Regulating LED Current Using an Inverting Buck-Boost Topology

若需控制LED 亮度，就必须具备能够提供恒定、稳压电流的驱动器。而要达到此目标，驱动器拓朴必须能产生足够的输出电压来顺向偏置LED。那麽当输入和输出电压范围重叠时，设计人员又该如何选择呢？转换器有时可能需要逐渐降低输入电压，但有时也可能需要升高输出电压。以上情况通常出现在那些具有大范围"脏" (dirty) 输入功率来源的应用中，例如车载系统。在这种降压/升压的操作中，几种拓朴可以达到较好的效果，像是SEPIC 或四次切换升降压拓朴。这些拓朴一般需要大量的元件，设计的材料成本也因而增加。但由於它们可提供正输出电压，因此设计人员通常视其为可接受的方案。不过负输出电压转换器也是另一种不该被忽略的替代解决方案。

图1 显示在恒定电流配置中驱动3 个LED 的反相升降压电路示意图。该电路拥有诸多优点。首先，它使用了标准降压控制器，不但能将成本降到最低，并有助於所有系统级的再利用。如果需要，设计人员也可以轻松改造该电路以利用整合型FET 降压控制器或同步降压拓朴来提升效率。这种拓朴使用的功率级元件数目与简易降压转换器相同，因此可将切换稳压器的元件数降至最低，同时达到相对於其他拓朴的最低总体成本。由於LED 本身的输出为光线，就系统级而言LED 因受到负电压而产生偏压并不会造成影响，跟正电压的情况不同，也因此使其成为一种值得考虑的电路设计。

Figure 1. Using a negative output voltage to regulate constant LED current with a buck-boost topology.

LED 电流的调节是透过感应感测电阻R1 两端的电压并将其用作控制电路的反馈。控制器接地接脚必须为负输出电压的参考电压，以便让该直接反馈正常运作。如果控制器为系统接地的参考电压，则需要一个电平移位电路。这种"负接地"对电路构成了一些限制。功率MOSFET、二极体和控制器的额定电压必须高於输入与输出电压的总和。

Second, external connections to the controller (such as enable) require level shifting of the signal from system ground to controller ground, thus requiring more components. For this reason alone, eliminating or minimizing unnecessary external controls is the best approach.

最後相较於四次切换的升降压拓朴，反相升降压拓朴中的功率装置会受到额外的电压和电流压力，进而降低了相关效率，但该效率与SEPIC 相当。即便如此，这种电路还是能够达到89% 的效率。藉由该电路的完全同步化，效率还可以再提高2%～3%。

A simple way to adjust the LED brightness is to quickly turn the converter on and off by shorting the soft-start capacitor C5. Figure 2 shows the PWM input signal and the actual LED current. This PWM brightness adjustment method is more effective because the converter is off and consumes very little power when the SS pin is shorted. However, this method is also relatively slow because the converter must gradually increase the output current in a controlled manner each time it turns on, resulting in a non-linear, finite dead-time before the output current rises. At the same time, this also reduces the minimum duty cycle of the on time to 10%-20%. In some LED applications that do not require high speed and 100% PWM regulation, this method may be sufficient.

This inverting buck-boost circuit provides engineers with an alternative method to drive LEDs. The use of a low-cost buck controller and low component count make it an ideal alternative to more complex topologies.

Figure 2 PWM drive (top) efficiently controls LED current (bottom)