【正文】 while suffering a higher operating current while the LEDs are off. Optimizing light control with PWM requires minimum slewup and slewdown 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 softstart and often a softshutdown, 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 voltagemode or currentmode PWM (not to be confused with the dimming via PWM) faster than the boost regulator or the various buckboost topologies. Power delivery during the control switch39。一些應(yīng)用只是簡(jiǎn)單地來實(shí)現(xiàn)“開”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。對(duì)于單色LED來說，其主波長(zhǎng)會(huì)改變。同樣延伸到光源應(yīng)用里，混合多個(gè)單色LED也會(huì)存在同樣的問題。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。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)開始提高輸出電流的時(shí)候的過渡延遲。用開關(guān)調(diào)節(jié)器調(diào)光 基于開關(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)掉和開啟成百上千次。其一，Buck調(diào)節(jié)器是唯一能夠在控制開關(guān)打開的時(shí)候?yàn)檩敵龉╇姷拈_關(guān)變換器。這些應(yīng)用需要高PWM調(diào)光頻率和高對(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。要關(guān)掉LED，驅(qū)動(dòng)電流要經(jīng)過開關(guān)分流，這個(gè)開關(guān)就是一個(gè)典型的nMOSFET。另外，Boost調(diào)節(jié)器不允許輸出電壓下降到輸入電壓以下。為了避免因?yàn)檫^壓失敗，我們需要輸出鉗制電路和/或誤差放大器。如果調(diào)光頻率必須很高或者信號(hào)轉(zhuǎn)換率必須很快，或者二者都需要，那么Buck調(diào)節(jié)器就是最好的選擇。另外，必須要特別考慮系統(tǒng)回應(yīng)。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。對(duì)于真正的高速PWM調(diào)光，這兩個(gè)問題都需要解決。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。降低tSU 和 tSN要從硅設(shè)計(jì)和開關(guān)調(diào)節(jié)器拓?fù)鋬煞矫嫒胧?。在機(jī)器視覺和工業(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。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）。幾乎每個(gè)LED驅(qū)動(dòng)都要用到某種串聯(lián)電阻來辨別電流。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。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。s output voltage to rise without bound. Clamping circuits for the output and/or the error amplifier are required to prevent failure due to overvoltage. These clamps are difficult to realize with external circuitry, hence series FET dimming is practical only with dedicated boost/buckboost 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 bachelor39。附錄AA matter of light：PWM dimming By Sameh Sarhan and Chris Richardson, National Semiconductor Whether you drive LEDs with a buck, boost, buckboost or linear regulator, the mon thread is drive circuitry to con