96A High Efficiency, Non-Inverting,Buck-Boost DC-DC Converter
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96A High Efficiency, Non-Inverting,Buck-Boost DC-DC Converter
AHighEfficiency,Non-Inve；MarkGaboriault；AllegroMicroSystems115No；Abstract―Portableelectro；Theoptimalsolutiontothis；Keywords-Buck,Boost,Conv；AndrewNotman；AllegroMicroSystemsStuar；A.
A High Efficiency, Non-Inverting, Buck-Boost DC-DC Converter Mark GaboriaultAllegro MicroSystems 115 Northeast Cutoff Worcester, MA 01606 USA
Abstract― Portable electronics applications often require a system voltage that is within the range of fully-charged to semi-discharged batteries, for example 3.3V output from a 2.8V to 4.2V Lithium-Ion battery input.The optimal solution to this requirement is a high-efficiency, non-inverting buck-boost, DC-DC converter, providing a programmable constant output voltage, typically from single cell Li-ion, multi-cell NiMH, or Alkaline power sources. This solution uses a control scheme which provides automatic and smooth transition through boost, buck-boost and buck modes.Keywords - Buck, Boost, Converter, Regulator, Cuk, SEPICAndrew NotmanAllegro MicroSystems Stuart House, Eskmills Park Musselburgh, East Lothian EH21 7PB ScotlandA. Drawbacks of Cascaded Boost & Buck Converter A schematic of this solution is shown in Fig. 1.LX1LX2I.
INTRODUCTIONA common power management problem, especially for battery powered portable electronics applications, is the need to provide a regulated output voltage from a battery voltage which, when charged or discharged, can be greater than, less than, or equal to the desired output voltage.There are several existing solutions to this problem, each with significant drawbacks. However, Allegro MicroSystems has developed a solution (patent pending) for a buck-boost converter IC which maximizes efficiency, minimizes ripple noise on input and output, and minimizes external component requirements and associated cost.II.EXISTING SOLUTIONSBecause this topology incorporates two DC-DC converters butted together, there is twice the losses associated with just a single converter and it therefore invariably provides poor efficiency.There is also a high number of external components required - inductors and decoupling capacitors, as well as the compensation networks required for each of the two controllers. These components use up valuable circuit board area and add to the cost associated with this function.At power up there is also a tendency for a large instantaneous current demand from the input supply, since the first stage is normally a boost converter and has to charge its output capacitor. Each of the supplies must also be sequenced properly to ensure proper operation at start-up.B. Drawbacks of Classic 4-Switch Buck-Boost Converter A schematic of this solution is shown in Fig. 2.VinThere are several solutions being utilized today, including: cascaded boost & classic 4-switch buck- cascaded boost converter & SEPIC and Cuk converter.The proposed solution has advantages over all of these alternatives. Most notably are improved efficiency and the simplification of external components required for control loop(s) and/or compensation. A more detailed discussion of the cascaded boost & buck converter and classic 4-switch buck-boost converter follows. Figure 2.
Classic 4-Switch Buck-Boost Converter This converter is based on an H-Bridge topology where each of the input and output pairs of transistors are activated as diagonal pairs in every cycle. The first path (A-D pair) enables charging of the inductor, while the second path (B-C pair) shows the inductor charging the capacitor.Notice that in any cycle, each of the four switches turn on and off precisely once every cycle. In either a Buck or a Boost converter, only two switches turn on and off per cycle. Therefore the switching losses of the Classic 4-Switch converter may be double those of a Buck or Boost converter. The output to input voltage ratio is given by: D/(1-D) where D is the duty cycle. When the input voltage equals the output voltage, D is 0.5, or 50%. The inductor current (neglecting ripple current) is given by Iload/(1-D), in this case 2*Iload.The controller changes modes to make the H-bridge look like a Buck converter for Vin&&Vout and a Boost converter for Vin&&Vout. The solution introduces a Buck-Boost Mode for Vin~Vout, where the direct connection of input to output via the inductor is maximized. The direct connection ensures a more continuous DC current, as opposed to the high peak current experienced in a classic Four-switch Buck-Boost. It also minimizes stress on both input and output capacitors and reduces ripple voltage.Fig. 3 illustrates the three modes of operation. The input and output voltages correspond to the 2.5V to 5.5V rating of the A8440 buck-boost converter IC.A. DesignThe Buck and Boost transfer functions are combined into aBecause the inductor current is twice the load current, the single differential modulation scheme, and controlled from one resistive losses are four times that of a Buck or low duty cycle source - labeled “VCONT”. The Buck-Boost region is Boost converter.
