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LM3485 evaluation board

Posted: 13 Nov 2002     Print Version  Bookmark and Share



LM3485 Evaluation Board Introduction The LM3485 is a Hysteretic P-FET Buck Controller, which uses a pulse-frequency modulation (PFM) scheme to regu- late 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. TABLE 1. Bill Of Materials Code Description Manufacturer C1 Input Capacitor CAP-Tantalum 225F 35V EEJL1VD226R Panasonic C2 Output Capacitor CAP-POSCAP 1005F 6.3V 6TPC100M Sanyo C3 CADJ CAP-Ceramic Chip 1nF 50V GRM39X7R102K50 Murata C4 Cff CAP-Ceramic Chip 100pF 50V GRM39X7R101K50 Murata D1 Catch Diode Schottky Diode 1A 30V MBRS130T3 On Semiconductor L1 Inductor 225H LQH66SN220M01L Murata Q1 P-channel MOSFET -60V FDC5614P Fairchild R1 Feedback high side resistor Chip Resistor 33K MCR10EZHF3302 Rohm R2 Feedback low side resistor Chip Resistor 20K MCR10EZHF2002 Rohm R3 RADJ Chip Resistor 24K MCR10EZHF2402 Rohm U1 Buck Controller LM3485 National Semiconductor AN200416-1 FIGURE 1. Regulator with 3.3V Output at 0.5A National Semiconductor Application Note 1227 Naomi Sugiura May 2002 LM3485EvaluationBoardAN-1227 ) 2002 National Semiconductor Corporation AN200416 Output Voltage Current Limit Setting AN200416-2 FIGURE 2. Component Location (Top Side) AN-1227 2 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 LM3485. 3. Separate any noise sensitive traces, primarily in the voltage feedback path, from noise source traces asso- ciated with the inductor. 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. LIFE SUPPORT POLICY NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Corporation Americas Email: National Semiconductor Europe Fax: +49 (0) 180-530 85 86 Email: Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Frangais Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 AN200416-3 Top Layer AN200416-4 Bottom Layer LM3485EvaluationBoardAN-1227 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

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