This document introduces the LED current source based on L5973AD and the basic principles and applications of LEDs. The brightness of LEDs is proportional to the forward current flowing through them. LEDs can be divided into low current LEDs and high current LEDs based on the typical forward current of the final application. The voltage drop across LEDs varies widely, so current mode driving is more convenient. The current source regulates the LED current and the output voltage is determined by the forward voltage of each LED.