无需外部开关的高功率LED驱动器

无需外部开关的高功率LED驱动器

　　随着最近更高功率及效率的新LED出现，LED被迅速应用到新的领域，例如手电筒及车载设备中。高功率LED甚至被应用到了长期由白炽灯和荧光灯占据的环境照明领域中。为高功率LED供电，最好方法是电流源。因为绝大部分能源，包括电池、发电机和工业干线都是电压源，而很少有电流源。于是就需要在LED和供电电源中间插入一些电路。这个电路像串联电阻一样简单，但是考虑到能源效率和其他因素，最好的选择是高效率带电压反馈的电流源。由于LED电流大于0.35A，因此电感式继电器通常是最好选择。

　　为了达到高效率和小型化的目标，基于单功率IC继电器衍生出了一系列的电路。电路设计者通过减少使用体积较大的部件，如外部晶体管，开关，大容量电容以及限流电阻等，并在维持正常运作的情形下尽可能将高亮度的灯光传递地越远越好，以此来达到高效及小型化的目标。

　　图1、图2、图3中的电路均适合应用在由三到四块镍氢电池或镍镉电池组成的电源中。图4和图5中的电路适合于供电系统线电压为12V、24V或是42V的车辆中。图4和图5的电路也可以应用在包括控制和应急子系统的24V电源和通信所用的－48V电源的工业系统中。

　　这些电路的设计者们在设计时都有同样的观念：完整的单一模式集成电路继电器和微功率运放。运放将最终的1.25V反馈到集成电路上。虽然节点都是以标准电压拓扑为目标，但是运放可以使其与微弱的电流敏感电压及有些许不同的电流拓扑结构相匹配。所有的这些电路都不需要外部的功率开关。这种设计去除了通常在开关电源附近的大滤波电容，因为没有必要对LED电流的高频谐波进行平滑处理。所有电路最普遍的选择是在运放的输入端引入可调偏置，依靠IC通过一个电阻和一个由内部调节电位计，来增加亮度调节能力。

　　高频开关调节器给LED的基本调节电路供电。如图1所示电路，输入电压范围为3.6V 到6.5V，可以提供高达1A的电流驱动LED，并用一个电流敏感电阻来控制电流调节闭环回路。图2中所示电路与图1中比较类似，但是电流敏感电阻被电感的寄生电阻代替。与图1中电路功能相同，图2电路也可以将3.6V 到6.5V的输入电压转换成驱动LED高达1A的电流。

　　对图3的单LED电路，MAX1685的启动电源决定了输入范围，最低到2.7V。相对于图1和图2中的1A电路而言，最大电流能力为0.5A。输入电压上限仍为6.5V。一旦图3电路开始运行，即使输入电压降到1.7V，仍可以驱动LED。以上三种电路可以应用在由碱性电池、三或四块镍氢/镍镉电池、锂电池驱动的前灯、手电筒和其它便携式灯光设备中。

　　图4和图5中电路适用于输入电压为8V到50V。假定一个12V系统中的所有部件都完全确定，由于集成电路输入端电压VIN最高可以达到76V，因此这两个电路有负载抑制。如果将输入电压的最小值提高到11.5V，那么最大输出电流为1A，最多可以驱动串连的三个LED。图4与图5中的电路很相似，除了图5中用电感作为电流敏感元件。这样的不利之处是由于铜的温度系数较大，造成输出电流对环境的依赖性很大。电感线圈是由铜缠绕而成的，外界温度变化1°C，它的直流阻抗就会变化千分之3.9。因此，当外界温度变化10°C的时候，输出电流就会减少大约4%。

　　英文原文：

　　High-power LED drivers require no external switches

　　Suiting a variety of applications, these circuits transform a switching regulator into a current source for driving power LEDs.

