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Digital lighting is becoming an increasingly important aspect of the automotive ecosystem, with the dual functions of improving visual appeal and exchanging information to improve lighting performance and driving safety.
The functionality of vehicle lights is undergoing a major transformation, and automakers are constantly exploring new trends and technologies in automotive digital lighting systems.
The ability to create personalized interactive light patterns for different functional lights such as side lights, brake lights, tail lights, turn signals, etc. is a major advantage of digital lighting. This not only enhances the visual appeal of the car, but also helps to improve the vehicle's recognition on the road, making it easier for other road users to spot the vehicle.
In addition to its aesthetic appeal, digital lighting systems also exchange information in a multifunctional way. For example, digital LED and OLED panels can send early warning information about vehicle and road conditions to other road users, which will greatly improve the safety of the car and help prevent traffic accidents.
Intelligent driving systems play a key role in on-board digital lighting monitoring. These complex and advanced intelligent driving systems are equipped with a set of advanced functions to ensure that the lighting system has detailed diagnosis, precise light adjustment and comprehensive safety protection functions, significantly improving the quality of vehicle lighting and improving the driving experience.
In the design of automotive lighting systems, choosing the right driver is an important product selection decision, especially the choice between constant current drivers and constant voltage drivers.
LED car light developers usually prefer constant current drivers because constant current drive solutions can make the lights evenly bright and flicker-free, while constant voltage drivers have difficulty controlling current fluctuations and cannot accurately control current, resulting in a shortened service life of LED car lights. In addition, constant voltage systems usually require the addition of rectifier components, which will reduce system energy efficiency. In contrast, constant current drivers are a self-sufficient solution that no longer requires any additional components, helping to improve the energy efficiency of LED lighting systems. Although constant current drivers have a higher initial cost, the long-term accumulated energy efficiency proves that this initial investment is worth it.
OLED display technology obviously requires constant current drivers. OLEDs use DC power to achieve higher performance and are very sensitive to the direction of current. Given the potential current variations, a constant voltage drive solution may affect the performance and life of the OLED panel. OLED panels have a steep voltage-brightness response curve and a relatively flat current-brightness response curve, which means that their light output is proportional to the operating current. Therefore, any fluctuation in current will result in a noticeable change in brightness.
Therefore, we recommend using OLED dedicated drivers because these products are specially designed, have higher current control accuracy, small output current ripple, provide transient overshoot protection, precise PWM dimming to achieve dynamic brightness control, and have diagnostic communication and short-circuit protection functions.
STMicroelectronics' L99LDLH32 can bring many benefits to car companies. This compact chip uses a 7mm x 7mm quad flat 48-pin (QFN) lead-free package. Although small in size, it is smart and powerful. It is specially designed to manage automotive OLED panels and LED lights, and meets the strict requirements of ISO 26262's automotive safety integrity level (ASIL B), ensuring that automotive lighting systems meet high standards of safety requirements.
The L99LDLH32 is a high-side linear current regulator with 32 current sources, as shown in Figure 1.
Figure 1: Smart lighting driver block diagram
Each output channel of the L99LDLH32 has a separate current source capable of adjusting the current from 1 mA to 15 mA. This current regulation function is achieved through an 8-bit digital-to-analog converter (DAC) dedicated to each channel. The current setting parameters are stored in the chip's non-volatile memory, which is designed to facilitate dynamic reconfiguration of the current output to meet real-time current needs. In addition, the L99LDLH32 can save the settings of a single light string in memory, thereby achieving consistent and repeatable light output, which is critical for maintaining uniform brightness throughout the vehicle lighting system. Uniform brightness is not only for aesthetic purposes, but also important for safety-related functions that require precise control of brightness.
The independent current setting function enables the L99LDLH32 driver to control each pixel of the LED lamp and OLED panel individually, accurately calibrate the brightness of each area on the panel, and customize the light pattern and light and shadow effects according to specific needs. This fine control provides excellent adjustment accuracy and adaptability, which is essential for complex lighting design and is particularly useful for dynamic display applications.
The L99LDLH32 is useful for dynamic displays that require fine-tuning of brightness, including display panels with 3D animations. The chip's flexible brightness adjustment makes it an ideal choice for automakers who want to develop advanced, responsive lighting systems that not only make cars more attractive to the eye, but also help provide a safer and more engaging driving experience.
In summary, the L99LDLH32 provides a versatile and powerful solution for the automotive industry to develop complex customized lighting systems, capable of easily handling complex lighting scenarios. In the next generation of vehicle design, lighting plays an important role in both functionality and aesthetics, and the L99LDLH32 will become a valuable resource for the next generation of vehicle design.