designated as a combination of Buck and Boost regions of The physical size of the inductor must also be larger to control. By using Time-Division-Multiplexing of fixed high accommodate this extra current without saturating. duty cycle Buck and low duty cycle Boost pulses, an overall Furthermore, as the output capacitor must carry the full output Buck-Boost transfer function is achieved.
current during the PWM on-time (D), and the charge current The mix of Buck and Boost pulses required to achieve the during the PWM off-time, the output capacitor must have low regulated output voltage is controlled by a Delta-Sigma equivalent series resistance (ESR). function that monitors one of the outputs from the error-amplifier.III. IDEAL SOLUTIONThe Buck-Boost region only allows a limit of 1:7 and 7:1To mitigate disadvantages as described in Section II, it is Buck to Boost pulse ratios. Outside these limits the ideal to have a four-switch DC-DC converter that changes conventional Boost or Buck mode control takes over. mode of operation depending on the input and output voltages.(Please note: Allegro patents pending regarding this control scheme)B. Features and FunctionsThe A8440 buck-boost converter IC incorporates the following features and functions:? ? ? ? ?Enable / Soft startUVLO C undervoltage lockout Thermal protection circuitUp to 2MHz clock generation and external synch Two modes of operation:o PWM (full power) and PFM/Hystereticmode (low power, e.g. 50mA or less)
Overcurrent Limits:o 2A in PWM mode(600mA operation in hysteretic mode) o -0.4A reverse current limit? Figure 3.
A8440 Operating Modes C. Control LawFig. 4 illustrates the control law of the DC-DC converter without an output load. The curve will scale as the load changes. The value of “VCONT” (when compared to reference voltages “VBUL” and “VBOL”) determines which mode is selected.3.51) Buck Mode Operation (Fig. 5): VCONT is below both VBUL and VBOL, so the slow comparators monitoring VCONT select Buck mode.32.5VBOM VBOL VCM VBUL VBUMVCONTBVout/Vin21.5VCONTVIN1A0.5BCACB CA CBCALX1 CLX2VOUTC000.2Control Voltage VCONT (V)0.40.60.811.2Figure 5.
Differential Modulation - Buck ModeFigure 4.
Buck-Boost Control FunctionAs VCONT rises, VCONTB falls and the A-C region decreases while the A-D region increases. Only the C-D switch pair changes state during the cycle.2) Boost Mode Operation (Fig. 6): VCONT is above both VBUL and VBOL, so the system selects Boost mode.The Buck region is a perfectly linear function of the control voltage, the Buck-Boost region has a slight curvature, and the Boost region is very non-linear as one would expect from a function like 1/(1-D).Any discontinuities built in due to the piecewise linear nature of the control law are eliminated by the high loop gain of the system.In practice, the control regions actually overlap so all Vout/Vin ratios can be achieved. This, taken together with the very small amount of hysteresis in the slow mode-comparators at the cross-over points, ensures that a single mode is selected. VTOPVBOM VBOL VCM VBUL VBUMD. Differential ModulationThe control voltage VCONT is the output of the error amplifier which controls the duty cycle. VCONT & VCONTB are differential error amplifier signals with a common-mode voltage of VCM = VTOP/2, where VTOP is a bandgap reference voltage.The mode of operation is controlled by comparing VCONT with both VBUL (the control voltage limit for Buck mode) and VBOL (the control voltage limit for Boost mode).The switching waveforms are generated from the outputs of two fast comparators that compare the ramp waveform with VCONT and VCONTB. The following sections describe the differential modulation scheme which selects one of the three operating modes.VCONTVCONTBVINACADACADACADACLX1VOUTLX2Figure 6.
