1、<p><b>　　附錄A</b></p><p>　　A matter of light：PWM dimming </p><p>　　By Sameh Sarhan and Chris Richardson, National Semiconductor </p><p>　　Whether you drive LEDs with

2、 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 th

3、e 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

4、 the eye to average the light out</p><p>　　Figure 1: LED driver using PWM dimming, with waveforms.</p><p>　　PWM dimming preferred </p><p>　　Analog dimming is often simpler to implem

5、ent. 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 fundame

6、ntal 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)

7、 changes. It's diff</p><p>　　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

8、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

9、 white LED, a difference in CCT between two adjacent LEDs can be both obvious and unpleasant. That concept</p><p>　　LED manufacturers specify a certain drive current in the electrical characteristics tables

10、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.

11、Such precise control is particularly important in RGB applications where we blend light of different colors to produce white. </p><p>　　From the driver IC perspective, analog dimming presents a serious chall

12、enge 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 dis

13、sipation 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</p>

14、<p>　　Dimming frequency vs. contrast ratio </p><p>　　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 th

15、ese delays, the lower the achievable contrast ratio (a measure of control over lighting intensity). </p><p>　　Figure 2: Dimming delays.</p><p>　　As shown, tn represents the propagation delay fro

16、m 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 t

17、he 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

18、dimming per</p><p>　　Contrast ratio is typically expressed as the inverse of the minimum on-time, i.e., </p><p>　　CR = 1 / tON-MIN : 1 </p><p>　　where tON-MIN = tD + tSU. Applicatio

19、ns 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

20、 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. </p><p>　　Dimming with a switchi

21、ng regulator </p><p>　　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

22、 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 cur

23、rent over response time. Dedicated switching regulations for driving LEDs will do the opposite, </p><p>　　Optimizing light control with PWM requires minimum slew-up and slew-down delays not only for best con

24、trast 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 so

25、ft-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 swi</p><p>　　Buck regulators ar

26、e 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

27、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 deliv

28、ery during the control switch's on-time al</p><p>　　Faster than the enable pin </p><p>　　Even a pure hysteretic buck regulator without an output capacitor will not be capable of meeting the

29、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

30、 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 beyo</p><p>　　Consider a fast bu

31、ck 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, bo

32、th 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-MOSF

33、ET. The IC continues to operate and the induct</p><p>　　Figure 3: Shunt FET circuit, with waveforms.</p><p>　　Dimming with a shunt FET causes rapid shifts in the output voltage, to which the IC&

34、#39;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

35、best response. </p><p>　　Fast PWM with boost and buck-boost </p><p>　　Neither the boost regulator nor any of the buck-boost topologies are well suited to PWM dimming. That's because in the c

36、ontinuous 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 plan

37、e 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 v</p><p>　　Figure 4: Boost regulator with series

38、 DIM switch.</p><p>　　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 trans

39、ient 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 ove

40、r-voltage. These clamps are difficult to realize with external circuitry, hence series FET dimming </p><p>　　In summary, proper control of LED lighting requires careful attention right from the start of the

41、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

42、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. </p><p>　　About the authors </p><p>　　Sameh

43、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.

44、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 (Egy

45、pt). </p><p>　　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 ne

46、w 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 regu

47、lators. In the last three years he has focused on products for the emerging high brightn</p><p>　　Source: National Semiconductor Corporation</p><p><b>　　附錄B</b></p><p>　

48、　LED照明知識(shí)：PWM調(diào)光</p><p>　　不管你用Buck, Boost, Buck-Boost還是線性調(diào)節(jié)器來(lái)驅(qū)動(dòng)LED，它們的共同思路都是用驅(qū)動(dòng)電路來(lái)控制光的輸出。一些應(yīng)用只是簡(jiǎn)單地來(lái)實(shí)現(xiàn)“開(kāi)”和“關(guān)”地功能，但是更多地應(yīng)用需求是要從0到100%調(diào)節(jié)光的亮度，而且經(jīng)常要有很高的精度。設(shè)計(jì)者主要有兩個(gè)選擇：線性調(diào)節(jié)LED電流（模擬調(diào)光），或者使用開(kāi)關(guān)電路以相對(duì)于人眼識(shí)別力來(lái)說(shuō)足夠高的頻率工作來(lái)改變光輸出