Drill down into performance
By dynamically adjusting the intensity and brightness of individual light strings, the L99LDLH32 can manage digital lighting configurations and achieve animation effects. Precise control of light intensity is achieved by presetting high-frequency pulse width modulation (PWM) technology and setting high-resolution current for each light string individually. The dual control method of PWM modulation technology and specific current settings enables complex lighting patterns and effects, giving designers the flexibility to create a variety of visual experiences. The high-resolution current control method ensures that each light string emits light that meets the brightness requirements, while the high-frequency PWM technology provides smooth light intensity transitions and changes, a feature that is critical to improving the quality of automotive lighting and displays.
The high-frequency PWM dimming function of L99LDLH32 can not only create a variety of lighting effects, but also play an important role in improving the energy efficiency of the driver. By adjusting the duty cycle of the PWM signal, the brightness of each light string can be controlled without changing the current, allowing the lighting system to maintain high energy efficiency.
This dimming method can minimize the dissipated power, and the high-frequency PWM operation ensures that the human eye cannot perceive the change in light intensity, bringing a flicker-free experience that is both comfortable and beautiful.
Additionally, this advanced lighting control functionality reduces thermal stress and power consumption, helping to extend the life of lighting components, thereby reducing maintenance costs and improving the sustainability of lighting solutions throughout the life of the vehicle.
In summary, the L99LDLH32's high-frequency PWM dimming technology combined with high-resolution current control methods provides an advanced and energy-efficient solution for automotive lighting systems, allowing developers to freely design advanced lighting effects while ensuring the system's energy efficiency, safety and sustainability.
In addition to these features, the L99LDLH32 is also equipped with advanced safety protection functions to provide important safety protection for the driver and lighting components, including any abnormal detection mechanisms such as open circuit or short circuit within the load. These protection functions can issue prompt signals in time when potential problems are found.
The chip also monitors temperature, issues overheating warnings, and proactively handles overheating risks. If the safe temperature limit is exceeded, the L99LDLH32 automatically intervenes based on the measurements of the external negative temperature coefficient (NTC) thermistor and the internal temperature sensor, shuts down the system, reduces the output current, and initiates thermal derating safety protection.
These protection features provide a comprehensive safety solution for automotive lighting applications, preventing potential electrical hazards and overheating hazards from compromising the lighting system.
The L99LDLH32 has fail-safe and limp-home modes. The fail-safe mode protects the system and user safety in the event of a fault. The fail-safe function puts the system into a safe state, preventing faults from damaging the system and avoiding serious faults that could cause personal injury or damage.
When a fault is detected, limp-home mode reduces the amount of system functionality to maintain lighting levels, which is a better approach than shutting down the system completely. In this mode, lighting levels are adjusted to safe levels despite the fault.
Fail-safe and limp-home functions are essential for OLED screens and LED lighting systems and play a key role in safety. These two functions are also key to meeting the requirements of ISO 26262 standard.
Versatility and flexibility
As an LED string and OLED panel driver, the L99LDLH32's dual control capabilities make it a versatile component in automotive lighting and display applications. The device's precise current control provides the current required for LEDs and OLEDs to emit uniform light, manages transient inrush currents, and maintains the functional integrity of the vehicle's electrical system.
Electromagnetic interference is also an important aspect of automotive lighting system design. The L99LDLH32 is specially designed with electromagnetic interference (EMI) suppression mechanism.
The device introduces a method of slightly delaying the turn-on of the illumination channels. The driver turns on each illumination channel sequentially instead of turning on all channels at the same time. Demonstration waveform examples are provided for sequential turn-on of channels 0 to 7, and these plots show that the output voltage and output current behavior are consistent (see Figure 2).
▲Figure 2: Channel output voltage rising edge measurement value using the gradual delay turn-on method
The delayed turn-on feature for all channels can be enabled or disabled with just one dedicated data bit, giving designers the flexibility to tailor the performance of the lighting system to the specific requirements of each application. This feature reduces the potential electromagnetic energy surge caused by switching all channels on and off simultaneously, thereby reducing the risk of electromagnetic interference affecting other in-vehicle electronic systems.
In addition to managing the rising edge of the output voltage, the L99LDLH32 can also regulate the falling edge with the same accuracy. Figure 3 depicts this balanced voltage transition control method.
Figure 3: Measured value of the falling edge of the channel output voltage using the delayed turn-on method
In addition, the L99LDLH32 ensures that the output voltage transitions smoothly during channel switching. This feature helps reduce the electrical stress on the system and helps enhance EMI performance. However, in order to effectively utilize this feature, a higher data bandwidth is required. The advanced CAN (Controller Area Network) FD (Flexible Data Rate) lighting protocol supports data bandwidths up to 1Mbit/s, which can meet this requirement.
In addition, the device also integrates clock dithering function to improve EMI suppression. Specifically, this technology applies a triangle wave on the embedded 20 MHz oscillator, as shown in Figure 4.
▲Figure 4: Oscillator jitter curve
The triangular waveform produces a jitter effect on the oscillator frequency, dispersing the noise over a wider frequency range, distributing the fundamental frequency and higher-order harmonic energy over a wider bandwidth, and reducing the peak amplitude of the noise generated at any single frequency. Jitter technology helps filter noise and effectively reduce the overall EMI impact of the chip.
