An application technique for automobile lighting design

火辣西米秀

An application technique for automobile lighting design [Copy link]

Given the many disciplines involved in lighting design, the development process of the final product naturally involves many design choices.

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　　As with home lighting, exterior automotive lighting was previously dominated by incandescent lamps. As light-emitting diode (LED) lamps become more common in homes, they are also gaining traction in the automotive market. An example of this is LED automotive tail and brake lights. Designers often use a shared set of LED strings as the light source for both types of lights. When the driver brakes, the brake lights brighten and the tail lights dim. Designers achieve this functionality by using dimming functions in the electronics that drive the LED light sources.

　　Figure 1 shows a block diagram of the electronics that drive the LED headlights and taillights.

　　Some key considerations in dimming automotive LED headlight and taillight systems include:

　　LED Driver with PWM: As shown in Figure 1, the LED driver, the electronic circuit that provides current to the LED, receives a pulse width modulation (PWM) signal from the timer circuit. The duty cycle of this PWM signal controls the average current driven by the LED driver, which in turn controls the brightness of the LED light source. Therefore, when the driver brakes, the light from the LED becomes brighter, while the light is dimmed only to alert surrounding vehicles and pedestrians. Electronic designers typically generate PWM signals using TI's 555 timer integrated circuit (IC), which is widely used in automotive, industrial and many other industries.

　　Duty Cycle: The disadvantage of 555 timer-based PWM signals is the lack of duty cycle. The 555 timer IC parameters that affect the duty cycle vary from one IC to another; even if every component value in the design is the same, the duty cycle generated by one 555 IC timer may be different from the duty cycle generated by another 555 IC. In a car, this means that the brightness of the taillights on the left and right sides may be different. One way to solve this problem is to replace the 555 IC with a single-chip microcontroller that uses a crystal as the clock source to generate a duty cycle PWM signal. However, this solution requires the use of more peripherals and more complex software programming, and also has to deal with the problem of electromagnetic interference from the inside of the car. Another method is to calibrate the brightness of the taillights during the manufacturing process. However, this method requires the electronics to support calibration, which takes longer to produce and also increases the overall product cost.

　　Feedback: A third possibility is to use feedback. This method is depicted in Figure 2. The principle is simple: compare the PWM duty cycle with a reference value and adjust the timer circuit output accordingly. This method is simple to operate and cost-effective.

Figure 2: Block diagram showing feedback added to increase the accuracy of the PWM signal generated by the 555 timer circuit .

　　This useful technique in automotive lighting design, using feedback to improve duty cycle is simpler and more cost effective than other methods.