A Fast, Efficient and High-precision LED Lighting Scheme Is Proposed

Thanks to excellent lighting characteristics and efficiency, high-power LED is becoming more and more popular in automobile exterior lighting design. Electronic devices supporting led must be fast, efficient and accurate to control lighting intensity, direction and focus. These devices must support a wide input voltage range and be able to work outside the AM band range of automotive radio to avoid electromagnetic interference (EMI). The electronics must also support the complex lighting modes required in the LED matrix to support adaptive headlighting systems. This paper reviews the typical LED power management schemes, and introduces the innovative Buck Controller IC supporting fast, efficient and high-precision LED lighting schemes.

Figure 1. LED car headlights

Application of LED in automobile exterior lighting

Due to its significant advantages over traditional technology, LED is setting off a storm in the automotive industry. The white light in the LED headlights has excellent clarity, thus reducing the driver's reaction time. Adaptive headlighting system (AFS) is supported by LED matrix, which can produce rapid and complex lighting mode changes and improve the driver's visibility under poor lighting conditions. At night, AFS can automatically adjust the lighting mode according to the beam to the vehicle to prevent our drivers from being blinded by strong light. The rise time of LED lighting is 2 times faster than that of incandescent light source, so the brake light based on LED lights up faster, warn the driver in advance and improve road safety. Finally, compared with equivalent incandescent lamps, LED has lower power consumption, so it has obvious advantages in energy consumption. LED controller is an electronic device responsible for operating LED. It plays an important role in maintaining and enhancing the inherent clarity, speed and efficiency of LED.

LED power supply

LED is widely used in the automotive field and is widely used in various configurations from single led to LED lamp string and matrix. In order to achieve optimal performance, high brightness (HB) LEDs require constant current. The current is related to junction temperature and color. Therefore, Hb led must be driven by current rather than voltage. The power supply that supports the long lamp string can be any power from 12V battery to up to 60V boost converter. When the engine is started, the battery voltage drop of the vehicle with start / stop technology is relatively large, resulting in the battery voltage falling below the typical 12V, even 6V or lower.

Dimming

Dimming is a widely used function in many automotive applications, and it is also an important safety feature of LED headlights. When the lamp is dimmed from 100% to 50%, it is almost invisible to human eyes. To ensure clear discrimination, it must be dimmed to 1% or lower. With this in mind, it will not be surprising why the dimming ratio is as high as 1000:1 or higher. Because the human eye can sense a single photon under appropriate conditions, this function is actually unlimited.

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 time segmented switching of the current, rather than changing the amplitude. To prevent the LED from flashing, the PWM frequency must be kept above 200Hz.

When PWM dimming is adopted, the factor limiting the minimum "on / off" time of the LED is the rise / fall time of the current in the inductor of the switching regulator. This can result in dozens of subtle response times, which are too slow for LED headlights that rely on fast and complex dimming methods. At this time, the only way to realize dimming is to use a special MOSFET switch (sw1-k in Figure 2) to independently turn on / off each LED in the lamp string. The challenge for the current control loop is to recover from the transient change of output voltage quickly enough, which is caused by diode switching.

LED controller characteristics

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

Headlight system

The sophisticated headlight system uses a boost converter as the front end to manage the changes of input voltage (load shedding or cold start) and EMI radiation. The boost converter provides a stable and high enough output voltage (Fig. 2). The dedicated step-down converter uses the stable voltage as the input, allowing each step-down converter to control a single function, such as high beam, low beam, fog lamp, daytime running lamp (DRL), direction, etc., so as to overcome the complexity of controlling lighting brightness and direction.

Figure 2. Advanced LED lighting system

In this application, the main control loop of each buck converter sets the current in its LED string, and the two auxiliary loops implement overvoltage and overcurrent protection.

Typical high power buck LED driver scheme

A typical buck LED driver scheme is shown in Figure 3. This scheme uses p-channel and high side MOSFET, and its RDSON is higher than that of n-channel transistor; The non synchronous structure is adopted, and Schottky diode D is used for current reflux. Both are inefficient factors.

Figure 3. Typical asynchronous buck LED driver

Typical transient response

Figure 4 shows another disadvantage of the typical scheme in terms of transient response. In the light string composed of 12 LEDs in this test, the number of power on diodes surged from 8 to 12. As a result, the output voltage step produces current and voltage fluctuations, which takes tens of subtle time to extinguish. The PWM dimming circuit with high dimming ratio will sample the current only in the initial few minutes. At this time, the amplitude is decreasing, resulting in incorrect dimming brightness and color.

Figure 4. Typical transient response with hysteretic Buck

Synchronous high power buck LED driver scheme

The ideal scheme 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 5. Synchronous high power buck LED driver

Max20078 LED controller adopts a proprietary average current mode control method to keep the switching frequency close to constant while adjusting the inductance current. The device operates in a wide input voltage range of 4.5V to 65V, the switching frequency is up to 1MHz, and supports analog and PWM dimming functions at the same time. The device adopts space saving (3mm x 3mm) 16 pin TQFN package (normal or SW) or 16 pin tssop package.

Efficient

Fig. 6 shows the relationship between the efficiency of LED Driver Based on max20078 and power supply voltage. Two 107m Ω synchronous rectifier MOSFET transistors ensure high efficiency over a wide input voltage range.

Figure 6. Relationship between efficiency of max20078 scheme and power supply voltage

High precision light intensity control

Compared with the example shown in Figure 4, the proprietary architecture of max20078 provides almost error free transient response. In Figure 7, the number of diodes increased from 8 to 12, and the output voltage or current did not fluctuate significantly.

Figure 7. Max20078 transient response

The on time of high operating frequency max20078 is programmable, and the switching frequency range is 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 outside the AM band range. While reducing RF interference, the spread spectrum characteristics meet the EMI standard. summary

We reviewed many challenges in the power supply of complex LED lighting systems and the requirements for optimizing the performance of LED systems. We introduce 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, high efficiency and minimizing power consumption. These features support excellent vehicle exterior lighting with higher efficiency, complex lighting modes and higher precision lighting brightness, direction and focus control.