　　Alfredo H Saab and Steve Logan, Maxim Integrated Products, Sunnyvale, CA; Edited by Charles H Small and Fran Granville -- EDN, 7/19/2007

　　As the latest generation of new LEDs achieves higher levels of power and efficiency, use of these devices extends to new areas, such as flashlights and vehicular applications. High-power LEDs are finding use even in ambient lighting, long the sole province of incandescent bulbs and fluorescent tubes. A current source is the best way to power LEDs. Because most energy sources, including batteries, generators, and industrial mains, look more like voltage sources than current sources, LEDs require that you insert some electronic circuitry between them and the source of power. This circuitry can be as simple as a series resistor, but a better choice, considering energy efficiency and other factors, is a high-efficiency, voltage-fed current source. For LEDs with currents greater than 0.35A, an i
nductive switching regulator is usually the best choice.

　　This Design Idea presents a series of circuits based on single-power-IC switching regulators, with efficiency and miniaturization as the main objectives. The circuits’ designers approach these objectives by minimizing the use of large components, such as external power transistors, switches, high-value capacitors, and current-sense resistors, and by maintaining regular operation by delivering constant, high-intensity light over as extended a range as possible.

　　The circuits in figure 1, figure 2, and figure 3 are suitable for applications in which the power source comprises three or four alkaline, NiMH (nickel-metal-hydride), or NiCd (nickel-cadmium) cells. Those in figure 4 and figure 5 are for vehicular applications in which the nominal line voltage for the power-distribution system is 12, 24, or 42V. The circuits of figure 4 and figure 5 are also useful in industrial systems that include a 24V distribution line for control and emergency subsystems and in telecom applications for which the system power is distributed as a –48V line.

　　The designers of these circuits based them on the same concept: a fully integrated, single-die-IC switching regulator and a micropower operational amplifier. The op amp drives the 1.25V feedback terminal on the IC. Although that node targets the topology of a standard voltage regulator, the op amp matches it to the much smaller current-sense voltage and the slightly different topology of a current regulator. None of the circuits requires the use of external power switches. The design eliminates the use of the large-valued filter capacitors you usually find in a switching regulator, because there is no need to smooth out high-frequency ripple in the LED current. Common to all circuits is the option of adding a dimming capability by introducing adjustable bias at an op-amp input through a resistor and a potentiometer powered from the internal regulator—the VD or CVL terminal, depending on the IC.

　　A high-frequency switching regulator powers the basic regulator circuit for LEDs (Figure 1). It operates with input voltages of 3.6 to 6.5V, drives a single LED with currents as high as 1A, and uses a current-sense resistor to control the current-regulation loop. The circuit of Figure 2 is similar, but, in place of a current-sense resistor, it employs the parasitic resistance of the inductor as a current-sensing element. Like the circuit in Figure 1, it operates with 3.6 to 6.5V inputs and drives one LED with currents as high as 1A.

　　For the single-LED circuit of Figure 3, the starting voltage of the MAX1685 defines the input range, which goes as low as 2.7V. Its maximum current capability is 0.5A versus 1A for the circuits in figure 1 and figure 2. The upper operating limit remains 6.5V. Once this circuit is operating, it maintains power to the LED even for input voltages as low as 1.7V. Applications for the circuits of figure 1, figure 2, and figure 3 include headlights, flashlights, and any other portable lights powered by three or four alkaline primary cells, three or four NiMH/NiCd secondary cells, or a single lithium secondary cell.

　　The circuits of figure 4 and figure 5 operate over 8 to 50V. Assuming a 12V system in which all the components are properly specified, these circuits can survive load dumps, thanks to the 76V absolute maximum rating for the IC’s input-power terminal, VIN. The maximum available current is 1A, and the circuits can drive as many as three LEDs in series, provided that you increase the lower limit of the operating range to 11.5V. These two circuits are similar, except for the use of the inductor resistance as a current sensor in Figure 5. The disadvantage of using the inductor resistance in this way is the resulting dependence of output current on temperature, due to the large temperature coefficient of copper resistivity. The inductor winding is made of copper, and its dc resistance has a first-order temperature coefficient of 3.9 parts/1000/°C. As a result, the regulated current decreases about 4% for each 10°C increase in operating temperature.

　　英文原文地址：http://www.edn.com/article/CA6459061.html

来源：EDN/作者：Alfredo H Saab and Steve Logan, Maxim Integrated Products, Sunnyvale, CA