49、的平均值（數(shù)字調(diào)光）。使用脈沖寬度調(diào)制（PWM）來(lái)設(shè)置周期和占空度（圖1）可能是最簡(jiǎn)單的實(shí)現(xiàn)數(shù)字調(diào)光的方法，并且Buck調(diào)節(jié)器拓?fù)渫軌蛱峁┮粋€(gè)最好的性能。 </p><p>　　圖1：使用PWM調(diào)光的LED驅(qū)動(dòng)及其波形。</p><p><b>　　推薦的PWM調(diào)光 </b></p><p>　　模擬調(diào)光通常可以很簡(jiǎn)單的來(lái)實(shí)現(xiàn)。我們可以通過(guò)

50、一個(gè)控制電壓來(lái)成比例地改變LED驅(qū)動(dòng)的輸出。模擬調(diào)光不會(huì)引入潛在的電磁兼容/電磁干擾（EMC/EMI）頻率。然而，在大多數(shù)設(shè)計(jì)中要使用PWM調(diào)光，這是由于LED的一個(gè)基本性質(zhì)：發(fā)射光的特性要隨著平均驅(qū)動(dòng)電流而偏移。對(duì)于單色LED來(lái)說(shuō)，其主波長(zhǎng)會(huì)改變。對(duì)白光LED來(lái)說(shuō)，其相關(guān)顏色溫度（CCT）會(huì)改變。對(duì)于人眼來(lái)說(shuō)，很難察覺(jué)到紅、綠或藍(lán)LED中幾納米波長(zhǎng)的變化，特別是在光強(qiáng)也在變化的時(shí)候。但是白光的顏色溫度變化是很容易檢測(cè)的。 </p

51、><p>　　大多數(shù)LED包含一個(gè)發(fā)射藍(lán)光譜光子的區(qū)域，它透過(guò)一個(gè)磷面提供一個(gè)寬幅可見(jiàn)光。低電流的時(shí)候，磷光占主導(dǎo)，光趨近于黃色。高電流的時(shí)候，LED藍(lán)光占主導(dǎo)，光呈現(xiàn)藍(lán)色，從而達(dá)到了一個(gè)高CCT。當(dāng)使用一個(gè)以上的白光LED的時(shí)候，相鄰LED的CCT的不同會(huì)很明顯也是不希望發(fā)生的。同樣延伸到光源應(yīng)用里，混合多個(gè)單色LED也會(huì)存在同樣的問(wèn)題。當(dāng)我們使用一個(gè)以上的光源的時(shí)候，LED中任何的差異都會(huì)被察覺(jué)到。 </p

52、><p>　　LED生產(chǎn)商在他們的產(chǎn)品電氣特性表中特別制定了一個(gè)驅(qū)動(dòng)電流，這樣就能保證只以這些特定驅(qū)動(dòng)電流來(lái)產(chǎn)生的光波長(zhǎng)或CCT。用PWM調(diào)光保證了LED發(fā)出設(shè)計(jì)者需要的顏色，而光的強(qiáng)度另當(dāng)別論。這種精細(xì)控制在RGB應(yīng)用中特別重要，以混合不同顏色的光來(lái)產(chǎn)生白光。 </p><p>　　從驅(qū)動(dòng)IC的前景來(lái)看，模擬調(diào)光面臨著一個(gè)嚴(yán)峻的挑戰(zhàn)，這就是輸出電流精度。幾乎每個(gè)LED驅(qū)動(dòng)都要用到某種串聯(lián)電阻

