The ZMM55C3V9 is a Zener diode with an SOD-80 package, manufactured by Formosa. It is compliant with RoHS directive, meaning it is free of hazardous substances. This diode is used for voltage regulation and reference purposes, where it acts as a controlled, low-voltage voltage source that helps protect other components from high voltage and reverse currents. It operates with a maximum reverse voltage of 3.9V and is rated for up to 500mW power dissipation. Features of the ZMM55C3V9 include high performance and stability in a wide range of temperatures, and a surge current capability of up to 200mA.