel LED driver that can be programmed by a DAC for precise lighting control applications. It is also important to make appropriate adjustments to specific needs to avoid malfunctioning.

in conclusion

The circuit described in this article shows an easier way to create a programmable LED driver that is ideal for precision lighting control applications that require a compact, scalable, easy-to-power, and highly linear power supply. However, the size must be adapted to the requirements of the application to avoid any failures due to various inductances present, such as line inductance and parasitic inductance.