Differential Modulation - Boost ModeAs VCONT rises, VCONTB falls and the A-C region decreases while the A-D region increases. Only the C-D switch pair changes state during the cycle.3) Buck-Boost Mode Operation (Fig. 7): VCONT is between VBUL and VBOL, so the system selects Buck-Boost mode. The example in Fig. 7 is for Vout just below Vin, such that there are more Buck pulses (AC-BC) than Boost pulses (AD-AC).Ripple Current (A)0.060.040.02034567-0.02-0.04-0.06VTOPVBOM VBOL VCM VBUL VBUM-0.08-0.1VCONTTime (uSec) VINFigure 8.
Buck-Boost Mode Ripple CurrentACADACBCACB CACLX1VOUTLX2Figure 7.
Differential Modulation - Buck-Boost Mode2) Measured Efficiency: Fig. 9 shows actual data taken at Vout
3.3V@100mA, 1MHz switching frequency.1009896Efficiency (%) The Delta-Sigma circuit is active, and gives a data stream with respect to the VCONT level. The
Delta-Sigma gives a data stream that is a given mixture of ones and zeroes:? ?‘1’ = 15% duty cycle - Boost pulse ‘0’ = 85% duty cycle - Buck pulse82802.533.54Input Voltage (V)4.55When VCONT is equal to VBOL, the resultant output is 11101…or when VCONT is equal to VBUL, the result is 0001…In the case of Fig. 7, this result is …F. Performance1) Inductor Current in Buck-Boost Mode: Fig. 8 shows an example of the ripple current for the extreme case of seven Buck pulses followed by one Boost pulse. “Real-world” data is shown for a system operating at 1MHz with a 10uH inductor and regulating the output to 3.3V.Overall ripple current is no greater than that of a Buck converter operating at 5.5V input and regulating to 2.5V output (though at a much lower frequency). Figure 9.
Measured Efficiency at 100mA load, 1MHzNote that an efficiency of 95% efficiency in the Buck-Boost region is achieved. There is a slight dip due to the Delta-Sigma function circuitry as Vout nears Vin in buck-boost mode. However, this dip is far less significant than that which would appear for a four-switch buck-boost converter.IV. CONCLUSIONAllegro’s A8440 Buck-Boost DC-DC converter IC provides a regulated output voltage from an input voltage above, below, or equal to the input voltage - thus solving a problem encountered often in designing power supplies for portable electronics.The design incorporates two n-channel and two p-channel power switches and employs synchronous rectification to maximize efficiency with minimum external components.
This solution is optimal because only two switches are in operation during any clock cycle. In the buck-boost region the output capacitor is less stressed, as the time the converter connects the input to the output via the inductor is maximized.
Therefore the key advantages of this solution are:1) Improved Efficiency - No need to run at double load current, synchronous rectification utilized, and only two switches needed in any given clock period. 2) Reduced Output Noise C Minimizes the time when the output capacitor supplies the load. 3) Minimal External Components - Thus minimizing costs. Figure 10.
A8440 Buck-Boost IC Block DiagramFigure 11.Typical Application Using A8440 Buck-Boost Converter IC包含各类专业文献、生活休闲娱乐、行业资料、高等教育、文学作品欣赏、中学教育、专业论文、各类资格考试、96A High Efficiency, Non-Inverting,Buck-Boost DC-DC Converter等内容。
　and also meets high efficiency, rapid dynamic response...a low voltage high current DC-DC converter ...同步整流结构图 图 6 四相交错并联 Buck 同步整流... 　DC/DC 变换器(Bi-directional DC/DC Converter, ...(32) 3.2 非隔离型双向 Buck-Boost 变换器的小...Performance Characterization of a High-Power Dual ... 　(3)Buck―Boost Converter 降压/升压式变换器,含极性反转(Inverting)式变换器; (4)Cuk Converter 升压,升压串联式变换器; (5)SEPIC(Single Endcd Pdimary ... 　DC-DC Converter The DC-DC converter converting a...Buck Regulators Boost Regulators Buck-Boost ...The efficient is very high, but they are ... 　