A simpler way to implement a programmable LED driver

In response to the requirements of new energy regulations, LEDs are increasingly being used as energy-saving light sources. Compared with traditional lamps, they have decisive advantages: lower energy consumption, longer life, and a variety of colors to choose from. For example, with the help of LEDs, the largest church in the world-St. Peter’s Basilica in Rome, can now be presented under a new light. Through the intelligent control system, even the smallest details of its important collections can be presented one by one through preset lighting scenes. These digital control systems integrate programmable LED drivers, so LEDs can be activated on demand. Figure 1 shows an example of a 3-channel LED driver configuration.

In response to the requirements of new energy regulations, LEDs are increasingly being used as energy-saving light sources. Compared with traditional lamps, they have decisive advantages: lower energy consumption, longer life, and a variety of colors to choose from. For example, with the help of LEDs, the largest church in the world-St. Peter’s Basilica in Rome, can now be presented under a new light. Through the intelligent control system, even the smallest details of its important collections can be presented one by one through preset lighting scenes. These digital control systems integrate programmable LED drivers, so LEDs can be activated on demand. 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, AD5686 from Analog Devices) controls a voltage-current converter stage, and separate LEDs are placed in the load path of each stage, For each LED channel. All three converter stages are implemented by an operational amplifier (op amplifier) ​​ADA4500-2 and connected to a MOSFET used to control the LED current. Theoretically, this LED current can be as high as several amperes, depending on the voltage source (VS) and load resistance, which is 2 Ω in this circuit. Therefore, it is very important to choose the right MOSFET.

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

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

A simpler way to implement a programmable LED driver

figure 1. Simplified schematic of an LED driver used to control three independent LEDs.

A simpler way to implement a programmable LED driver

figure 2. A simplified version of the rail-to-rail bipolar transistor input stage in an operational amplifier.

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

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

In this type of circuit, you must pay attention to the inductance generated by the LED wiring in the load/current path. The wire is usually several meters long, and if the correct compensation is not provided, it may cause abnormal oscillations. The compensation in this circuit is achieved through the feedback path, which returns the current measured by the shunt resistor to the input of the operational amplifier. The existing resistor and capacitor circuits on the ADA4500-2 should be adjusted according to the generated inductance.

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

in conclusion

The circuit described in this article shows a simpler way to create a programmable LED driver that is ideal for precise lighting control applications that require compact, scalable, easy-to-power, and highly linear power supplies. However, the size must be adapted to the requirements of the application to avoid any malfunctions due to various existing inductances (such as line inductance and parasitic inductance).

The Links:   2MBI150UA-120 AA104SG04

Author: Fymicohuang

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