High-brightness LED applications bring new opportunities and challenges to LED driver ICs

At present, there is a big difference in market forecasts for high-brightness LEDs (light-emitting diodes). Although the forecast data is different, the trend is clear: the high-brightness (HB) LED market is growing at an alarming rate. Some forecast data have a compound annual growth rate of 15%, while others have a compound annual growth rate of 35%. In 2007, LED lighting in the automotive sector was in its infancy, with sales of US$670 million, but is expected to grow at a compound annual growth rate of 15%, reaching US$1.2 billion in 2011 (1). If non-automotive applications are taken into account, the compound annual growth rate is close to 35%, and by 2011, the total high-brightness LED market is likely to easily exceed $2.5 billion.

What drive is this high growth potential? First, LEDs produce light 10 times more efficiently than incandescent lamps and almost twice as much as fluorescent lamps, thus greatly reducing the amount of electrical power required to deliver a specific amount of light output (in lumens). Second, incandescent lamps need to be replaced after 1,000 hours of use. Fluorescent lamps can last for 10,000 hours, while LEDs last for more than 100,000 hours. In most applications, this allows the LED to be permanently embedded in the final application without the need for an attachment. Examples include display screens for car bodies or LCD screens for HDTVs, as these LEDs do not need to be replaced during the life of a car, HDTV or sports stadium display. Third, LEDs are orders of magnitude smaller than other lighting products, and the form factor can be made very flat so that they can be permanently embedded in automotive interior and exterior applications. In addition, countless colors can be achieved by using red, green and blue LED configurations. Finally, the LEDs are dimmable, and their turn-on and turn-off times are fast and undetectable by the human eye, which greatly improves the backlighting of HDTVs and other types of displays. Without these LEDs, it is impossible to have a high-resolution LCD HDTV with extremely high contrast and sharpness.

Still, one of the biggest challenges for lighting system designers is how to take advantage of all the benefits of the latest generation of LEDs. Because they typically require an accurate DC current source and require dimming, the LED driver IC must be designed to meet the needs of a variety of applications. Power solutions must be efficient, reliable, and as always, very compact and economical. There is reason to believe that one of the most demanding LED driver applications is in automotive applications, including LED-based interior lighting, LCD display backlighting, brake/steering indicator lights, and even headlights.

Automotive LED lighting

The advantages of small size, low power consumption, and short turn-on time have made high-brightness LEDs widely available in today's automobiles. The original LED application was the Central High Brake Light (CHMSL), which uses red LEDs to provide a very small array of illumination that is easy to install and never needs to be replaced.

Figure 1: Audi R8 / Lexus LED headlights / turn signal lights / driving lights.

Traditionally, incandescent lamps are the most economical source of light, and many cars are still in use. However, the reduced space available for lighting components and the need for long working life, combined with LEDs for bright natural colors and simplified design, have quickly replaced incandescent lamps in many applications. White LED arrays replace traditional cold cathode fluorescent lighting (CCFL) TFT-LCD backlighting is becoming more and more common. White LEDs provide more accurate and adjustable backlighting, and the working life will easily exceed the life of the car itself. Even the headlights, which are primarily used in halogen/氙 filament design applications, are gradually adopting electronic “controllable” high-current LED arrays (see Figure 1). All automotive lighting applications, including internal/external and backlighting applications, are almost turning to LEDs. There are several advantages to using LEDs in this environment. First, they never need to be replaced because their reliable life span of more than 100,000 hours (11.5 years of service) exceeds the life of the car itself. This allows automakers to permanently embed them in "in-cabin" lighting systems without the need for a corresponding design for easy replacement. The style can also vary greatly, as the LED lighting system requires less depth and area than incandescent lamps.

Figure 2 shows the various LED lighting applications in modern cars. Inside the car, there are several “standard built-in” lighting modules that use various types of LEDs. Some are single LEDs, while others such as navigation dashboard backlighting require an LED array. LEDs are also recognized for external lighting. More than 40% of central high-position brake lights now use red LEDs. In addition, the Audi 2008 A8 uses a high current LED array as the daytime running light (DRL). All external “forward lighting” including the Lexus 600 sedan and the Audi R8, including the headlights, are done by LEDs. Similarly, even more mid-range cars and many motorcycles use color LED arrays as brake/turn signal indicators.

Figure 2: Typical application of LED lighting in modern cars.

Automotive LED lighting design parameters

Table 1: Forward voltage drop and drive current for high current white LEDs.

To ensure optimum performance and long life, LEDs require an efficient drive circuit. These drive circuits must be able to adapt to the most demanding automotive power bus and be both economical and space efficient. To ensure a longer life, the first thing is to not exceed the current and temperature limits of the LED. Table 1 shows the typical forward voltage and drive current for high current white LEDs.

Figure 3: 50W headlight circuit with LT3755.