Single-inductor four-switch non-inverting buck-boost dc-dc converter Chin-Wei...A high-efficiency, dual-mode, dynamic, buck-boost power supply IC for po... 　build a continuous mode (CCM), under voltage-controlled BUCK / BOOST structure DC / DC converter linear model, to achieve non-linear transformation to th... 　high power, low value, reliability,low cost, small size, a wide range ...the buc k DC-DC converters, step-up DC-DC converter, buck-boost type ... 　efficiency of driving systems by using bidirectional DC/DC converter in ...D1 S1 D2 S2 V1 C1 L C2 V2 (a) 双向 Buck/Boost 变换器 S2 L D1 ... 　)及(b)情形二(Buck&Boost)的电感电流波形图 A. ...“A single-inductor switching DC-DC converter ...DC-DC converters with high light-load efficiency ...led照明知识(第三部分)：boost和buck-boost时机_半导体材料_中国百科网
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led照明知识(第三部分)：boost和buck-boost时机
    如同线性和buck衍生led驱动一样，boost led驱动设计中的主要技术挑战是要给阵列中的每个led提供一个可控前向电流if。理想状态下，每个led都有安装一个单组链来确保通过每个的电流都相同。当需要把输入dc电压提升到一个高dc输出电压的时候，boost调节器是最简单的选择，因为它允许在给定电压下串联更多的led。通用照明系统设计者通常需要把线路电压设计成110 vac 或者 220 vac。如果功率因数校正（pfc）、隔离和线路谐波滤波都不需要的话，那么单级非隔离转换器（buck, boost, 或各种buck-boost拓扑）就可以使用ac电压的校正输出来直接驱动长串的串接led。然而，在很多情况下，我们需要使用一个中间dc总线电压，它是由一个采用了通用ac输入并且pfc、隔离和滤波的ac/dc调节器产生的。包括法律要求在内，一个低中间电压总线降低了电介质击穿和电弧问题，使维修人员的的工作更安全。欧盟提出了世界上最严格的法律规定：任何高于25瓦的光源都要具有pfc。没有几年，北美和亚洲也做出了同样的规定。诸如ul和ce这样的安全标准电气规定限制了供给boost led驱动的ac/dc供电输出电压。通常电压规定为12和24v，有时是48v。这些中间电压总线很少超过60v，也就是ul class 2 定为dc电压的最高值。boost挑战不管我们是否要控制输出电压或输出电流，boost调节器都要比buck调节器更难设计。持续导通状态（ccm）boost转换器中的平均感应电流等于负载电流（led电流）乘以1/(1 - d)，这里d是占空度。boost电压调节器需要设计者考虑到输入电压的限制来保证电感的正确设计，特别是额定峰值电流。boost led驱动加了一个可变输出电压，这个电压影响了占空比，因此也影响了主电感器的电感值和额定电流。为了避免电感饱和，最大平均值和电流峰值必须由vin-min 和 vo-max同时求出。例如，历数加工、驱动电流和模具温度，一个标准的白ingan led的vf可以从3v变到4v。串联的led越多，vo-min 和 vo-max的间距就越大。不同于带有输出电感的bulk调节器，boost转换器有一个非持续输出电流。因此，输出电容需要输出电压要持续（输出电流也如此）。这里，电压调节器中的输出电容被设计成兼有滤波器并且在负载瞬变时可以保持输出电压，在电流调节中，它只是起到了类似一个ac电流滤波器的作用。电容值要尽量低，并且要与所期望的led波动电流保持一致。输出电容越小（同时也可以尽量降低成本和大小），转换器对输出电流的回应就越快，这样led的调光反应就越好。boost转换器的另外一个严峻挑战是控制环。buck调节器允许电压模式的pwm控制、峰值电流模式的pwm控制、constant/controlled on-time以及其它的滞后控制。注意到处于ccm的boost调节器（低功率、便携设备除外）的右半平面零和在控制开关关闭的时候还在向输出供电的特性，它们几乎被限定在峰值电流模式pwm控制。要设计一个控制输出电流的boost led 驱动，控制环必须要把led看作是负载来分析，这与boost电压调节器的典型负载非常不同。在峰值电流模式控制中，负载阻抗对dc增益和控制到输出转换函数的低频极点有很大影响。对电压调节器来说，负载阻抗由输出电压与输出电流的比值来决定。 led是个拥有动态电阻的二极管。这个动态电阻只能通过做出vf (if)曲线，然后用切线来找到希望的前向电流的斜率来决定。如图1所示，电流调节器使用负载本身来作为反馈分频器来闭环。这就使dc增益降低了(rsns/ (rsns + rd))倍。我们趋向于用一个简单的积分器牺牲稳定带宽来补偿boost led驱动。事实上是大多数或者说许多led驱动应用需要调光。无论调光是通过if 的线性调节(模拟调光)来完成，还是通过高频打开或切断输出（数字或pwm调光）来实现，系统都需要像电压调节器实现的高带宽和快速瞬变回应。buck-boost挑战照明用led的开发要比固态光源标准的发展快得多。大量不同种类的led拥有很多不同的供电电压。串联的led的数目、种类及其不同的加工和模具温度都产生了不同的输出电压。例如，高端汽车正在过渡到利用led来作为日间行驶灯。三个3瓦白色led组成了一个12v1a的负载。汽车电压系统通常需要持续工作于9到16v，并且可以延伸到6到42v，使系统可以无损运行，但是其性能可能要有折扣。通常来说，buck调节器是最好的led驱动器，其次是boost, 但是在这个应用中，他们没有优劣之分。如果一定要用buck-boost调节器，最难的决定就是采用哪种拓扑。任何拓扑的buck-boost调节器和buck调节器或boost调节器的最基本的区别是buck-boost从来没有把输入供电直接连接到输出。在一部分转换环中，buck和boost调节器把vin 连接到 vo（通过电感和开关/二极管），这个直连使它们更有效率。所有的buck-boost都把所有要传送给负载的能量储存或者磁场（电感或）或者电场（电容）中，这样就导致了电源转换中的高峰值电流或者更高电压。特别的一点是要考虑在输入电压和输出电压的拐角，因为峰值转换电流发生在vin-min 和 vo-max，但是峰值转换电压发生在vin-max、 vin-max 和 vo-max。一般来说，这意味着拥有一个这样的输出功率的buck-boost调节器要比一个同样输出功率的buck或boost调节器更大且效率更低。单电感buck-boost可以像buck或boost调节器一样组建，使它在系统成本的角度来讲很吸引人。这种拓扑的一个缺点是vo被反置（图2a）或者以vin为参照（图2b）。测平移动或者反偏电路必须要用一些转换器。像boost转换器，它们有一个不连续输出电流，并且需要一个输出电容来维持一个持续led电流。功率mosfet要承受一个峰值为iin加上if的电流还有一个峰值为vin 加上 vo的电压。图2：高端buck-boost (a); 低端buck-boost (b)。其它拓扑sepic转换器拥有连续输入电流的优点，这个连续输入电流是由输入电感和正输出电压产生的。像boost和单电感buck-boost，它们需要一个输出电容来维持一个平滑led电流。另外一个sepic转换器的优点是几乎任何一个低端调节器或者都可以被设置成为一个毋需反偏或测平移动电路的sepic。图3：sepic led 驱动。很少被用作电压调节的cuk转换器作为led驱动而崭露头角。输入和输出电流都是连续的。输出电压的极性就像高端buck-boost一样被反置，但是输出电容像buck转换器一样被消除。除buck-boost 和boost 以外，cuk是拥有这种能力的唯一的实用型非隔离调节器。图4：cuk调节器。由于boost和buck-boost调节器的高度复杂性及其外围电路、低效（特别是buck-boost）和控制拓扑的选择不足，致使它们都不是转换led驱动的首选。但是它们都是led越来越多的照明应用必不可少的。某些系统结构可以用buck或者甚至是线性以调节器为基础的led驱动来替代。比如类似于街灯的大型光源需要一百甚至更多的1w＋ led。一般来说，针对通用照明的led从低功耗走向高功耗，并且在其中间舞台，比如汽车前灯和小型光部件，boost和buck-boost调节器代表了常电流驱动的最佳选择。