53、來(lái)辨別電流。電流辨別電壓（VSNS）通過(guò)折衷低能耗損失和高信噪比來(lái)選定。驅(qū)動(dòng)中的容差、偏移和延遲導(dǎo)致了一個(gè)相對(duì)固定的誤差。要在一個(gè)閉環(huán)系統(tǒng)中降低輸出電流就必須降低VSNS。這樣就會(huì)反過(guò)來(lái)降低輸出電流的精度，最終，輸出電流無(wú)法指定、控制或保證。通常來(lái)說(shuō)，相對(duì)于模擬調(diào)光，PWM調(diào)光可以提高精度，線性控制光輸出到更低級(jí)。 </p><p>　　調(diào)光頻率VS對(duì)比度 </p><p>　　LED驅(qū)動(dòng)

54、對(duì)PWM調(diào)光信號(hào)的不可忽視的回應(yīng)時(shí)間產(chǎn)生了一個(gè)設(shè)計(jì)問(wèn)題。這里主要有三種主要延遲（圖2）。這些延遲越長(zhǎng)，可以達(dá)到的對(duì)比度就越低（光強(qiáng)的控制尺度）。 </p><p><b>　　圖2：調(diào)光延遲。</b></p><p>　　如圖所示，tn表示從時(shí)間邏輯信號(hào)VDIM提升到足以使LED驅(qū)動(dòng)開(kāi)始提高輸出電流的時(shí)候的過(guò)渡延遲。另外，tsu輸出電流從零提升到目標(biāo)級(jí)所需要的時(shí)間，相

55、反，tsn是輸出電流從目標(biāo)級(jí)下降到零所需要的時(shí)間。一般來(lái)說(shuō)，調(diào)光頻率（fDIM）越低，對(duì)比度越高，這是因?yàn)檫@些固定延遲消耗了一小部分的調(diào)光周期（TDIM）。fDIM的下限大概是120Hz，低于這個(gè)下限，肉眼就不會(huì)再把脈沖混合成一個(gè)感覺(jué)起來(lái)持續(xù)的光。另外，上限是由達(dá)到最小對(duì)比度來(lái)確定的。 </p><p>　　對(duì)比度通常由最小脈寬值的倒數(shù)來(lái)表示： </p><p>　　CR = 1 / tO

56、N-MIN : 1 </p><p>　　這里tON-MIN = tD + tSU。在機(jī)器視覺(jué)和工業(yè)檢驗(yàn)應(yīng)用中常常需要更高的PWM調(diào)光頻率，因?yàn)楦咚傧鄼C(jī)和傳感器需要遠(yuǎn)遠(yuǎn)快于人眼的反應(yīng)時(shí)間。在這種應(yīng)用中，LED光源的快速開(kāi)通和關(guān)閉的目的不是為了降低輸出光的平均強(qiáng)度，而是為了使輸出光與傳感器和相機(jī)時(shí)間同步。</p><p><b>　　用開(kāi)關(guān)調(diào)節(jié)器調(diào)光 </b></

57、p><p>　　基于開(kāi)關(guān)調(diào)節(jié)器的LED驅(qū)動(dòng)需要一些特別考慮，以便于每秒鐘關(guān)掉和開(kāi)啟成百上千次。用于通常供電的調(diào)節(jié)器常常有一個(gè)開(kāi)啟或關(guān)掉針腳來(lái)供邏輯電平PWM信號(hào)連接，但是與此相關(guān)的延遲（tD）常常很久。這是因?yàn)楣柙O(shè)計(jì)強(qiáng)調(diào)回應(yīng)時(shí)間中的低關(guān)斷電流。而驅(qū)動(dòng)LED的專用開(kāi)關(guān)調(diào)節(jié)則相反，當(dāng)開(kāi)啟針腳為邏輯低以最小化tD時(shí)，內(nèi)部控制電路始終保持開(kāi)啟，然而當(dāng)LED關(guān)斷的時(shí)候，控制電流卻很高。 </p><p>

