introduction
With the strengthening of consumers' environmental protection concepts, solid-state lighting solutions have quickly become the lighting technology of choice in automotive applications. According to market research firm Strategies Unlimited, general lighting, backlighting and automotive applications will be the main growth drivers for the high-brightness LED (HB LED) market in the next few years. By 2011, the entire market will reach $1.9 billion. 1.
Compared to traditional lighting technology, HB LEDs have several key advantages: they do not contain any harmful substances, such as mercury in CCFLs; they consume less power and have a longer life. In addition, HB LEDs have a higher cost competitive advantage over traditional solutions, greatly reducing the overall cost of the system (eg, operating and maintenance costs). Of course, the use of HB LEDs also faces some special challenges, especially in noisy work environments such as automobiles. This article discusses the basic principles of HB LED driver selection, compares different driver topologies, and provides configuration options for different automotive lighting applications, including: car dome lights, daytime running lights (DRL), taillights (RCL), fog lights And low beam/high beam lights, etc.
HB LED requires constant current drive
The HB LED driver is used to manage the power supply of the HB LED, and it is critical that the drive circuit maintain a constant current and minimal voltage fluctuations. Excessive drive current will increase the junction temperature of the HB LED and accelerate the degradation of the HB LED. In lighting applications, high power HB LEDs are required for higher lumens. The forward conduction current of these HB LEDs is typically 350mA to 1A. The forward voltages of white, blue, and green HB LEDs range from 2.8 to 4.5V, and the forward voltages of red and amber HB LEDs range from 2.3 to 3.5V. In order to maintain a fixed chromatogram and brightness, HB LED drivers must meet specific current rating requirements. Driving a HB LED with a voltage source and series resistor current limiting can result in unacceptable changes in brightness and spectrum.
HB LED brightness adjustment
The LED color of the LED changes with the change of current. Therefore, the pulse width modulation (PWM) method is better for adjusting the fixed current than the actual current, that is, the DC current is fixed at the HB LED manufacturer. The value is chopped at a certain frequency and duty cycle, and the brightness can be adjusted by pulse width modulation to maintain a consistent spectrum at different brightness levels. In order to avoid visual flicker, the dimming frequency should be higher than 100Hz. The dimming range depends on the minimum duty cycle allowed by the HB LED driver.
Most LED drivers require a brightness control signal generated by a microprocessor or an external timer. The HB LED driver, such as the MAX16806, internally generates a PWM signal that is modulated by an external voltage applied to the DIM input (Figure 1). This configuration eliminates the need for microprocessors in applications such as automotive interior lighting.
Figure 1: 350mA Linear HB LED Driver IC, MAX16806 Eliminates Microcontroller or Switch Mode Converter
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