The first switching LED driver that we will study is the buck converter. The buck converter is the simplest of the switching drivers, and is a step-down converter for applications where the load voltage is never more than about 85% of the supply voltage. The limit of about 85% is due to switching delays in the control system. In a buck converter circuit, a power MOSFET is usually used to switch the supply voltage across an inductor and LED load connected in series. The inductor is used to store energy when the MOSFET is turned on; this energy is then used to provide current for the LED when the MOSFET is turned off. A diode across the LED and inductor circuit provides a return path for the current during the MOSFET off time. A simple schematic is shown in Figure 5.1.

Figure 5.1: Buck LED Driver

Buck converters are an attractive choice for LED drivers in offline and in low voltage applications as they can produce a constant LED current at very high efficiencies. A peak-current-controlled buck converter can give reasonable LED current variation over a wide range of input and LED voltages and needs no design effort in feedback control design. Coupled with the fact that these converters can be designed to operate at above 90% efficiencies, the buck-based driver becomes an attractive solution to drive high brightness LEDs.