作者简介sameh sarhan：加利福尼亚州圣克拉拉县，主要从事于中压/高压电源管理的应用工程师。从1998年开始，开始涉及电源电子。曾经在联邦无线电委员会和vicor公司工作。工作经验包括设计软/硬开关电源，从几瓦到600瓦开关电源。sameh1996年在开罗大学(埃及)电子工程系取得学士学位。chris richardson：中压和高压的应用工程师。他的主要工作是划分任务，包括实验室工作、新ic评估、规格书的书写和应用注意事项等文档工作、培训现场工程师工作。2001年加入国家半导体以来，chris工作的重点主要是同步控制器和调整器。在最近的3年，他主要是关注于高亮度led在汽车和工业方面的应用。chris拥有维吉尼亚工学院和州立大学的学士学位。下页为英文原文参考：a matter of light, part 3---when to boost and buck-boostby sameh sarhan and chris richardson, national semiconductorin part one of this series, we looked at the basics of led lighting sources and their driving requirements. in part 2, we discussed why a constant-current buck converter should be your first preference when it comes to switch-mode led drivers. in this third installment, we investigate larger led displays and the applications space for other converter topologies.manufacturers and designers of led lighting often refer to applications with clear advantages for solid state illumination as "low-hanging fruit." examples such as garden path lighting or mr16 bulb replacement often require only a few leds, or just one (fig. 1). the most common voltages for low-voltage lighting are 12 vdc, 24 vdc, and 12 vac. these applications often use a buck regulator. although the buck is preferred, as previously discussed, the boost regulator is finding more use as the number of leds increase for led lighting applications. not content to pick off flashlights or single bulb replacements anymore, designers are targeting large-scale general illumination, systems that require thousands of lumens. examples include street lighting, residential and commercial lighting, stadium lighting, and decorative or architectural lighting of spaces both interior and exterior.figure 1: buck and boost led driver buck: vo = n x vf, vo & boost: vo = n x vf, vo > vin。constant-current still requiredas with linear and buck-derived led drivers, the main technical challenge in boost led driver design is providing a controlled forward current, if, to each led of the array. ideally every led would be placed in a single series chain, ensuring that the same current flows through each device. a boost regulator is the simplest choice when stepping up a dc input voltage up to a higher dc output voltage, as it allows more leds to be placed in series from a given input voltage. a system designer for general illumination usually draws line power at 110 vac or 220 vac. if power factor correction (pfc), galvanic isolation, and line harmonic filtering aren't required, then single stage, non-isolated switching converters (buck, boost, or various buck-boost topologies) can use the rectified output of ac line voltage and directly drive long strings of series connected leds.in many cases, however, an intermediate dc bus voltage is used, derived from an ac/dc regulator that takes a universal ac input and provides pfc, isolation, and filtering. besides solving legal requirements, a lower intermediate voltage bus reduces problems with dielectric breakdown, arcing, and improves the safety of service people working with lighting. the european union leads the world with the strictest legal requirements, including pfc for any lighting over 25 watts. north america and asia consistently follow europe's lead, albeit some years later. safety standards and electrical codes such as ul and ce limit the output voltage of the ac/dc power supply that forms the input of the boost led driver. common rails are 12 and 24 volts, and in some cases 48 volts. rarely are these intermediate bus rails higher than 60 volts, which is the cutoff for dc voltages under ul class 2.the boost challengeboost regulators are more difficult to design than buck regulators, regardless of whether we control the output voltage or the output current. the average inductor current in a continuous conduction mode (ccm) boost converter