AN-1227| Application Note

LM3485 Evaluation Board

National Semiconductor Application Note 1227 Naomi Sugiura May 2002

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

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FIGURE 1. Regulator with 3.3V Output at 0.5A

TABLE 1. Bill of Materials Code C1 C2 C3 C4 D1 L1 Q1 R1 R2 R3 U1 Description Input Capacitor CAP-Tantalum 22uF 35V EEJL1VD226R Output Capacitor CAP-POSCAP 100uF 6.3V 6TPC100M CADJ CAP-Ceramic Chip 1nF 50V GRM39X7R102K50 Cff CAP-Ceramic Chip 100pF 50V GRM39X7R101K50 Catch Diode Schottky Diode 1A 30V MBRS130T3 Inductor 22uH LQH66SN220M01L P-channel MOSFET -60V FDC5614P Feedback low side resistor Chip Resistor 20K MCR10EZHF2002 RADJ Chip Resistor 24K MCR10EZHF2402 Buck Controller LM3485 Manufacturer Panasonic Sanyo Murata Murata On Semiconductor Murata Fairchild Rohm Rohm National Semiconductor

Feedback high side resistor Chip Resistor 33K MCR10EZHF3302 Rohm

AN-1227

2002 National Semiconductor Corporation

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