High-brightness LED control design for low-cost microprocessors

In the automotive industry, HB LED technology differentiates vehicles from styling, safety, and fuel economy, from simple switch lighting, LCD backlighting to high-intensity headlamp applications. However, efficient and reliable control of the brightness of HB LEDs is not an easy task; power stage efficiency, thermal design and EMC are the most critical design challenges in applications involving HBLEDs. Typically, a dedicated Constant Current driver (CCD) is used to drive the HBLED string to solve most important design problems and simplify the design. However, CCDs are usually more expensive than microcontroller-based solutions. This article describes the use of 8-bit microcontrollers (MCUs) and low-cost separation solutions to implement intelligent HBLED lighting control, thereby avoiding the use of expensive analog drives or CCDs.

Important features of high-brightness LEDs

As is the case with low-intensity LEDs, the luminous intensity of a high-brightness LED is proportional to the electrical flow that passes through it. This current is often referred to as forward current (IF) and is in the range of 100 mA to 1000 mA in the HBLED. At the same time, whenever the HBLED is polarized, a voltage drop occurs, called the forward voltage (VF). In HBLEDs, luminosity and chromaticity are directly proportional to IF, so precise control of the current through the HBLED is critical.

HBLEDs with the same part number and technical specifications do not necessarily have exactly the same VF value. When the current IF through the two HBLEDs is the same, their backward voltage VF may be different. Therefore, controlling the intensity of the LED by means of a constant voltage may result in a difference in density between the HBLED and the HBLED, and to ensure that all HBLEDs have the same brightness, a current control must be provided.

Not only is the luminous intensity related to the current through the HBLED, but the chromaticity is also related to the HBLED current. In order to maintain the HBLED color, the HBLED must be driven with a constant current. This solution will use PWM (Pulse Width Modulation) to provide a lower average current in HBLED (light intensity) while maintaining the same instantaneous current (LED color).

As the HBLED current increases, power consumption will also increase. The current is 350mA. A HBLED with a voltage drop of 3V consumes about 1 watt of power. If proper thermal management is not performed, this dissipation can cause HBLED overheating and long-term performance degradation. Another important aspect of thermal design is that the intensity of HBLED illumination is inversely proportional to the junction temperature of the LED. As the temperature increases, the color of the emitter enters a higher wavelength.

The challenge of driving high-brightness LEDs

In low-intensity LEDs, it is very common to use resistors to limit IF current. In HBLEDs, the rated power of the resistor must be higher, which can lead to inefficient systems. Therefore, in HBLED systems, Switch Mode Power Supplies (SMPS) are used to increase efficiency and reduce power consumption. Since SMPS requires energy storage components (inductors and capacitors), the price is usually more expensive; at the same time, SMPS can also cause noise or EMI problems.

A group of HBLEDs can be driven simultaneously in parallel or in series. Parallel driving allows each HBLED to have a different illumination intensity. However, if a control loop is required, each HBLED will require a dedicated control, so the cost is too high for a large number of HBLEDs.

When connecting HBLEDs in parallel, each string requires only one drive and control loop, and the current through all of the HBLEDs in the series is the same, providing them with a relatively constant brightness. Depending on the number of LEDs in series, the voltage required for line penetration may be lower or higher than the input voltage.

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