用开关调节器调光
基于开关调节器的LED驱动需要一些特别考虑，以便于每秒钟关掉和开启成百上千次。用于通常供电的调节器常常有一个开启或关掉针脚来供逻辑电平PWM信号连接，但是与此相关的延迟（tD）常常很久。这是因为硅设计强调回应时间中的低关断电流。而驱动LED的专用开关调节则相反，当开启针脚为逻辑低以最小化tD时，内部控制电路始终保持开启，然而当LED关断的时候，控制电流却很高。
用PWM来优化光源控制需要最小化上升和下降延迟，这不仅是为了达到最好的对比度，而且也为了最小化LED从零到目标电平的时间（这里主导光波长和CCT不能保证）。标准开关调节器常常会有一个缓开和缓关的过程，但是LED专用驱动可以做所有的事情，其中包括降低信号转换速率的控制。降低tSU 和 tSN要从硅设计和开关调节器拓扑两方面入手。
Buck调节器能够保持快速信号转换而又优于所有其它开关拓扑主要有两个原因。其一，Buck调节器是唯一能够在控制开关打开的时候为输出供电的开关变换器。这使电压模式或电流模式PWM（不要与PWM调光混淆）的Buck调节器的控制环比Boost调节器或者各种Buck-Boost拓扑更快。控制开关开启的过程中，电力传输同样可以轻易地适应滞环控制，甚至比最好的电压模式或电流模式的控制环还要快。其二，Buck调节器的电导在整个转换周期中连在了输出上。这样保证了一个持续输出电流，也就是说，输出电容被删减掉。没有了输出电容，Buck调节器成了一个真正的高阻抗电流源，它可以很快达到输出电压。Cuk和zeta转换器可以提供持续的输出电感，但是当更慢的控制环（和慢频）被纳入其中的时候，它们会落后。
比开启针脚更快
即使是一个单纯的无输出电容的滞后Buck调节器，也不能满足某些PWM调光系统的需要。这些应用需要高PWM调光频率和高对比度，这就分别需要快速信号转换率和短延迟时间。对于机器视觉和工业检验来说，系统实例需要很高的性能，包括LCD板的背光和投影仪。在某些应用中，PWM调光频率必须超过音频宽，达到25kHz或者更高。当总调光周期降低到微秒级时，LED电流总上升和下降时间（包括传输延迟），必须降低到纳秒级。
让我们来看看一个没有输出电容的快速Buck调节器。打开和关断输出电流的延迟来源于IC的传输延迟和输出电感的物理性质。对于真正的高速PWM调光，这两个问题都需要解决。最好的方法就是要用一个电源开关与LED链并联（图3）。要关掉LED，驱动电流要经过开关分流，这个开关就是一个典型的n-MOSFET。IC持续工作，电感电流持续流动。这个方法的主要缺点是当LED关闭的时候，电量被浪费掉了，甚至在这个过程中，输出电压下降到电流侦测电压。


图3：分流电路及其波形。
用一个分流FET调光会引起输出电压快速偏移，IC的控制环必须回应保持常电流的请求。就像逻辑针脚调光一样，控制环越快，回应越好，带有滞环控制的Buck调节器就会提供最好的回应。
用Boost和Buck-Boost的快速PWM
Boost调节器和任何Buck-Boost拓扑都不适合PWM调光。这是因为在持续传导模式中（CCM），每个调节器都展示了一个右半平面零，这就使它很难达到时钟调节器需要的高控制环带宽。右半平面零的时域效应也使它更难在Boost或者Buck-Boost电路中使用滞后控制。另外，Boost调节器不允许输出电压下降到输入电压以下。这个条件需要一个输入端短电路并且使利用一个并联FET实现调光变得不可能。。在Buck-Boost拓扑中，并联FET调光仍然不可能或者不切实际，这是因为它需要一个输出电容（SEPIC，Buck-Boost和flyback），或者输出短电路（Cuk和zeta）中的未受控制得输入电感电流。当需要真正快速PWM调光的时候，最好的解决方案是一个二级系统，它利用一个Buck调节器作为第二LED驱动级。如果空间和成本不允许的时候，下一个最好的原则就是一个串联开关（图4）。


图4：带有串联DIM开关的Boost调节器。
LED电流可以被立即切断。另外，必须要特别考虑系统回应。这样一个开路事实上是一个快速外部退荷暂态，它断开了反馈环，引起了调节器输出电压的无界上升。为了避免因为过压失败，我们需要输出钳制电路和/或误差放大器。这种钳制电路很难用外部电路实现，因此，串联FET调光只能用专用Boost/Buck-Boost LED驱动IC来实现。
总而言之，LED光源的单纯控制需要设计的初始阶段就要非常小心。光源越复杂，就越要用PWM调光。这就需要系统设计者谨慎思考LED驱动拓扑。Buck调节器为PWM调光提供了很多优势。如果调光频率必须很高或者信号转换率必须很快，或者二者都需要，那么Buck调节器就是最好的选择。

