FR9809 is a high-efficiency synchronous step-down DC/DC converter that employs a special process technique to obtain very low RDS(ON) for the internal metal–oxide–semiconductor field-effect transistor (MOSFET). The input operation voltage is in a wide range from 4.75V to 21V, and continuous load current capability is 5A. Control circuit is designed by a particular current mode which provides fast transient response and eases loop stabilization.