Texas Instruments LM3485 User Manual

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AN-1227 LM3485 Evaluation Board

User's Guide

Literature Number: SNVA052A

May 2004–Revised May 2004

Contents

1 Introduction ........................................................................................................................ 5

2 Output Voltage Current Limit Setting ..................................................................................... 6

3 Layout Fundamentals .......................................................................................................... 6

Table of Contents SNVA052A–May 2004–Revised May 2004

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1 Regulator with 3.3V Output at 0.5A ...................................................................................... 5

2 Component Location (Top Side) .......................................................................................... 6

3 Top Layer .................................................................................................................... 7

4 Bottom Layer ................................................................................................................ 7

1 Bill of Materials.............................................................................................................. 5

List of Figures

List of Tables

SNVA052A–May 2004–Revised May 2004 List of Figures

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User's Guide

SNVA052A–May 2004–Revised May 2004

AN-1227 LM3485 Evaluation Board

The LM3485 is a Hysteretic P-FET Buck Controller, which uses a pulse-frequency modulation (PFM) scheme to regulate the output voltage. This LM3485 demo board and the recommended components are intended to demonstrate the performance with a 3.3V output from a 12V source. The demo board can be used with source voltages from 7V to 28V to deliver output load currents up to 1A. By changing the size of a single resistor, regulated output voltages from 1.242V to 5V can be obtained.

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AN-1227 LM3485 Evaluation Board SNVA052A–May 2004–Revised May 2004

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1 Introduction

The circuit schematic is shown in Figure 1 and the bill of materials is given in Table 1.

Introduction

Figure 1. Regulator with 3.3V Output at 0.5A

Table 1. Bill of Materials

Code Description Manufacturer

C1 Input Capacitor CAP-Tantalum 22µF 35V EEJL1VD226R Panasonic C2 Output Capacitor CAP-POSCAP 100µF 6.3V 6TPC100M Sanyo C3 C C4 CffCAP-Ceramic Chip 100pF 50V GRM39X7R101K50 Murata D1 Catch Diode Schottky Diode 1A 30V MBRS130T3 On Semiconductor

L1 Inductor 22µH LQH66SN220M01L Murata Q1 P-channel MOSFET -60V FDC5614P Fairchild R1 Feedback high side resistor Chip Resistor 33KΩ MCR10EZHF3302 Rohm R2 Feedback low side resistor Chip Resistor 20KΩ MCR10EZHF2002 Rohm R3 R U1 Buck Controller LM3485 National Semiconductor

CAP-Ceramic Chip 1nF 50V GRM39X7R102K50 Murata

ADJ

Chip Resistor 24KΩ MCR10EZHF2402 Rohm

ADJ

SNVA052A–May 2004–Revised May 2004 AN-1227 LM3485 Evaluation Board

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Output Voltage Current Limit Setting

2 Output Voltage Current Limit Setting

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3 Layout Fundamentals

The LM3485 can work in a wide range of applications. For your application circuit, proper layout for the buck regulator should be implemented by following a few simple guidelines.

1. Place the power components, which are the MOSFET, diode, inductor and filter capacitors, close together. Make the traces between them as short and as wide as possible.

2. Place the trace for the Gate of the external PFET as close as possible to the PGATE pin of the

AN-1227 LM3485 Evaluation Board SNVA052A–May 2004–Revised May 2004

Figure 2. Component Location (Top Side)

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3. Separate any noise sensitive traces, primarily in the voltage feedback path, from noise source traces

4. Keep the trace short between the ground pin of the input capacitor and the anode of the diode.

5. Ensure the ground is low impedance.

Layout Fundamentals

LM3485.

associated with the inductor.

Figure 3. Top Layer

Figure 4. Bottom Layer

SNVA052A–May 2004–Revised May 2004 AN-1227 LM3485 Evaluation Board

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Texas Instruments LM3485 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual