This document provides an overview and specifications for the LM5085Q constant on-time PFET buck switching controller from National Semiconductor. It includes key features such as a wide input voltage range, adjustable current limit, programmable switching frequency up to 1MHz, and integrated soft-start and thermal shutdown protection. The document describes the typical block diagram and performance characteristics, and provides guidance on shutdown implementation, current limiting, and applications including component selection for the inductor, output capacitor, and PFET.

3. Features ■ LM5085Q is an Automotive Grade product that is AECQ100 grade 1 qualified (-40°C to 125°C operating junction temperature) ■ Wide 4.5V to 75V input voltage range ■ Adjustable current limit using RDS(ON) or a current sense resistor ■ Programmable switching frequency to 1MHz ■ No loop compensation required ■ Ultra-Fast transient response ■ Nearly constant operating frequency with line and load variations ■ Adjustable output voltage from 1.25V ■ Precision ±2% feedback reference ■ Capable of 100% duty cycle operation ■ Internal soft-start timer ■ Integrated high voltage bias regulator ■ Thermal shutdown

4. Block Diagram LM5085 Block Diagram

5. Typical Performance Characteristics Efficiency vs. Load Current and VIN Current Limit vs. VIN

6. Alternate Output Ripple Configurations Reduced Ripple Configuration Lowest Cost Ripple Generating Configuration

8. Current Limiting Current Limit Sensing

12. Typical Application Example Circuit