A matter of light, Part 4 --- PWM dimming
By Sameh Sarhan and Chris Richardson, National Semiconductor
In part one of this series, we looked at the basics of LED lighting sources and their driving requirements. In part two, we discussed why a constant-current buck converter should be your first preference when it comes to switch-mode LED drivers. In part 3, we investigated larger LED displays and the applications space for other converter topologies. Here in the concluding part of this series, the authors take a look at how to best implement the dimming function.
Whether you drive LEDs with a buck, boost, buck-boost or linear regulator, the common thread is drive circuitry to control the light output. A few applications are as simple as ON and OFF, but the greater number of applications call for dimming the output between zero and 100 percent, often with fine resolution. The designer has two main choices: adjust the LED current linearly (analog dimming), or use switching circuitry that works at a frequency high enough for the eye to average the light output (digital dimming). Using pulse-width modulation (PWM) to set the period and duty cycle (Fig. 1) is perhaps the easiest way to accomplish digital dimming, and a buck regulator topology will often provide the best performance.


Figure 1: LED driver using PWM dimming, with waveforms.
PWM dimming preferred
Analog dimming is often simpler to implement. We vary the output of the LED driver in proportion to a control voltage. Analog dimming introduces no new frequencies as potential sources of EMC/EMI. However, PWM dimming is used in most designs, owing to a fundamental property of LEDs: the character of the light emitted shifts in proportion to the average drive current. For monochromatic LEDs, the dominant wavelength changes. For white LEDs, the correlated color temperature (CCT) changes. It's difficult for the human eye to detect a change of a few nanometers in a red, green, or blue LED, especially when the light intensity is also changing. A change in color temperature of white light, however, is easily detected.
Most white LEDs consist of a die that emits photons in the blue spectrum, which strike a phosphor coating that in turn emits photons over a broad range of visible light. At low currents the phosphor dominates and the light tends to be more yellow. At high currents the blue emission of the LED dominates, giving the light a blue cast, leading to a higher CCT. In applications with more than one white LED, a difference in CCT between two adjacent LEDs can be both obvious and unpleasant. That concept extends to light sources that blend light from multiple monochromatic LEDs. When we have more than one light source, any difference between them jars the senses.
LED manufacturers specify a certain drive current in the electrical characteristics tables of their products, and they guarantee the dominant wavelength or CCT only at those specified currents. Dimming with PWM ensures that the LEDs emit the color that the lighting designer needs, regardless of the intensity. Such precise control is particularly important in RGB applications where we blend light of different colors to produce white.
From the driver IC perspective, analog dimming presents a serious challenge to the output current accuracy. Almost every LED driver uses a resistor of some type in series with the output to sense current. The current-sense voltage, VSNS, is selected as a compromise to maintain low power dissipation while keeping a high signal-to-noise ratio (SNR). Tolerances, offsets, and delays in the driver introduce an error that remains relatively fixed. To reduce output current in a closed-loop system, VSNS, must be reduced. That in turn reduces the output current accuracy and ultimately the output current cannot be specified, controlled, or guaranteed. In general, dimming with PWM allows more accurate, linear control over the light output down to much lower levels than analog dimming.
Dimming frequency vs. contrast ratio
The LED driver's finite response time to a PWM dimming signal creates design issues. There are three main types of delay (Fig. 2). The longer these delays, the lower the achievable contrast ratio (a measure of control over lighting intensity).


