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.
The document explains that while standard DC-DC converters can drive LEDs by regulating voltage at an FB pin with a sense resistor, this method wastes significant power. The note proposes converting the voltage loop of the L5973AD into a current loop to improve efficiency.
Two topologies are detailed for implementing these high-efficiency current sources. Topology 1 uses two additional resistors to decrease the regulated voltage across the sense resistor. Topology 2 utilizes an external operational amplifier (specifically mentioning the TS321) to further increase overall conversion efficiency.
| Input voltage range | 4V to 36V |
|---|---|
| Reference voltage | 1.235 V |
The main drawback is the amount of wasted power on the sense resistor, which affects overall application efficiency.
By using two additional resistors to implement a trick that decreases the magnitude of the regulated voltage across the sense resistor.