is equal to the load current (led current) multiplied by 1/(1 - d), where d is the duty cycle. boost voltage regulators require design review at the limits of input voltage to ensure correct design of the inductor, especially the peak current rating. a boost led driver adds a variable output voltage that influences duty cycle and therefore the inductance and current rating of the main inductor. to prevent inductor saturation, the maximum average and peak currents must be evaluated at both vin-min and vo-max. for example, over the range of process, drive current, and die temperature, a typical white ingan led's vf can vary from 3 to 4 volts. the more leds are placed in series, the greater the gap between vo-min and vo-max.unlike the buck regulator with its output inductor, the boost converter has a discontinuous output current. for this reason an output capacitor is required to keep the output voltage (and hence the output current) continuous. where the output capacitor in a voltage regulator is designed to both filter and hold up the output voltage during load transients, in a current regulator it functions as an ac current filter only. the capacitance is made as low as possible, consistent with maintaining the desired led ripple current. the lower the output capacitance (which keeps cost and size to a minimum), the faster the converter's response to changes in output current, and consequently the led's dimming response is better.another serious challenge for boost converters is the control loop. buck regulators are available with voltage mode pwm control, peak current mode pwm control, constant/controlled on-time, and hysteretic control among others. boost regulators in ccm (with the exception of low-power/portable equipment) are almost universally constrained to peak current mode pwm control, owing to their right-half plane zeroes and the fact that they deliver power to the output when the control switch is off. to design a boost led driver that controls output current, the control loop must be analyzed using leds as the load, a case much different from the typical load of a boost voltage regulator. in peak current mode control, the impedance of the load has a strong effect on both the dc gain and the low-frequency pole of the control-to-output transfer function. for voltage regulators the load impedance is determined by dividing output voltage by output current. leds are diodes, with a dynamic resistance. this dynamic resistance can only be determined by plotting the vf versus if curve and then taking the tangent line to find the slope at the desired forward current. as shown in fig. 1, the current regulator uses the load itself as a feedback divider to close the control loop. this reduces the dc gain by a factor of (rsns/ (rsns + rd)). it is tempting to compensate a boost led driver with a simple integrator, sacrificing bandwidth for stability. the reality is that many, if not most led driver applications require dimming. whether dimming is done by linear adjustment of if (analog dimming) or by turning the output on and off at high frequency (digital, or pwm dimming) the system requires high bandwidth and fast transient response just as a voltage regulator does.the buck-boost challengeleds for lighting are being adopted much faster than the standards for solid state illumination have developed. a wide variety of input voltages power a wide variety of leds. the number of leds in series, the type of leds, and the variation of vf with both process and die temperature all contribute to a wide range of output voltage. for example, high-end automobiles are converting to leds for their daytime running lamps. three 3-watt white leds present a load of about 12 volts at a current of 1 amp. automotive voltage systems usually require continuous operation over a range of 9 to 16 volts, with