Dual-phase synchronous buck controller for multi-rail POL regulation
The LTC3728EG#PBF is a dual-output, dual-phase synchronous buck controller from Analog Devices. It drives two independent step-down channels from a single 3.5V to 30V supply rail, making it a fit for point-of-load regulation in systems that need separate core and I/O voltages from one input bus. Each channel uses synchronous rectification, which replaces the external Schottky catch diode with a second MOSFET — this cuts conduction losses in the low-side path and lifts efficiency, especially at lower output voltages where the diode drop would otherwise dominate the loss budget.
Switching frequency and duty cycle — inductor and headroom decisions
The switching frequency is programmable from 260 kHz to 550 kHz. A lower frequency reduces switching losses and allows a larger inductor for lower ripple current; a higher frequency shrinks the inductor footprint but increases gate-drive losses in the external MOSFETs. The frequency range is set by a single resistor to ground — no external clock input is required, and the part does not support external clock synchronisation. Maximum duty cycle is 99.4%, meaning the high-side MOSFET can stay on for nearly the entire switching period. This is useful when the input voltage is only slightly above the output voltage — for example, generating 3.3V from a 3.6V rail — because the controller does not skip cycles or drop out early. The practical limit is the minimum off-time needed to refresh the bootstrap capacitor for the high-side gate drive.
Control features and output configuration
The controller includes Enable, Power Good, and Soft Start. The soft-start pin sets the output ramp time with an external capacitor, preventing inrush current from tripping the input supply. Power Good is an open-drain output that asserts when both output channels are within regulation — useful for sequencing downstream loads or driving a reset line. Output configuration is positive voltage, transistor-driver type. The controller generates the gate-drive signals for external N-channel MOSFETs — both the high-side and low-side FETs are external, so the designer selects the FETs based on the load current and switching frequency. The two phases operate 180 degrees out of phase, reducing input ripple current and allowing a smaller input capacitor than a single-phase controller of the same total output current.
Package, temperature grade, and compliance
The supplier device package is 28-SSOP.