Better automotive exterior lighting with high-power Buck LED controllers

High-power LEDs are becoming increasingly popular in automotive exterior lighting designs due to their superior lighting characteristics and efficiency. The electronics supporting LEDs must be fast, efficient, and highly accurate to control lighting intensity, direction, and focus. These devices must support a wide input voltage range and be able to operate outside the AM band of the car radio to avoid electromagnetic interference (EMI). The electronics must also support the complex lighting patterns required in LED matrices to support adaptive front lighting systems. This article reviews typical LED power management schemes and introduces innovative buck controller ICs that support fast, efficient, and highly accurate LED lighting schemes.

LED Applications in Automotive Exterior Lighting

LEDs are taking the automotive industry by storm due to their significant advantages over traditional technologies. The white light in LED headlights has excellent clarity, which reduces driver reaction time. Adaptive front lighting systems (AFS), powered by LED matrices, are able to produce rapid, complex changes in lighting patterns, improving driver visibility in poor lighting conditions. At night, AFS can automatically adjust the lighting pattern based on the beam of oncoming vehicles to prevent drivers from being blinded by strong light. The rise time of LED lighting is 2 times faster than that of incandescent light sources, so LED-based brake lights illuminate faster, alerting drivers in advance and improving road safety. Finally, LEDs have a clear advantage in energy consumption because they consume less power than comparable incandescent lamps. LED controllers are the electronics responsible for operating LEDs and play an important role in maintaining and enhancing the inherent clarity, speed and efficiency of LEDs.

Figure 1. LED car headlight

LED Power Supply

LEDs are widely used in the automotive field and are widely used in various configurations from single LEDs to LED strings and matrices. For optimal performance, high-brightness (HB) LEDs require constant current. Current is related to junction temperature and color. Therefore, HB LEDs must be driven by current rather than voltage. The power supply to support long light strings can be anything from a 12V car battery to a boost converter up to 60V. When the engine of a car with start/stop technology is started, the battery voltage drops significantly, causing the battery voltage to drop below the typical 12V, even to 6V or lower.

Dimming

Dimming is a common feature in many automotive applications and is an important safety feature of LED headlights. The human eye can barely detect when a light is dimmed from 100% to 50%. To ensure clear discrimination, dimming to 1% or less is necessary. With this in mind, it is not surprising that dimming ratios of 1000:1 or more are possible. Since the human eye can perceive single photons under the right conditions, there is virtually no limit to this feature.

To ensure the color, the current must remain constant. The best LED dimming strategy is PWM (pulse width modulation), which modulates the light intensity by switching the current on and off in segments over time, rather than changing the amplitude. To prevent LED flicker, the PWM frequency must be kept above 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 can result in a response time of tens of microseconds, which is too slow for LED headlight clusters that rely on fast, complex dimming methods. The only way to achieve dimming at this time is to use a dedicated MOSFET switch (SW1-K in Figure 2) to independently turn each LED in the light string on and off. The challenge for the current control loop is to be able to recover quickly enough from the output voltage transient caused by the diode switching.

LED Controller Features

To achieve optimal results, the LED controller must support a wide input voltage range and have a fast transient response as described above. To reduce RF interference and meet  EMI  standards, a high, well-controlled switching frequency is required, which is outside the AM band. Finally, high efficiency can reduce heat and improve the reliability of LED lighting systems.

Headlight system

A sophisticated headlight system uses a boost converter as the front end to manage variations in input voltage (load dump or cold start) and  EMI  emissions. The boost converter provides a stable, sufficiently high output voltage (Figure 2). A 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 lamp, daytime running lamp (DRL), direction, etc., thereby overcoming the complexity of controlling lighting 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 Drive Solution

A typical buck LED driver solution is shown in Figure 3. This solution uses a p-channel, high-side MOSFET, which has a higher RDSON than an n-channel transistor, and a non-synchronous structure, with a Schottky diode D for current return. Both are factors that contribute to low efficiency.

Figure 2. Advanced LED lighting system

Figure 3. Typical non-synchronous Buck LED driver

Typical transient response

Figure 4 shows another shortcoming of the typical solution in terms of transient response. In the 12-LED string tested in this test, the number of powered diodes increased dramatically from 8 to 12. The resulting output voltage step produces current and voltage fluctuations that take tens of microseconds to extinguish. The high dimming ratio PWM dimming circuit will only sample this current in the first few microseconds, when 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 solution. (Figure 5).

Figure 4. Typical transient response of a buck with hysteresis

Figure 5. Synchronous high power Buck LED driver

The MAX20078 LED controller uses a proprietary average current mode control method to keep the switching frequency nearly constant while regulating the inductor current. The device operates over 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 (regular or SW) or a 16-pin TSSOP package.

Efficient

Figure 6 shows the efficiency vs. supply voltage of a MAX20078-based LED driver. Two 107mΩ synchronous-rectifier MOSFET transistors ensure high efficiency over a wide input-voltage range.

Figure 6. MAX20078 solution efficiency vs. supply voltage

Figure 7. MAX20078 transient response.

High operating frequency

The MAX20078 has a programmable on-time and a switching frequency range of 100kHz up to 1MHz. The device's on-time is proportional to the input voltage and the output voltage, which means that the switching frequency is almost constant. The MAX20078 has a high and well-controlled switching frequency that is easy to set outside the AM band. The spread spectrum feature meets EMI standards while reducing RF interference.

Summarize

We reviewed the many challenges in powering complex LED lighting systems and the requirements for optimizing LED system performance. We described how the MAX20078 overcomes these challenges through an innovative LED controller architecture that provides high-precision average current control, high operating frequency outside the AM band, good transient response for high dimming accuracy, and high efficiency to minimize power consumption. These features enable superior automotive exterior lighting with higher efficiency, complex lighting patterns, and more accurate control of lighting brightness, direction, and focus.