Datasheet LM2734 (Texas Instruments) - 8

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LM2734
SNVS288J – SEPTEMBER 2004 – REVISED DECEMBER 2014
www.ti.com 7 Detailed Description 7.1 Overview
The LM2734 device is a constant frequency PWM buck regulator IC that delivers a 1-A load current. The regulator has a preset switching frequency of either 550 kHz (LM2734Y) or 1.6 MHz (LM2734X). These high frequencies allow the LM2734 device to operate with small surface-mount capacitors and inductors, resulting in DC-DC converters that require a minimum amount of board space. The LM2734 device is internally compensated, so it is simple to use, and requires few external components. The LM2734 device uses current- mode control to regulate the output voltage. The following operating description of the LM2734 device will refer to the Simplified Block Diagram () and to the waveforms in Figure 12. The LM2734 device supplies a regulated output voltage by switching the internal NMOS control switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS control switch. During this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and the inductor current (IL) increases with a linear slope. IL is measured by the current-sense amplifier, which generates an output proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and VREF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through Schottky diode D1, which forces the SW pin to swing below ground by the forward voltage (VD) of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage. VSW D = TON/TSW VIN SW Voltage TON TOFF 0 V t D TSW IL IPK Inductor Current 0 t
Figure 12. LM2734 Waveforms of SW Pin Voltage and Inductor Current
8 Submit Documentation Feedback Copyright © 2004–2014, Texas Instruments Incorporated Product Folder Links: LM2734 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 Performance Characteristics 7 Detailed Description 7.1 Overview 7.2 Functional Block Diagram 7.3 Feature Description 7.3.1 Output Overvoltage Protection 7.3.2 Undervoltage Lockout 7.3.3 Current Limit 7.3.4 Thermal Shutdown 7.4 Device Functional Modes 7.4.1 Enable Pin / Shutdown Mode 7.4.2 Soft-Start 8 Application and Implementation 8.1 Application Information 8.1.1 Boost Function 8.2 Typical Applications 8.2.1 LM2734X (1.6 MHz) VBOOST Derived from VIN 5V to 1.5 V/1 A 8.2.1.1 Design Requirements 8.2.1.2 Detailed Design Procedure 8.2.1.3 Application Curves 8.2.2 LM2734X (1.6 MHz) VBOOST Derived from VOUT 12 V to 3.3 V /1 A 8.2.2.1 Design Requirements 8.2.2.2 Detailed Design Procedure 8.2.2.3 Application Curves 8.2.3 LM2734X (1.6 MHz) VBOOST Derived from VSHUNT 18 V to 1.5 V /1 A 8.2.3.1 Design Requirements 8.2.3.2 Detailed Design Procedure 8.2.3.3 Application Curves 8.2.4 LM2734X (1.6 MHz) VBOOST Derived from Series Zener Diode (VIN) 15 V to 1.5 V / 1 A 8.2.4.1 Design Requirements 8.2.4.2 Detailed Design Procedure 8.2.4.3 Application Curves 8.2.5 LM2734X (1.6 MHz) VBOOST Derived from Series Zener Diode (VOUT) 15 V to 9 V /1 A 8.2.5.1 Design Requirements 8.2.5.2 Detailed Design Procedure 8.2.5.3 Application Curves 8.2.6 LM2734Y (550 kHz) VBOOST Derived from VIN 5 V to 1.5 V / 1 A 8.2.6.1 Design Requirements 8.2.6.2 Detailed Design Procedure 8.2.6.3 Application Curves 8.2.7 LM2734Y (550 kHz) VBOOST Derived from VOUT 12 V to 3.3 V / 1 A 8.2.7.1 Design Requirements 8.2.7.2 Detailed Design Procedure 8.2.7.3 Application Curves 8.2.8 LM2734Y (550 kHz) VBOOST Derived from VSHUNT 18 V to 1.5 V / 1 A 8.2.8.1 Design Requirements 8.2.8.2 Detailed Design Procedure 8.2.8.3 Application Curves 8.2.9 LM2734Y (550 kHz) VBOOST Derived from Series Zener Diode (VIN) 15 V to 1.5 V / 1 A 8.2.9.1 Design Requirements 8.2.9.2 Detailed Design Procedure 8.2.9.3 Application Curves 8.2.10 LM2734Y (550 kHz) VBOOST Derived from Series Zener Diode (VOUT) 15 V to 9 V / 1 A 8.2.10.1 Design Requirements 8.2.10.2 Detailed Design Procedure 8.2.10.3 Application Curves 9 Power Supply Recommendations 10 Layout 10.1 Layout Guidelines 10.2 Layout Example 11 Device and Documentation Support 11.1 Third-Party Products Disclaimer 11.2 Trademarks 11.3 Electrostatic Discharge Caution 11.4 Glossary 12 Mechanical, Packaging, and Orderable Information