Datasheet UC1846, UC1847, UC2846 UC2847, UC3846, UC3847 (Texas Instruments) - 8

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UC1846, UC1847 , UC2846 UC2847, UC3846 , UC3847
SLUS352C – JANUARY 1997 – REVISED DECEMBER 2015
www.ti.com 7 Detailed Description 7.1 Overview
The UCx846/7 family of control devices provides the necessary features to implement off-line or DC-to-DC fixed- frequency, current-mode control schemes with a minimal external parts count. Internally implemented circuits include under-voltage lockout featuring start-up current less than 1 mA, a precision reference trimmed for accuracy at the error amplifier input, logic to insure latched operation, a PWM comparator which also provides current limit control, and a totem pole output stage designed to source or sink high-peak current. The output stage, suitable for driving either N-Channel MOSFETs or bipolar transistor switches, is low in the off state.
7.2 Functional Block Diagram
5.1-V VIN 15 REFERENCE 2 VREF REGULATOR 13 VC SYNC 10 UVLO LOCKOUT F/F Q 11 A OUT RT 9 T OSC Q UC1846 CT 8 Output Stage C/S- 3 COMP X3 + + S R UC1847 C/S+ 4 Q Output Inverted S 0.5 V 14 B OUT + 0.5 mA NI 5 12 GND +E/A INV 6 CURRENT LIMIT 1 ADJUST + 16 SHUTDOWN COMP 7 350 mV 6 k:
7.3 Feature Description 7.3.1 Current Sense Amplifier
The current sense amplifier may be used in a variety of ways to sense peak switch current for comparison with an error voltage. Referring to Functional Block Diagram, maximum swing on the inverting input of the PWM comparator is limited to approximately 3.5 V by the internal regulated supply. Accordingly, for a fixed gain of 3, maximum differential voltages must be kept below 1.2 V at the current sense inputs. 8 Submit Documentation Feedback Copyright © 1997–2015, Texas Instruments Incorporated Product Folder Links: UC1846 UC1847 UC2846 UC2847 UC3846 UC3847 Document Outline 1 Features 2 Applications 3 Description Table of Contents 4 Revision History 5 Pin Configuration and Functions 6 Specifications 6.1 Absolute Maximum Ratings 6.2 ESD Ratings 6.3 Recommended Operating Conditions 6.4 Thermal Information 6.5 Electrical Characteristics 6.6 Typical Characteristics 7 Detailed Description 7.1 Overview 7.2 Functional Block Diagram 7.3 Feature Description 7.3.1 Current Sense Amplifier 7.3.2 Oscillator 7.4 Device Functional Modes 7.4.1 Current Limit 7.4.2 Shutdown 8 Application and Implementation 8.1 Application Information 8.2 Typical Application 8.2.1 Design Requirements 8.2.2 Detailed Design Procedure 8.2.2.1 Design Switching Frequency 8.2.2.2 Error Amplifier Output Configuration 8.2.2.3 Parallel Operation Configuration 8.2.2.4 Design Pulse by Pulse Current Limit Threshold 8.2.2.5 Soft-Start and Shutdown, Restart Function Design 8.2.3 Application Curves 9 Power Supply Recommendations 10 Layout 10.1 Layout Guidelines 10.2 Layout Example 11 Device and Documentation Support 11.1 Related Links 11.2 Community Resources 11.3 Trademarks 11.4 Electrostatic Discharge Caution 11.5 Glossary 12 Mechanical, Packaging, and Orderable Information