"Feedback Input. The MP3302 regulates the voltage across the current sense resistor between FB and GND. Connect a current sense resistor from the bottom of the LED string to GND. Connect the bottom of the LED string to FB. The regulation voltage is 200mV."你要接上背光，形成回路，LED电流流过sense resistor,也即你图中的R19，FB端口才会出现200mV的电压。另外，你的EN端口的电压是否符合要求，仔细看datasheet，FB端口的电压和EN的电压也有关系的。