【正文】 sn是輸出電流從目標(biāo)級下降到零所需要的時(shí)間。要在一個(gè)閉環(huán)系統(tǒng)中降低輸出電流就必須降低VSNS。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)被察覺到。對白光LED來說，其相關(guān)顏色溫度（CCT）會(huì)改變。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），或者使用開關(guān)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(shù)字調(diào)光）。s ontime also adapts easily to hysteretic control, which is even faster than the best voltagemode or currentmode control loops. Second, the buck regulator39。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 bees 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 e from the IC39。使用脈沖寬度調(diào)制（PWM）來設(shè)置周期和占空度（圖1）可能是最簡單的實(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。對于人眼來說，很難察覺到紅、綠或藍(lán)LED中幾納米波長的變化，特別是在光強(qiáng)也在變化的時(shí)候。 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)電流來產(chǎn)生的光波長或CCT。這樣就會(huì)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。一般來說，調(diào)光頻率（fDIM）越低，對比度越高，這是因?yàn)檫@些固定延遲消耗了一小部分的調(diào)光周期（TDIM）。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?？刂崎_關(guān)開啟的過程中，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。在某些應(yīng)用中，PWM調(diào)光頻率必須超過音頻寬，達(dá)到25kHz或者更高。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)過程中，輸出電壓下降到電流偵測電壓。在BuckBoost拓?fù)渲?，并?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，BuckBoost和flyback），或者輸出短電路（Cuk和zeta）中的未受控制得輸入電感電流。 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。這就需要系統(tǒng)設(shè)計(jì)者謹(jǐn)慎思考LED驅(qū)動(dòng)拓?fù)?。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)開關(guān)（圖4）。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)。 讓我們來看看一個(gè)沒有輸出電容的快速Buck調(diào)節(jié)器。這樣保證了一個(gè)持續(xù)輸出電流，也就是說，輸出電容被刪減掉。 用PWM來優(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)光波長和CCT不能保證）。另外，上限是由達(dá)到最小對比度來確定的。 調(diào)光頻率VS對比度 LED驅(qū)動(dòng)對PWM調(diào)光信號的不可忽視的回應(yīng)時(shí)間產(chǎn)生了一個(gè)設(shè)計(jì)問題。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產(chǎn)生白光。 大多數(shù)LED包含一個(gè)發(fā)射藍(lán)光譜光子的區(qū)域，它透過一個(gè)磷面提供一個(gè)寬幅可見光。推薦的PWM調(diào)光 模擬調(diào)光通?？梢院芎唵蔚膩韺?shí)現(xiàn)。s control loop must respond in an attempt to keep the output current constant. As with logicpin 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 buckboost Neither the boost regulator nor any of the buckboost topologies are well suited to PWM dimming. That39。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 t