58、;　　用PWM來(lái)優(yōu)化光源控制需要最小化上升和下降延遲，這不僅是為了達(dá)到最好的對(duì)比度，而且也為了最小化LED從零到目標(biāo)電平的時(shí)間（這里主導(dǎo)光波長(zhǎng)和CCT不能保證）。標(biāo)準(zhǔn)開(kāi)關(guān)調(diào)節(jié)器常常會(huì)有一個(gè)緩開(kāi)和緩關(guān)的過(guò)程，但是LED專用驅(qū)動(dòng)可以做所有的事情，其中包括降低信號(hào)轉(zhuǎn)換速率的控制。降低tSU 和 tSN要從硅設(shè)計(jì)和開(kāi)關(guān)調(diào)節(jié)器拓?fù)鋬煞矫嫒胧帧?</p><p>　　Buck調(diào)節(jié)器能夠保持快速信號(hào)轉(zhuǎn)換而又優(yōu)于所有其它開(kāi)關(guān)拓?fù)?/p>

59、主要有兩個(gè)原因。其一，Buck調(diào)節(jié)器是唯一能夠在控制開(kāi)關(guān)打開(kāi)的時(shí)候?yàn)檩敵龉╇姷拈_(kāi)關(guān)變換器。這使電壓模式或電流模式PWM（不要與PWM調(diào)光混淆）的Buck調(diào)節(jié)器的控制環(huán)比Boost調(diào)節(jié)器或者各種Buck-Boost拓?fù)涓??？刂崎_(kāi)關(guān)開(kāi)啟的過(guò)程中，電力傳輸同樣可以輕易地適應(yīng)滯環(huán)控制，甚至比最好的電壓模式或電流模式的控制環(huán)還要快。其二，Buck調(diào)節(jié)器的電導(dǎo)在整個(gè)轉(zhuǎn)換周期中連在了輸出上。這樣保證了一個(gè)持續(xù)輸出電流，也就是說(shuō)，輸出電容被刪減掉。沒(méi)

60、有了輸出電容，Buck調(diào)節(jié)器成了一個(gè)真正的高阻抗電流源，它可以很快達(dá)到輸出電壓。Cuk和zeta轉(zhuǎn)換器可以提供持續(xù)的輸出電感，但是當(dāng)更慢的控制環(huán)（和慢頻）被納入其中的時(shí)候，它們會(huì)落后。 </p><p><b>　　比開(kāi)啟針腳更快 </b></p><p>　　即使是一個(gè)單純的無(wú)輸出電容的滯后Buck調(diào)節(jié)器，也不能滿足某些PWM調(diào)光系統(tǒng)的需要。這些應(yīng)用需要高PWM調(diào)光

61、頻率和高對(duì)比度，這就分別需要快速信號(hào)轉(zhuǎn)換率和短延遲時(shí)間。對(duì)于機(jī)器視覺(jué)和工業(yè)檢驗(yàn)來(lái)說(shuō)，系統(tǒng)實(shí)例需要很高的性能，包括LCD板的背光和投影儀。在某些應(yīng)用中，PWM調(diào)光頻率必須超過(guò)音頻寬，達(dá)到25kHz或者更高。當(dāng)總調(diào)光周期降低到微秒級(jí)時(shí)，LED電流總上升和下降時(shí)間（包括傳輸延遲），必須降低到納秒級(jí)。 </p><p>　　讓我們來(lái)看看一個(gè)沒(méi)有輸出電容的快速Buck調(diào)節(jié)器。打開(kāi)和關(guān)斷輸出電流的延遲來(lái)源于IC的傳輸延遲和輸

