Features

• Maintains Constant Frequency at Low Output Currents
• Dual N-Channel MOSFET Synchronous Drive
• Programmable Fixed Frequency (PLL Lockable)
• Wide VIN Range: 3.5V to 36V Operation
• Ultrahigh Efficiency
• Very Low Dropout Operation: 99% Duty Cycle
• Low Dropout, 0.5A Linear Regulator for VPP
• Generation or Low Noise Audio Supply
• Built-In Power-On Reset Timer
• Programmable Soft Start
• Low-Battery Detector
• Remote Output Voltage Sense
• Foldback Current Limiting (Optional)
• Pin Selectable Output Voltage
• 5V Standby Regulator Active in Shutdown: IQ < 200 µA
• Output Voltages from 1.19V to 9V
• Available in 28- and 36-Lead SSOP Packages

Typical Application

Description

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 Power TM output 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/f CLK (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).