A design tip: accurately and easily dimming auto LED lights

While you probably don’t even think about your car’s headlights and rear lights when driving in the dark, I am fascinated by the many smart minds that are involved in the design of these lighting systems:

  • Car body stylists create awesome headlight and rear light shapes that attract drivers to buy a car
  • Government regulators are concerned about the patterns and brightness of the light beam
  • System architects decide on the light source and functionality
  • Optical engineers develop  the reflectors and the glass characteristics,
  • Mechanical engineers choose the materials and design the physical structure of the light source
  • Electrical engineers design circuits to drive the light sources and to communicate with vehicle electronics.

With so many disciplines involved in the design of lights, there are obviously many permutations of design choices available to create the end product. I would like to shed light on one of these choices: LED lights.

Let there be . . . LED light

Just like lights in homes,  exterior lighting in cars were traditionally based on incandescent bulbs. Just as Light Emitting Diode (LED)-based lights are becoming popular in homes, LED-based based lights in automotive are gaining popularity as well. One such application for LEDs as the light source is in car taillight and brake light function. Designers often utilize one shared set of LED strings as the light source for both these light functions. The light from the LED becomes bright when the driver applies the brakes and is otherwise dimmed for taillight function. Designers accomplish this with the help of dimming features implemented in the electronics that drive the LED light source.

Figure 1 shows a block diagram of the electronics for driving LED lights in automotive headlight and rear lights.

Figure 1:  Block diagram showing a typical architecture for driving LEDs in automotive lighting.

A few key considerations of dimming in automotive LED headlight and rear light systems include:

  • LED driver with PWMs: As you can see in Figure 1, the LED driver, which is an electronic circuit that supplies current to the LED, receives a pulse-width modulated (PWM) signal from a timer circuit. The duty cycle of this PWM signal controls the average current driven by the LED driver in the LED, which in turn controls the brightness of the light emitted from the LED light source. Therefore the light from the LED becomes bright when the driver applies the brakes and is otherwise dimmed when simply indicating the presence of the car.  Electronics design engineers commonly generate the PWM signal using TI’s 555 timer Integrated Circuit (IC), which is a popular IC in many industries – automotive, industrial and many others.
  • Duty-cycle accuracy: A disadvantage with 555 timer-based PWM signals is the accuracy of the duty cycle. The parameters of 555 timer ICs that affect the duty cycle vary from part to part; this means that the duty cycle generated by one 555 IC timer may not be the same as the duty cycle generated by another 555 IC, even if every component value in the design is identical. In a car, that means that the brightness of the right-side taillight could differ from the left-side taillight. Ugh! To resolve this problem, one approach to generate an accurate duty cycle PWM signal is by replacing the 555 IC with a microcontroller that uses a crystal as the clock source. However this solution involves more expensive components and software programming. Another approach is to calibrate the brightness of the taillights during manufacturing. Unfortunately this approach requires that the electronics support calibration requiring additional time on the manufacturing line and greater overall product cost.
  • Feedback: A third possibility is to use feedback. Figure 2 shows this approach. The idea is simple: compare the PWM duty cycle against a precise reference and adjust the timer’s circuit output. This is a clever idea that is both elegant and cost-effective.

Figure 2:  Block diagram showing the addition of feedback to improve the accuracy of the PWM signal generated by the TI 555-based timer circuit.

I’m sure you’re asking: Does this really work? The answer is yes! The precision PWM dimming LED driver reference design for automotive lighting includes design details and test results showing that this solution actually works.

I hope I have given you one more trick (in a bag of many) available to automotive lighting designers. I believe this trick of employing feedback to improve duty cycle accuracy is simpler and has cost advantage compared to alternate approaches. Now, use it and give me your feedback!

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