The LTC1538-AUX/LTC1539 are dual, synchronous step-down switching regulator controllers which drive external N-channel power MOSFETs in a phase-lockable fixed frequency architecture. The Adaptive PowerTMoutput stage selectively drives two N-channel MOSFETs at frequencies up to 400kHz while reducing switching losses to maintain high efficiencies at low output currents.An auxiliary 0.5A linear regulator using an external PNP pass device provides a low noise, low dropout voltage source. A secondary winding feedback control pin (SFB1) guarantees regulation regardless of load on the main output by forcing continuous operation.A 5V/20mA regulator, internal 1.19V reference and an uncommitted comparator remain active when both controllers are shut down. A power-on reset timer (POR) is included which generates a signal delayed by 65536/fCLK(typ 300ms) after the controller’s output is within 5% of the regulated first voltage. Internal resistive dividers provide pin selectable output voltages with remote sense capability on one of the two outputs.The operating current levels are user-programmable via external current sense resistors. Wide input supply range allows operation from 3.5V to 30V (36V maximum).ApplicationsNotebook and Palmtop Computers, PDAsPortable InstrumentsBattery-Operated DevicesDC Power Distribution Systems