Text: Jeff Gruetter
(The author of this article works at Linear Technology)
The market for high-brightness light-emitting diodes (HB LEDs ) will grow at an alarming rate. According to French market research firm Yole Developpement, the overall LED market size will reach 10.3 billion US dollars in 2012, of which high-brightness and ultra-high-brightness LEDs are about It accounted for $4.45 billion, almost 5.5 times the market size of $783 million in 2007 (based on packaged LEDs).
What is the power that drives its growth potential? First of all, the luminous efficiency of LEDs is ten times that of incandescent bulbs and twice that of fluorescent lamps. Therefore, when a given amount of light output (in lumens) is provided, the amount of electrical energy and heat generated by it is greatly reduced; With the development of LEDs, the efficiency of its luminous flux from electrical energy will continue to rise. Secondly, LED lighting is not required to operate, expose and handle the toxic mercury vapors commonly found in fluorescent bulbs. The third is that incandescent bulbs need to be replaced every 1,000 hours. Fluorescent bulbs last for 10,000 hours, while LEDs last for more than 100,000 hours, allowing LEDs to be permanently embedded in the final product or system without the need for a certain Kind of fixture.
LED achieves high contrast ratio / high resolution
Examples of applications include automotive body panels or liquid crystal displays (LCDs) in high definition televisions (HDTVs), as they will never need to be replaced during the life of a car or HDTV. In addition, LEDs are smaller in area and height than comparable alternatives, and can be fabricated into a very flat profile that can be permanently embedded in automotive interior and exterior devices and flat consumer electronics. In addition, an unlimited number of colored lights are available through the red, green and blue LED configurations. The dimming and turn-on/turn-off operation of LEDs is far faster than the human eye's resolution, resulting in dramatic improvements in backlighting applications such as HDTV and other types of displays. High-contrast and high-resolution HDTV will not be possible without LEDs.
However, one of the biggest challenges facing lighting system designers is how to optimize the advantages of the latest generation of LEDs? Since LEDs typically require an accurate and efficient direct current (DC) current source and a dimming method, LED driver ICs must be designed to meet these requirements in a wide range of applications. Power solutions must be efficient, robust, small, and cost effective. Therefore, for LED driver ICs, backlighting applications for automotive applications and large HDTV liquid crystal displays are two examples of many demanding applications.
Due to the comprehensive advantages of LED in automotive applications, various forms of automotive lighting have been fully adopted, from the headlights to the backlight of the instrument panel/navigation device and the general interior/external lighting of the body. HDTV backlighting applications traditionally offered by cold cathode tubes (CCFLs) are increasingly being replaced by high-intensity LED arrays that can be dimmed with separate LED strings for precise local dimming. Compared to traditional HDTVs with CCFL backlights, the contrast ratio will be increased by one level. Thanks to the local dimming capability and ultra-fast response time, real-time adjustment of LED brightness can be achieved, thus solving the problem of moving image blur inherent in CCFL backlighting HDTV.
Automotive lighting applications are gradually adopting LEDs
High-brightness LEDs are widely used in automobiles because of their small size, low power consumption, and fast turn-on time. The initial application of LEDs in automobiles was the Central Overhead Parking Light (CHMSL); this application uses a red LED to provide a very flat array of illumination that is easy to install and never needs to be replaced.
Traditionally, incandescent bulbs are the most economical source of light and are still used by many cars. However, as the available lighting space shrinks and the life expectancy of lighting sources continues to increase, in many applications, the color and compact design of LEDs is rapidly replacing incandescent bulbs. Traditional CCFL backlights in infotainment systems have been replaced by white LED arrays, which provide a more accurate and adjustable backlight that easily exceeds the life of the car.
In addition, people are still developing headlights using a "steerable" high current LED array. This field has always been monopolized by halogen/æ°™ filament design. Almost all automotive lighting applications, including interior/external lighting and backlighting applications, will gradually adopt LEDs. The benefits of using LEDs in this environment have many positive implications. First, LEDs never need to be replaced because their solid-state life of more than 100,000 hours (11.5 years of use) is longer than the life of the car. This allows automakers to permanently embed them in the "in-cabin" lighting system without the need to replace the entrance to the filament bulb. Since the LED lighting system does not require the installation depth or area required for incandescent bulbs, it can also make a noticeable change in the shape of the car.
Figure 1 shows the recently launched Lexus LS600h LED headlights. Audi's R8 and General Motors' (GM) Escalade also have similar options. The overall lighting configuration of all of these vehicles is similar. Each headlight unit contains five LED beams optimized for all lighting requirements, including low beam, high beam, curved light, daytime running lights and directional lights, all supplied by LEDs. A standard beam will typically require 35 to 50 watts of power. It doesn't sound too much, but the luminous flux provided by LEDs is ten times that of halogen lamps. Therefore, the light output of LEDs is equivalent to a 500-watt halogen lamp. The power required for the high beam is the same or slightly higher than the standard beam, while the cornering light, the daytime running light and the steering signal require less power. These beams can be driven with a single HB LED driver, and since power consumption can exceed 200 watts, the importance of using high efficiency LED drivers with minimal heat generation can be seen.
Figure 1 HB LED Lexus LS600h headlights / direction lights / headlights
LED driver circuit improves product performance
To ensure optimum performance and long operating life, LEDs require an efficient drive circuit. These drive circuits must be able to draw operating power from the rather demanding automotive power bus, but also cost and space "efficiency." In order to maintain its long working life, the current and temperature limits of the LED must not be exceeded.
Most headlamp applications require approximately 50 watts of LED current. Designed for this application, manufacturers can boost the automotive bus voltage (nominally 12 volts) up to 60 volts to drive up to fourteen amps of 1 amp LEDs in series (Figure 2).
Figure 2 50 watt headlamp circuit diagram with LED driver solution
Figure 3 shows that an LED driver solution can be as efficient as 93%, eliminating the need to dissipate any power components, resulting in a very small pin footprint. Although the circuit in Figure 2 is a boost mode architecture, it uses a unique high-side current sense design that allows it to be configured for boost, buck mode, buck-boost mode, or flyback depending on the application's specific requirements. Architecture.
Figure 3 Figure 2 Efficiency curve of the circuit shown in Figure 2.
The solution is designed to drive a low-voltage side external N-channel MOSFET from an internal regulated 7 volt supply. The fixed frequency, current mode architecture provides stable and accurate operation over a wide range of power and output voltages. If the LED driver IC wants to achieve constant LED brightness with unstable input voltage, a constant current source must be provided in the design. This is especially important for automotive applications because the input voltage can experience large swings during transients such as cold start and load dump, so the maximum input voltage requirements of the associated solution, even if the main automotive bus is subjected to transient voltages. In this case, it is also necessary to adjust the LED current and voltage.
In addition, a voltage feedback pin that references ground potential can act as an input to numerous LED protection functions (such as open-circuit LED protection) and cause the converter to generate a voltage source. A frequency adjustment pin allows the user to set the frequency from 100k to 1MHz to optimize efficiency and performance while minimizing external component size.
Currently, the True Color Pulse Width Modulation (PWM) dimming function in the relevant manufacturers' designs can achieve a dimming ratio of up to 3,000:1, while the color of the emitted light does not change (Figure 4), thus enabling the PWM duty cycle to be utilized. LED headlamps are constantly adjusted to suit various environmental conditions. Since Linear Technology's high current LED drivers are current mode regulators, they do not directly regulate the duty cycle of the power switch. Instead, the feedback loop is responsible for controlling the peak current in the switch during each cycle. Compared to voltage mode control, current mode control improves loop dynamics and provides cycle-by-cycle current limiting.
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