The LED has been available for many years now, initially as a red colored indictor. Later, yellow/ amber, green and finally blue colored LED light became available, which triggered an explosion in applications. Applications included traffic lights, vehicle lights and wall-washed (mood lighting). The amount of light available from LED lights has also increased steadily, and now power levels of 1W, 3W and 5W are fairly common.
Driving a single LED, or a long string of LED light connected in series, has relatively few problems when the current is low (may be 20mA). High current LED lights are tougher to drive, requiring 350mA, 700mA, 1A or higher. Of course, a simple linear regulator could be used if power dissipation was not an issue or a simple resistor if current regulation is not critical. However, in most applications, an efficient switching regulator is used. A switching regulator is essential if the LED string voltage is higher than the supply voltage, or if the supply voltage has wide variation. But switching the LED lights means that electro-magnetic interference has to be considered.
Constant voltage supply, we need to have some form of current control in series with the LED light. With a series resistor or active regulator circuit we are trying to create a constant current supply. In fact, a short circuit in any part of the circuit could lead to a catastrophic failure so we may have to provide some protection.
