In-depth analysis of typical high-power Buck LED driver solutions
“Thanks to excellent lighting characteristics and efficiency, high-power LEDs are becoming more and more popular in automotive exterior lighting design. Electronic devices supporting LEDs must be fast, efficient, and high-precision to control the intensity, direction, and focus of the illumination. These devices must support a wide input voltage range and be able to work outside the AM frequency range of car radios to avoid electromagnetic interference (EMI). Electronic devices must also support the complex lighting patterns required in the LED matrix to support adaptive headlight lighting systems. This article reviews typical LED power management solutions and introduces innovative bucs that support fast, efficient, and high-precision LED lighting solutions
Author: Nazzareno (Rossrtti), Yin Wu
Thanks to excellent lighting characteristics and efficiency, high-power LEDs are becoming more and more popular in automotive exterior lighting design. Electronic devices supporting LEDs must be fast, efficient, and high-precision to control the intensity, direction, and focus of the illumination. These devices must support a wide input voltage range and be able to work outside the AM frequency range of car radios to avoid electromagnetic interference (EMI). Electronic devices must also support the complex lighting patterns required in the LED matrix to support adaptive headlight lighting systems. This article reviews typical LED power management solutions and introduces innovative buck controller ICs that support fast, efficient, and high-precision LED lighting solutions.
Application of LED in automotive exterior lighting
Due to its significant advantages over traditional technologies, LEDs are causing a storm in the automotive industry. The white light in the LED headlights has excellent clarity, thereby reducing driver response time. The adaptive headlight lighting system (AFS) is supported by an LED matrix, which can produce rapid and complex lighting pattern changes and improve driver visibility under poor lighting conditions. At night, AFS can automatically adjust the lighting mode according to the beam of the oncoming vehicle to prevent the driver from being blinded by the strong light. The rise time of LED lighting is 2 times faster than that of incandescent light sources, so LED-based brake lights light up faster, warning drivers in advance and improving road safety. Finally, compared with the equivalent incandescent lamp, LED power consumption is lower, so it has a clear advantage in terms of energy consumption. The LED controller is the electronic device responsible for operating the LED and plays an important role in maintaining and enhancing the inherent clarity, speed and efficiency of the LED.
LED power supply
LEDs are widely used in the automotive field and are widely used in various configurations from single LEDs to LED light strings and matrices. In order to achieve optimal performance, high-brightness (HB) LEDs require constant current. The current is related to junction temperature and color. Therefore, HB LEDs must be driven by current rather than voltage. The power supply that supports long light strings can be anything from a 12V car battery to a boost converter up to 60V. When a car with start/stop technology starts the engine, the battery voltage drop is relatively large, causing the battery voltage to drop below the typical 12V, or even 6V or lower.
Dimming is a commonly used function in many automotive applications and an important safety feature of LED headlights. When the light is dimmed from 100% to 50%, the human eye can hardly detect it. To ensure clear recognition, it must be dimmed to 1% or less. Knowing this, it is not surprising why the dimming ratio is as high as 1000:1 or higher. Since the human eye can perceive a single photon under appropriate conditions, this function is actually not limited.
In order to ensure the color, the current must be kept constant. The best LED dimming strategy is PWM (Pulse Width Modulation), which modulates the light intensity by switching the current in segments in time instead of changing the amplitude. To prevent the LED from flickering, the PWM frequency must be kept higher than 200Hz.
When using PWM dimming, the factor that limits the minimum “on/off” time of the LED is the rise/fall time of the current in the switching regulator Inductor. This may result in tens of subtle response times, which is too slow for LED headlight groups that rely on fast and complex dimming methods. At this time, the only way to achieve dimming is to use a dedicated MOSFET switch (SW1-K in Figure 2) to independently turn on/off each LED in the light string. The challenge to the current control loop is to be able to recover quickly enough from the output voltage transient, which is caused by the diode switching.
LED controller features
To achieve the best results, the LED controller must support a wide input voltage range and have a fast transient response as described above. In order to reduce radio frequency interference and meet EMI standards, a high, well-controlled switching frequency is required, and it is outside the AM frequency band. Finally, high efficiency can reduce heat generation and improve the reliability of LED lighting systems.
Front light system
The sophisticated headlight system uses a boost converter as the front end to manage changes in input voltage (load dump or cold start) and EMI radiation. The boost converter provides a stable, sufficiently high output voltage (Figure 2). The dedicated buck converter uses this stable voltage as input, allowing each buck converter to control a single function, such as high beam, low beam, fog light, daytime running light (DRL), direction, etc., thereby overcoming the control of lighting The complexity of brightness and direction.
In this application, the main control loop of each buck converter sets the current in its LED string, and two auxiliary loops implement overvoltage and overcurrent protection.
Typical high-power Buck LED driver solution
A typical buck LED driver solution is shown in Figure 3. This scheme uses p-channel, high-side MOSFET, its RDSON is higher than n-channel transistor; and adopts non-synchronous structure, Schottky diode D is used for current return. Both of these are inefficient factors.
Figure 2. Advanced LED lighting system
Figure 3. Typical non-synchronous Buck LED driver
Typical transient response
Figure 4 shows another shortcoming of a typical solution in terms of transient response. In the light string composed of 12 LEDs in this test, the number of power-on diodes increased sharply from 8 to 12. As a result, the output voltage step produces current and voltage fluctuations, and it takes tens of microseconds to extinguish. The high dimming ratio PWM dimming circuit will only sample the current within the initial few microseconds. At this time, the amplitude is decreasing, resulting in incorrect dimming brightness and color.
Synchronous high-power Buck LED driver solution
The ideal solution should meet the requirements of wide input voltage range, fast transient response, high and well-controlled switching frequency, and support high efficiency through synchronous rectification. The MAX20078 LED controller supports such a scheme. (Figure 5).
Figure 4. Typical transient response of Buck with hysteresis
Figure 5. Synchronous high-power Buck LED driver
The MAX20078 LED controller uses a proprietary average current mode control method to adjust the inductor current while keeping the switching frequency close to constant. The device operates in a wide input voltage range of 4.5V to 65V, with a switching frequency of up to 1MHz, and supports both analog and PWM dimming functions. The device is available in a space-saving (3mm x 3mm) 16-pin TQFN package (normal or SW) or 16-pin TSSOP package.
Figure 6 shows the relationship between the efficiency of the MAX20078-based LED driver and the power supply voltage. Two 107mΩ synchronous rectification MOSFET transistors ensure high efficiency across a wide input voltage range.
Figure 6. The relationship between MAX20078 scheme efficiency and power supply voltage
Figure 7. MAX20078 transient response
High operating frequency
The on-time of MAX20078 is programmable, and the switching frequency ranges from 100kHz to up to 1MHz. The on-time of the device is proportional to the input voltage and output voltage, which means that the switching frequency is almost constant. MAX20078 has a high and well-controlled switching frequency, and it is easy to set up outside of the AM frequency band. While reducing radio frequency interference, the spread spectrum characteristic meets EMI standards.
We reviewed the many challenges in the power supply of complex LED lighting systems and the requirements for optimizing LED system performance. We introduced how MAX20078 overcomes these challenges through innovative LED controller architecture, providing high-precision average current control, high operating frequency outside the AM frequency band, good transient response supporting high dimming accuracy, and high efficiency, maximum To reduce power consumption. These features support excellent automotive exterior lighting, and are more efficient, support complex lighting patterns and higher-precision lighting brightness, direction and focus control.