1、附錄AA matter of light：PWM dimming By Sameh Sarhan and Chris Richardson, National Semiconductor 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 n

2、umber 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 (d

3、igital 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

4、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

5、 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, esp

6、ecially 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 l

7、ow 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 an

8、d 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, a

9、nd 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

10、 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 maint

11、ain 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 ul

12、timately 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 d

13、imming 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

14、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

15、the dimming frequency, fDIM, the higher contrast ratio, as these fixed delays consume a smaller portion of the dimming period, TDIM.The 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 th

16、e 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-spe

17、ed 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 switchi

18、ng 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 appli

19、ed, 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 min

20、imize 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

21、 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 i

22、s 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

23、 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

24、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 i

25、mpedance 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

26、 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

27、 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 cu

28、rrent, 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

29、 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 continu

30、es 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,

31、 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 b

32、oost 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

33、 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 di

34、mming 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

35、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

36、 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.

37、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

38、 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

39、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 w

40、ith 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 (Egyp

41、t). 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 semi

42、nars. 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 Poly

43、technic Institute and State University. Source: National Semiconductor Corporation附錄BLED照明知識：PWM調光不管你用Buck, Boost, Buck-Boost還是線性調節器來驅動LED，它們的共同思路都是用驅動電路來控制光的輸出。一些應用只是簡單地來實現“開”和“關”地功能，但是更多地應用需求是要從0到100%調節光的亮度，而且經常要有很高的精度。設計者主要有兩個選擇：線性調節LED電流（模擬調光），或者使用開關電路以相對于人眼識別力來說足夠高的頻率工作來改變光輸出的平均值（數字調光）。使用脈沖寬度調制

44、（PWM）來設置周期和占空度（圖1）可能是最簡單的實現數字調光的方法，并且Buck調節器拓撲往往能夠提供一個最好的性能。 圖1：使用PWM調光的LED驅動及其波形。推薦的PWM調光 模擬調光通常可以很簡單的來實現。我們可以通過一個控制電壓來成比例地改變LED驅動的輸出。模擬調光不會引入潛在的電磁兼容/電磁干擾（EMC/EMI）頻率。然而，在大多數設計中要使用PWM調光，這是由于LED的一個基本性質：發射光的特性要隨著平均驅動電流而偏移。對于單色LED來說，其主波長會改變。對白光LED來說，其相關顏色溫度（CCT）會改變。對于人眼來說，很難察覺到紅、綠或藍LED中幾納米波長的變化，特別是在光強也

45、在變化的時候。但是白光的顏色溫度變化是很容易檢測的。 大多數LED包含一個發射藍光譜光子的區域，它透過一個磷面提供一個寬幅可見光。低電流的時候，磷光占主導，光趨近于黃色。高電流的時候，LED藍光占主導，光呈現藍色，從而達到了一個高CCT。當使用一個以上的白光LED的時候，相鄰LED的CCT的不同會很明顯也是不希望發生的。同樣延伸到光源應用里，混合多個單色LED也會存在同樣的問題。當我們使用一個以上的光源的時候，LED中任何的差異都會被察覺到。 LED生產商在他們的產品電氣特性表中特別制定了一個驅動電流，這樣就能保證只以這些特定驅動電流來產生的光波長或CCT。用PWM調光保證了LED發出設計者需

46、要的顏色，而光的強度另當別論。這種精細控制在RGB應用中特別重要，以混合不同顏色的光來產生白光。 從驅動IC的前景來看，模擬調光面臨著一個嚴峻的挑戰，這就是輸出電流精度。幾乎每個LED驅動都要用到某種串聯電阻來辨別電流。電流辨別電壓（VSNS）通過折衷低能耗損失和高信噪比來選定。驅動中的容差、偏移和延遲導致了一個相對固定的誤差。要在一個閉環系統中降低輸出電流就必須降低VSNS。這樣就會反過來降低輸出電流的精度，最終，輸出電流無法指定、控制或保證。通常來說，相對于模擬調光，PWM調光可以提高精度，線性控制光輸出到更低級。 調光頻率VS對比度 LED驅動對PWM調光信號的不可忽視的回應時間產生了一

47、個設計問題。這里主要有三種主要延遲（圖2）。這些延遲越長，可以達到的對比度就越低（光強的控制尺度）。 圖2：調光延遲。如圖所示，tn表示從時間邏輯信號VDIM提升到足以使LED驅動開始提高輸出電流的時候的過渡延遲。另外，tsu輸出電流從零提升到目標級所需要的時間，相反，tsn是輸出電流從目標級下降到零所需要的時間。一般來說，調光頻率（fDIM）越低，對比度越高，這是因為這些固定延遲消耗了一小部分的調光周期（TDIM）。fDIM的下限大概是120Hz，低于這個下限，肉眼就不會再把脈沖混合成一個感覺起來持續的光。另外，上限是由達到最小對比度來確定的。 對比度通常由最小脈寬值的倒數來表示： CR =

48、 1 / tON-MIN : 1 這里tON-MIN = tD + tSU。在機器視覺和工業檢驗應用中常常需要更高的PWM調光頻率，因為高速相機和傳感器需要遠遠快于人眼的反應時間。在這種應用中，LED光源的快速開通和關閉的目的不是為了降低輸出光的平均強度，而是為了使輸出光與傳感器和相機時間同步。用開關調節器調光 基于開關調節器的LED驅動需要一些特別考慮，以便于每秒鐘關掉和開啟成百上千次。用于通常供電的調節器常常有一個開啟或關掉針腳來供邏輯電平PWM信號連接，但是與此相關的延遲（tD）常常很久。這是因為硅設計強調回應時間中的低關斷電流。而驅動LED的專用開關調節則相反，當開啟針腳為邏輯低以最小化tD時，內部控制電路始終保持開啟，然而當LED關斷的時候，控制電流卻很高。 用PWM來優化光源控制需要最小化上升和下降延遲，這不僅是為了達到最好的對比度，而且也為了最小化LED從零到目標電平的時間（這里主導光波長和CCT不能保證）。標準開關調節器常常會有一個緩開和緩關的過程，但是LED專用驅動可以做所有的事情，其中包括降低信號轉換速率的控制。降低tSU 和 tSN要從硅設計和開關調節器拓撲兩方面入