When driving at night, you may rarely think of the car's headlights and taillights, and I am fascinated by the many design elements in these lighting systems:
Body stylists create stunning headlights and taillights silhouettes and shapes that inspire consumers' desire to buy
Government regulators focus on the shape and brightness of the beam
System architect determines the source and function
Optical engineers developed reflective bowls and glass properties
Mechanical engineers select materials and design the physical structure of the light source
The electrical engineer designs the circuit to power the light source and communicate with the in-vehicle electronics.
In view of the many disciplines involved in lighting design, the final product development process will naturally have many design choices. Today, I want to talk about the LED lights.
LED light
Like home lighting, incandescent lamps are used before the exterior lighting of cars. With the increasing use of light-emitting diode (LED) lamps in the home, it is also increasingly sought after in the automotive market. LED car taillights and brake lights are an example. Designers often use a shared set of LED strings as the source of light for both lights. When the driver brakes, the brake lights light up and the tail lights darken. This feature is achieved by the designer through the dimming function in the electronics that drive the LED light source.
Figure 1 shows a block diagram of the electronic components that drive the LED headlights and taillights.
Figure 1: Block diagram of a typical architecture for driving automotive lighting LEDs.
Some key considerations in dimming automotive automotive headlamps and taillight systems include:
LED driver with PWM: As shown in Figure 1, the LED driver, an electronic circuit that supplies current to the LED, receives a pulse width modulation (PWM) signal from the timer circuit. The duty cycle of the PWM signal controls the average current driven by the LED driver, which in turn controls the brightness of the LED source. Therefore, when the driver brakes, the light from the LED becomes bright, and when only the surrounding vehicles and pedestrians are prompted, the light is dimmed. Electronic design engineers typically use TI's 555 timer integrated circuits (ICs) to generate PWM signals, which are widely used in automotive, industrial, and many other industries.
Duty Cycle Accuracy: A disadvantage of the PWM signal based on the 555 timer is that the accuracy of the duty cycle is insufficient. The 555 timer IC parameters that affect the duty cycle will vary from IC to IC; this means that even if the value of each component in the design is the same, the duty cycle generated by one 555 IC timer is generated by another 555 IC. The duty cycle may also vary. 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 microcontroller with a clock source to generate an accurate duty cycle PWM signal. However, this solution requires more expensive components and more complex software programming. Another method is to calibrate the brightness of the taillights during the manufacturing process. However, this approach requires electronic equipment to support calibration, which results in longer production times and increases overall product cost.
Feedback: The third possibility is to use feedback. Figure 2 depicts this approach. The principle is simple: compare the PWM duty cycle to an accurate reference and adjust the timer's circuit output accordingly. This method is simple and cost effective.
Figure 2: Feedback added to the block diagram to improve the accuracy of the PWM signal generated by the TI 555 timer circuit.
I believe you will definitely ask: Is this method effective? The answer is yes! The precision PWM dimming LED driver reference design for automotive lighting contains relevant design details and test results.
I hope that I have provided a useful technique for automotive lighting designers. I believe that this method of using feedback to improve duty cycle accuracy is simpler and cost-effective than other methods. Now, give it a try, don't forget to give me feedback!
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