When designing automotive LED lighting systems, anti-static performance is an important consideration. LEDs are semiconductor devices that are very sensitive to electrostatic discharge (ESD), which can damage the device or degrade its performance, shortening its service life.
Vehicles may be disturbed by various static sources, and ESD protection is essential to ensure the reliability and service life of automotive LED systems, especially LED lamps used for safety functions such as brake lights and turn signals, because their continuous and stable operation is very important for driving safety. Therefore, measures are usually taken to enhance the anti-static ability of automotive LEDs to ensure that they remain in normal and safe working condition throughout the service life of the vehicle.
The L99LDLH32 driver is designed with comprehensive anti-electrostatic discharge (ESD) protection to ensure its durability and reliability in automotive applications. The device includes protection functions designed for multiple types of ESD, each of which is an ESD protection function specially designed to simulate different discharge scenarios.
Human body model (HBM) ESD protection simulates the static electricity that may be accumulated and released by the human body, preventing the device from being damaged by static discharge when a person touches the device. It is common for people to touch electronic components during assembly and maintenance in the workshop, so it is crucial to integrate this type of ESD protection function into the device.
Charged Device Model (CDM) ESD protection is used to address situations where the device itself may become charged due to friction or contact with other materials, and then discharge when it comes into contact with a conductive surface. CDM protection is particularly important in automated production processes, where workpieces on the production line may become charged and then discharge rapidly, which may cause damage to the workpiece.
Machine Model (MM) ESD protection is designed to protect against higher energy discharges generated by manufacturing equipment or machinery. Machine Model (MM) ESD events can be more severe than those typically caused by human contact and can be particularly costly if not adequately protected against.
L99LDLH32 has three electrostatic protection mechanisms: HBM, CDM and MM ESD, proving that its design is highly robust and can withstand various electrostatic discharge risks that may be encountered from production to actual use. Automotive parts must withstand harsh working conditions, and comprehensive ESD protection measures are essential basic functions. In this case, if the ESD electrostatic protection function fails, the potential consequences are very serious, not only causing economic losses, but more seriously affecting the safety of passengers.
By comparing L99LDLH32 with major competitors, we can see that the ST driver has better ESD protection performance (Table 1).
▲Table 1: ESD performance comparison
Comparative analysis shows that while all pins of the L99LDLH32 and its market counterparts meet HBM's ± 2 kV level of baseline ESD protection, the L99LDLH32's output pins excel by increasing the ESD voltage to ± 4 kV. This superior protection is particularly beneficial for automotive LED lights and OLED panels, which are critical to automotive safety functions such as brake lights, turn signals, and other signaling devices. The L99LDLH32's output pins' strong ESD immunity is an important feature that prevents ESD from causing premature light failure or functional abnormalities, thereby ensuring the reliability of these important components.
The advanced ESD protection of the L99LDLH32 is an important consideration for automotive engineers and designers, as it can add an extra layer of durability and reliability to automotive lighting systems in automotive environments where ESD issues are a concern.
Advanced features and product advantages make the L99LDLH32 an essential component for advanced automotive lighting systems to meet the visual aesthetics and safety requirements of modern vehicles. Excellent LED light and OLED panel management capabilities, combined with optimized EMI anti-static performance, are essential for generating customizable interactive light patterns, which not only improves the visual appeal of the vehicle, but also enhances driving safety.
Comprehensive ESD protection is a very prominent product highlight of the L99LDLH32, ensuring that the automotive lighting system has very high reliability. Excellent ESD robustness, especially the anti-static discharge function of the output pins, is essential for key safety-related lighting components such as brake lights and turn signals to maintain normal functions.
In short, L99LDLH32 provides a flexible and effective solution for the automotive industry to develop advanced customized lighting experiences. In future car designs, the lighting system is a key element of functional and aesthetic design, and the excellent ability to control complex lighting scenes makes L99LDLH32 a valuable resource for future car designs.
[1] P. Horsky, J. Plojhar, J. Daniel, "Adaptive peak average current control LED driver for automotive lighting", IEEE Solid-State Circuits Letters (Volume: 2, Issue: 9), Sep. 2019.
[2] Z. Liu, H. Lee, "A current-accuracy-enhanced wide-input-range DC–DC LED driver with feedforward synchronous current control", IEEE Transactions on Circuits and Systems I (Volume: 65, Issue: 11 ), Nov. 2018.
[3] J. Barthel, F. Rennig, M. Sanzà, D. Tagliavia, "CAN FD Light - A novel communication bus supporting digitalization and customization of automotive lighting for the broad market", AEIT International Conference of Electrical and Electronic Technologies for Automotive (AEIT AUTOMOTIVE), 02-04 July 2019.
[4] F. Rennig, "CAN FD light – A network protocol to control lighting", Driving Vision News (DVN) Workshop, Munich, 27-28 Feb. 2024.
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