How a Buck Regulator Becomes a Smart Dimmable LED Driver (Part 1)

LEDs are expected to transform the lighting industry with their long life and low power consumption, but a major barrier to their rapid adoption is the high cost of the LEDs themselves. The cost of LED luminaires (complete electric lighting fixtures) varies, but LEDs typically make up about 25% to 40% of the total luminaire cost and are expected to remain a high percentage for many years to come (Figure 1).

　　Figure 1. LED luminaire cost breakdown

　　One way to reduce the overall fixture cost is to drive the LED at the highest possible DC current within the product specifications. This current may be much higher than its "binning current." This can produce a higher lumen/cost ratio if driven properly.

　　Figure 2. LED light output and efficiency vs. drive current.

　　However, this requires a higher current driver. There are many solutions that drive LEDs at low currents (<500mA), but there are few options for higher currents (700mA to 4A). This may seem surprising, as the semiconductor industry has a large number of DC-DC solutions with capabilities up to 4A, but they are designed to control voltage, not LED current. This article will explore some simple techniques for converting an off-the-shelf DC-DC buck regulator into a smart LED driver.

　　The buck regulator chops the input voltage and passes it through an LC filter to provide a regulated output, as shown in Figure 3. It uses two active components and two passive components. The active components are the switch "A" from the input to the inductor, and the switch (or diode) "B" from ground to the inductor. The passive components are the inductor (L) and the output capacitor (COUT) which form an LC filter that reduces the ripple generated by the active components.

　　Figure 3. Basic buck scheme.