62、出電感的物理性質(zhì)。對(duì)于真正的高速PWM調(diào)光，這兩個(gè)問(wèn)題都需要解決。最好的方法就是要用一個(gè)電源開(kāi)關(guān)與LED鏈并聯(lián)（圖3）。要關(guān)掉LED，驅(qū)動(dòng)電流要經(jīng)過(guò)開(kāi)關(guān)分流，這個(gè)開(kāi)關(guān)就是一個(gè)典型的n-MOSFET。IC持續(xù)工作，電感電流持續(xù)流動(dòng)。這個(gè)方法的主要缺點(diǎn)是當(dāng)LED關(guān)閉的時(shí)候，電量被浪費(fèi)掉了，甚至在這個(gè)過(guò)程中，輸出電壓下降到電流偵測(cè)電壓。 </p><p>　　圖3：分流電路及其波形</p><p&g

63、t;　　用一個(gè)分流FET調(diào)光會(huì)引起輸出電壓快速偏移，IC的控制環(huán)必須回應(yīng)保持常電流的請(qǐng)求。就像邏輯針腳調(diào)光一樣，控制環(huán)越快，回應(yīng)越好，帶有滯環(huán)控制的Buck調(diào)節(jié)器就會(huì)提供最好的回應(yīng)。 </p><p>　　用Boost和Buck-Boost的快速PWM </p><p>　　Boost調(diào)節(jié)器和任何Buck-Boost拓?fù)涠疾贿m合PWM調(diào)光。這是因?yàn)樵诔掷m(xù)傳導(dǎo)模式中（CCM），每個(gè)調(diào)節(jié)器都展

64、示了一個(gè)右半平面零，這就使它很難達(dá)到時(shí)鐘調(diào)節(jié)器需要的高控制環(huán)帶寬。右半平面零的時(shí)域效應(yīng)也使它更難在Boost或者Buck-Boost電路中使用滯后控制。另外，Boost調(diào)節(jié)器不允許輸出電壓下降到輸入電壓以下。這個(gè)條件需要一個(gè)輸入端短電路并且使利用一個(gè)并聯(lián)FET實(shí)現(xiàn)調(diào)光變得不可能。。在Buck-Boost拓?fù)渲校⒙?lián)FET調(diào)光仍然不可能或者不切實(shí)際，這是因?yàn)樗枰粋€(gè)輸出電容（SEPIC，Buck-Boost和flyback），或者輸出短

65、電路（Cuk和zeta）中的未受控制得輸入電感電流。當(dāng)需要真正快速PWM調(diào)光的時(shí)候，最好的解決方案是一個(gè)二級(jí)系統(tǒng)，它利用一個(gè)Buck調(diào)節(jié)器作為第二LED驅(qū)動(dòng)級(jí)。如果空間和成本不允許的時(shí)候，下一個(gè)最好的原則就是一個(gè)串聯(lián)開(kāi)關(guān)（圖4）。 </p><p>　　圖4：帶有串聯(lián)DIM開(kāi)關(guān)的Boost調(diào)節(jié)器</p><p>　　LED電流可以被立即切斷。另外，必須要特別考慮系統(tǒng)回應(yīng)。這樣一個(gè)開(kāi)路事實(shí)上

66、是一個(gè)快速外部退荷暫態(tài)，它斷開(kāi)了反饋環(huán)，引起了調(diào)節(jié)器輸出電壓的無(wú)界上升。為了避免因?yàn)檫^(guò)壓失敗，我們需要輸出鉗制電路和/或誤差放大器。這種鉗制電路很難用外部電路實(shí)現(xiàn)，因此，串聯(lián)FET調(diào)光只能用專用Boost/Buck-Boost LED驅(qū)動(dòng)IC來(lái)實(shí)現(xiàn)。 </p><p>　　總而言之，LED光源的單純控制需要設(shè)計(jì)的初始階段就要非常小心。光源越復(fù)雜，就越要用PWM調(diào)光。這就需要系統(tǒng)設(shè)計(jì)者謹(jǐn)慎思考LED驅(qū)動(dòng)拓?fù)?。Buc