Backlight and lighting applications based on high power LED driver circuits

　　With the acceleration of urban construction, leisure squares and green parks, as the highlights of the city, represent the mood of urban people eager to return to nature. Water represents spirituality, and waterscapes and fountains are an indispensable part of leisure squares and green parks, so the demand for underwater lights is considerable. At present, colored underwater lights are generally traditional PAR lamps, which require additional filters to produce various colors, greatly reducing the luminous efficiency. By comparison, it can be seen that among red, green and blue underwater lights, the total luminous flux of traditional lamps is 1~2 times that of LED underwater lights. The packaging technology of LED determines that the directionality of its light source is very strong. In addition, the size of LED is very small and can be matched with optical lenses of various angles. These characteristics determine that the power consumption of traditional underwater lights is more than 13 times that of LED underwater lights. It can be seen that the energy-saving effect of LED underwater lights is quite significant. In addition, the spectral distribution of LED determines that its light color purity and saturation are incomparable to traditional PAR lamps, so it has a strong competitive advantage.

　　For example, the LT3474 and LT3475 are high voltage, high current, single and dual channel buck LED converters with wide PWM dimming ratios that can drive one or more LEDs at currents up to 1A and 1.5A to achieve 80 lumens to 120 lumens per LED (and more when higher output LEDs become available). These dedicated LED drivers feature onboard high voltage NPN power switches and internal sense resistors to minimize board space, reduce component count, and simplify design.

　　With their high-side sense resistors, the LT3474 and LT3475 are able to drive grounded LEDs, an important advantage in many systems. Current mode control and a precise reference voltage optimize loop dynamics to achieve a well-regulated, low ripple constant LED current. The thermally enhanced exposed pad package maintains low junction temperature during high power operation in stress environments. A PWM pin uses the dimming MOSFET gate signal to extend the converter's dimming ratio by maintaining a constant output capacitor voltage and controlling the loop state during the PWM dimming off-time. Shutdown and external analog current adjust pins provide simple interfaces for greater LED light and current control flexibility in any system.

　　Single Channel Step-Down 1A LED Driver

　　The LT3474 step-down converter 1A LED driver shown in Figure 1 has features that make it suitable for automotive applications (and other battery-powered applications) or industrial applications where board space is limited, high voltages and high ambient temperatures are present. The circuit uses a high-side integrated 100mΩ sense resistor (for true LED current sensing and regulation), which is superior to the common inefficient method of biasing the LED with a constant voltage and a dissipative bias resistor. The input voltage range of 4V to 36V makes it ideal for use with protection circuits with minimal (or no) input transients in automotive, industrial and aviation applications, where long cables from the battery can cause very high input spikes.

Figure 1: The LT3474 high voltage step-down LED driver can regulate 1A current.

　　The boost NPN power switch achieves high efficiency in both 1-LED and 2-LED applications (Figure 2). The boost diode is integrated to further reduce component count. Driving the shutdown pin to ground turns off the LED, and the LED brightness is controlled by 400:1 TrueColorPWM? dimming and an external MOSFET driver or by an analog 25:1 (or filtered PWM) signal on the VADJ pin. The switching frequency can be set from 200kHz to 2MHz with an external resistor, allowing the application to be optimized for maximum efficiency or minimum component size.

Figure 2: The LT3474 step-down converter can efficiently drive single or multiple LEDs.

　　LT3474的最大输出电压被箝位于13.8V，这可以防止LT3474输出发生LED开路。短路保护是使LT3474成为一款在各种LED故障条件下均能够安然无恙成为"防弹型"转换器的最后功能。双信道降压型1.5ALED驱动器。　　图3示出了一款采用LT3475的双通道1.5A降压型转换器LED驱动器，它基本上就是把两个LT3474转换器整合在单个IC中，并增加了少量部件。对于在同一个系统中需要两个顶灯或仪表板灯的汽车应用而言，这种简单的解决方案堪称理想之选。两个灯通道（各为单个LED或一串LED）均具有单独的VADJ电压和PWM信号以实现独立的操作，但是，单个停机引脚可通过在停机模式中把电路的总电池漏电流减小至2μA，从而进一步改善节省电池电量的微功率运作。

Figure 3 LT3475 dual-channel high voltage buck LED driver adjustable 1.5A circuit

　　对于功率更高的LED或需要较高电流和较低正向电压的LED（例如：红光或淡黄光刹车和信号LED），能够以高达1.5A的电流来对两路输出中每一路进行驱动。虽然LT3475的最大输出电压和LT3474一样被箝位于13.8V的电平上，但是，其最大功率输出能力却是三倍。PWM调光比也较高（达1200：1）或利用图4中的扩展调光比电路提升至更高的比值）。PWM调光方法的改进和较低的最小调光接通时间要求有助于该IC实现极端的汽车和夜间调光水平，同时保持与100%占空比时相同的真彩色。每个通道的独立仿真VADJ调光比为30：1（50mALED电流）。为了降低内部功耗，用于每个通道的升压二极管未被集成到IC之中。

Figure 4 When added to the circuit shown in Figure 3, the LT3475’s extended dimming range circuit provides a 1200:1

PWM dimming ratio can be as high as 3000:1 at 1.4MHz frequency

　　Compared to the LT3474, the LT3475 offers three times the power output capability, the same shutdown current, the same switching frequency range, a slightly higher input voltage (36V operating voltage, 40V maximum), a higher dimming ratio, higher LED current, and only a slightly larger package (the LT3474 uses a 16-pin exposed thermal pad TSSOP package, while the LT3475 uses the same 20-pin package), making it an excellent choice for high power solutions. In addition, the anti-phase switching operation of the two channels in the LT3475 also reduces the input ripple on the power supply and limits the need for adding high voltage input capacitors.

　　in conclusion

　　The application of architectural glass in modern buildings is increasing, with more and more varieties and more and more complete functions. The application amount, application types and functions of architectural glass in buildings can be used as one of the criteria for evaluating the degree of modernization of buildings. In particular, the decorative performance of architectural glass can be said to be ever-changing. Without architectural glass, there would be no modern buildings. However, because glass can transmit light, reflect light and absorb light, the landscape lighting design that matches the building has entered a pattern.

　　How can we reproduce the texture beauty of glass during the day through light? This is a common concern of landscape lighting designers. For the lighting treatment of modern buildings with large glass panels, "architectural lighting" is used to integrate light-emitting devices (light sources) and building components, which not only deepens and expands the connotation of lighting fixtures, but also provides a variety of forms that are refreshing. High-power LEDs have high light efficiency, small unit volume, and great design flexibility. They can be matched with the structural characteristics of the light carrier to the maximum extent, and are easy to install, providing a new lighting solution for such buildings.