Most headlamp applications require approximately 50W of LED current. Linear Technology's LT3755 is designed to serve this type of application. It boosts the car's bus voltage (nominally 12V) to 60V to drive up to 14 1A LEDs connected in series, as shown in Figure 3.

Figure 4: Efficiency of the LT3755 circuit shown in Figure 3.

Figure 4 illustrates that the LT3755 can be as efficient as 93%. This is very important because it eliminates the heat dissipation requirements of all power components and can result in a very compact solution footprint. Although the circuit shown in Figure 3 is a boost topology, the LT3755's unique high-side current sense design allows it to be configured for boost mode, buck mode, buck-boost mode, or vice versa, depending on the application requirements. Excited topology.

The LT3755 drives an external low-side N-channel MOSFET with a stable internal 7V supply. The fixed frequency, current mode architecture allows for stable and accurate operation over a wide range of supply and output voltages. The LT3755 provides a constant current source that is important for LED driver ICs to achieve constant LED brightness when the input voltage is unstable, and is especially important in automotive applications because of transients in the event of cold start and load dump. The input voltage may have a large swing. With a maximum input voltage of 40V, the LT3755 is capable of stabilizing LED current and voltage when the main automotive bus encounters a 40V transient. 40V transients are common in load dump situations.

The ground-referenced voltage feedback pin acts as an input to many LED protection circuits, such as LED open-circuit protection, and also makes it possible to use the converter as a constant voltage source. The frequency adjustment pin allows the user to program the frequency from 100kHz to 1MHz to optimize efficiency and performance while minimizing external component size. If external synchronization is required, the LT3755-1 version can be synchronized to an external clock over the entire frequency range of 100kHz to 1MHz.

The true color PWM dimming of the LT3755 achieves a dimming ratio of up to 3000:1, and the color that is illuminated during dimming does not change, allowing continuous adjustment of the LED headlights to suit a variety of environmental conditions. Because Linear Technology's high current LED drivers are current mode regulators, they do not directly modulate the duty cycle of the power switch. Instead, the feedback loop controls the peak switching current for each cycle. Compared to voltage mode control, current mode control improves loop dynamics and achieves cycle-by-cycle current limit.

Two LED applications

As shown in Figure 2, many embedded high current LED applications consist of a single or two high current (ILED from 1A to 1.5A) LEDs. Such applications include interior lighting such as ceiling lights, map lights, trunk lighting, and exterior lighting such as sills or "puddle" lights. Depending on the application, colored LEDs may be used for instrument panel illumination, or white LEDs for general illumination. Because these LEDs typically have a forward voltage of 3V to 4V and are powered by a 12V to 14V automotive bus, a buck regulator is required. The LT3475 dual step-down LED driver is ideal for this type of application.

The LT3475 is a dual 36V, 2MHz step-down DC/DC converter used as a dual constant current LED driver (see Figure 3). Each channel has an internal sense resistor and dimming control, making it ideal for driving LEDs that require up to 1.5A. The two channels are switched with a phase difference of 180o, which reduces the output ripple of both channels. Each channel maintains high output current accuracy independently over a wide current range of 50mA to 1.5A, while the unique True Color PWMTM circuit achieves a 3000:1 dimming range without the colors normally associated with LED current dimming change. With a wide input voltage range of 4V to 36V (transient up to 40V), the LT3475 is well suited for automotive power systems. Its switching frequency can be set from 200kHz to 2MHz, allowing the use of tiny inductors and ceramic capacitors while keeping the switching noise out of the AM radio band. The device is packaged in a thermally enhanced TSSOP-20 package and provides a very compact solution for driving high current LEDs.

The LT3475 uses a high-side side detection method to achieve a grounded LED cathode connection, so no ground wire is required in most applications. Each channel also features an integrated boost diode that further reduces solution footprint and cost. Other features include LED open and short circuit protection.

Figure 5: Schematic of the LT3475 dual 1.5A step-down LED driver and associated efficiency curves.

Conclusion In current and future vehicles, LED lighting applications will accelerate at an unprecedented rate, which has led to many very specific performance requirements for LED driver ICs in high current LED automotive applications. These applications range from headlights to interior lighting. In addition, there are many LED applications in a large number of commercial and industrial environments outside the automotive market, and most of the high performance requirements in automotive applications are also required in these applications. These LED drivers must provide a constant current to maintain consistent brightness as the input voltage or LED forward voltage changes, and must operate at high efficiency. They must also be able to withstand the harsh electrical characteristics of the automotive power bus. These applications also require very compact, high thermal efficiency solutions. Linear Technology has noted these automotive design needs and has developed a range of high current LED driver products to meet the needs of automotive and a variety of general purpose LED lighting applications. Lighting system designers now have a very convenient source of LED drivers for challenging lighting designs.

Editor: China Lighting Network - Wheat