The LTC3728L/LTC3728LX are dual high performance step-down switching regulator controllers that drive all N-channel synchronous power MOSFET stages. A constant frequency current mode architecture allows phaselockable frequency of up to 550kHz. Power loss and noise due to the ESR of the input capacitors are minimized by operating the two controller output stages out of phase. OPTI-LOOP compensation allows the transient response to be optimized over a wide range of output capacitance and ESR values. The precision 0.8V reference and power good output indicator are compatible with future microprocessor generations, and a wide 4.5V to 28V (30V maximum) input supply range encompasses all battery chemistries. A RUN/SS pin for each controller provides both soft-start and optional timed, short-circuit shutdown. Current foldback limits MOSFET dissipation during short-circuit conditions when overcurrent latchoff is disabled. Output overvoltage protection circuitry latches on the bottom MOSFET until VOUT returns to normal. The FCB mode pin can select among Burst Mode, constant frequency mode and continuous inductor current mode or regulate a secondary winding. The LTC3728L/LTC3728LX include a power good output pin that indicates when both outputs are within 7.5% of their designed set point.