an extended range of 6 to 42 volts where performance is reduced but the system can operate without suffering damage. in general, the buck regulator makes the best led driver, followed by the boost, but neither is appropriate for this case. if a buck-boost regulator must be used, the most difficult decision to make is often which topology to use.one fundamental difference between buck-boost regulators of any topology and the buck regulator or the boost regulator is that the buck-boosts never connect the input power supply directly to the output. both the buck and the boost regulator connect vin to vo (across the inductor and switch/diode) during a portion of their switching cycles, and this direct connection gives them better efficiency. all buck-boost regulators store the entire energy delivered to the load in either a magnetic field (inductor or transformer) or in an electric field (in a capacitor), which results in higher peak currents or higher voltage in the power switches. in particular, evaluation of the converter at the corners of both input voltage and output voltage is necessary because peak switch current occurs at vin-min and vo-max, but peak switch voltage occurs at vin-max and vin-max and vo-max. in general this means that a buck-boost regulator of a certain output power will be larger and less efficient than a buck or boost regulator of equal output power.the single inductor buck-boost can be built with the same parts count as a buck regulator or boost regulator, making it attractive from a system cost standpoint. one disadvantage of this topology is that the polarity of vo is inverted (figure 2a) or regulated with respect to vin (figure 2b). level-shifting or polarity inverting circuitry must be employed in these converters. like the boost converter, they have a discontinuous output current, and require an output capacitor to maintain a continuous led current. the power mosfet suffers a peak current of iin plus if and a peak voltage of vin plus vo.figure 2: high-side buck-boost (a); low-side buck-boost (b)。other topologiesthe sepic converter has the advantages of a continuous input current due to the input inductor and positive output voltage. like the boost and single inductor buck-boost it requires an output capacitor to maintain a smooth led current. a further advantage of sepic converters is that almost any low-side regulator or controller can be configured as a sepic without the need of polarity inversion or level shift circuitry.figure 3: sepic led driver。rarely used in voltage regulation, the cuk converter has emerged as an led driver. input and output currents are continuous. the polarity of the output voltage is reversed, as with the high-side buck-boost, but the output capacitor can be eliminated like the buck converter. the cuk is the only other practical non-isolated regulator with this ability.figure 4: cuk regulator。neither the boost nor the buck-boost regulator is preferred for switching led drivers, owing to their higher complexity and parts count, lower efficiency (especially for the buck-boosts) and scant choice of control topologies. however, both are 'necessary evils' as leds push into more and more lighting applications. in some cases the system architecture can be altered to allow buck or even linear regulator-based led drivers. examples include very large light sources, such as street lights, where a hundred or more 1w+ leds are required. in general, leds for general illumination are working their way from lower to higher power, and in the intermediate arena, such as automotive headlights and small lighting fixtures, boost and buck-boost regulators represent the best choice for constant-current driving.about the authorssameh 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.来源：美国国家半导体公司
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