Figure 2: Dimming delays.
As shown, tn represents the propagation delay from the time logic signal VDIM goes high to the time that the LED driver begins to increase the output current. In addition, tsu is the time needed for the output current to slew from zero to the target level, and tsn is the time needed for the output current to slew from the target level back down to zero. In general, the lower the dimming frequency, fDIM, the higher contrast ratio, as these fixed delays consume a smaller portion of the dimming period, XXXXXXXe lower limit for fDIM is approximately 120 Hz, below which the eye no longer blends the pulses into a perceived continuous light. The upper limit is determined by the minimum contrast ratio that is required.
Contrast ratio is typically expressed as the inverse of the minimum on-time, i.e.,
CR = 1 / tON-MIN : 1
where tON-MIN = tD + tSU. Applications in machine vision and industrial inspection often require much higher PWM dimming frequencies because the high-speed cameras and sensors used respond much more quickly than the human eye. In such applications the goal of rapid turn-on and turn-off of the LED light source is not to reduce the average light output, but to synchronize the light output with the sensor or camera capture times.
Dimming with a switching regulator
Switching regulator-based LED drivers require special consideration in order to be shut off and turned on at hundreds or thousands of times per second. Regulators designed for standard power supplies often have an enable pin or shutdown pin to which a logic-level PWM signal can be applied, but the associated delay, tD, is often quite long. This is because the silicon design emphasizes low shutdown current over response time. Dedicated switching regulations for driving LEDs will do the opposite, keeping their internal control circuits active while the enable pin is logic low to minimize tD, while suffering a higher operating current while the LEDs are off.
Optimizing light control with PWM requires minimum slew-up and slew-down delays not only for best contrast ratio, but to minimize the time that the LED spends between zero and the target level (where the dominant wavelength and CCT are not guaranteed). A standard switching regulator will have a soft-start and often a soft-shutdown, but dedicated LED drivers do everything within their control to reduce these slew rates. Reducing tSU and tSN involves both the silicon design and the topology of switching regulator that is used.
Buck regulators are superior to all other switching topologies with respect to fast slew rates for two distinct reasons. First, the buck regulator is the only switching converter that delivers power to the output while the control switch is on. This makes the control loops of buck regulators with voltage-mode or current-mode PWM (not to be confused with the dimming via PWM) faster than the boost regulator or the various buck-boost topologies. Power delivery during the control switch's on-time also adapts easily to hysteretic control, which is even faster than the best voltage-mode or current-mode control loops. Second, the buck regulator's inductor is connected to the output during the entire switching cycle. This ensures a continuous output current and means that the output capacitor can be eliminated. Without an output capacitor the buck regulator becomes a true, high impedance current source, capable of slewing the output voltage very quickly. Cuk and zeta converters can claim continuous output inductors, but fall behind when their slower control loops (and lower efficiency) are factored in.
Faster than the enable pin
Even a pure hysteretic buck regulator without an output capacitor will not be capable of meeting the requirements of some PWM dimming systems. These applications need high PWM dimming frequency and high contrast ratio, which in turn requires fast slew rates and short delay times. Along with machine vision and industrial inspection, examples of systems that need high performance include backlighting of LCD panels and video projection. In some cases the PWM dimming frequency must be pushed to beyond the audio band, to 25 kHz or more. With the total dimming period reduced to a matter of microseconds, total rise and fall times for the LED current, including propagation delays, must be reduced to the nanosecond range.
Consider a fast buck regulator with no output capacitor. The delays in turning the output current on and off come from the IC's propagation delay and the physical properties of the output inductor. For truly high speed PWM dimming, both must be bypassed. The best way to accomplish this is by using a power switch in parallel with the LED chain (Fig. 3). To turn the LEDs off, the drive current is shunted through the switch, which is typically an n-MOSFET. The IC continues to operate and the inductor current continues to flow. The main disadvantage of this method is that power is wasted while the LEDs are off, even through the output voltage drops to equal the current sense voltage during this time.


Figure 3: Shunt FET circuit, with waveforms.
Dimming with a shunt FET causes rapid shifts in the output voltage, to which the IC's control loop must respond in an attempt to keep the output current constant. As with logic-pin dimming, the faster the control loop, the better the response, and buck regulators with hysteretic control provide the best response.
Fast PWM with boost and buck-boost
Neither the boost regulator nor any of the buck-boost topologies are well suited to PWM dimming. That's because in the continuous conduction mode (CCM), each one exhibits a right-half plane zero, which makes it difficult to achieve the high control loop bandwidth needed in clocked regulators. The time-domain effects of the right-half plane zero also make it much more difficult to use hysteretic control for boost or buck-boost circuits. In addition, the boost regulator cannot tolerate an output voltage that falls below the input voltage. Such a condition causes a short circuit at the input, and makes dimming with a parallel FET impossible. Among the buck-boost topologies, parallel FET dimming is still impossible or at best impractical due to the requirement for an output capacitor (the SEPIC, buck-boost and flyback), or the uncontrolled input inductor current during output short circuits (Cuk and zeta). When true fast PWM dimming is required, the best solution is a two-stage system that uses a buck regulator as the second, LED driving stage. When space and cost do not permit this approach, the next best choice is a series switch (Fig. 4).


Figure 4: Boost regulator with series DIM switch.
LED current can be shut off immediately. On the other hand, special consideration must be given to the system response. Such an open circuit is in effect a fast, extreme unloading transient that also disconnects the feedback loop and will cause the regulator's output voltage to rise without bound. Clamping circuits for the output and/or the error amplifier are required to prevent failure due to over-voltage. These clamps are difficult to realize with external circuitry, hence series FET dimming is practical only with dedicated boost/buck-boost LED driver ICs.
In summary, proper control of LED lighting requires careful attention right from the start of the design process. The more sophisticated the light source, the more likely that PWM dimming will be used. This in turn requires the system designer to carefully consider the LED driver topology. Buck regulators offer many advantages for PWM dimming. If the dimming frequency must be high, or the slew rates must be fast, or both, then the buck regulator is the way to go.
About the authors
Sameh Sarhan is a staff applications engineer for the Medium Voltage/High Voltage Power Management group in Santa Clara, CA. He has been involved with power electronics in various forms since 1998, having worked for FRC Corp. and Vicor Corp. His experience includes the design of hard/soft switching power supplies from a few watts to 600 watts. Sameh received a bachelor's degree in electronics engineering in 1996 from Cairo University (Egypt).
Chris Richardson is an applications engineer in the Power Management Products group, Medium and High Voltage Division. His responsibilities are divided between lab work, bench evaluation of new ICs, written work such as datasheets and applications notes, and training for field engineers and seminars. Since joining National Semiconductor in 2001, Chris has worked mainly on synchronous buck controllers and regulators. In the last three years he has focused on products for the emerging high brightness LED market in the automotive and industrial areas. Chris holds a BSEE from the Virginia Polytechnic Institute and State University.