LM25018MR-EVM

User's Guide SNVU227 – April 2013 LM25018 Evaluation Board 1 Introduction The LM25018 evaluation board provides the design engineer with a fully functional buck regulator, employing the constant on-time (COT) operating principle. This evaluation board provides a 10 V output over an input range of 12.5 V to 48 V. The board’s specifications are: • Input Range: 12.5 V to 48 V • Output Voltage: 10 V • Output Current: 300 mA • Nominal Switching Frequency ~ 440 kHz • Measured Efficiency: 91% at 200 mA and VIN = 15 V • Board size: 2.3 inch x 1.4 inch Figure 1. Complete Evaluation Board Schematic for LM25018 Based Synchronous Buck Converter All trademarks are the property of their respective owners. SNVU227 – April 2013 Submit Documentation Feedback LM25018 Evaluation Board Copyright © 2013, Texas Instruments Incorporated 1 Theory of Operation 2 www.ti.com Theory of Operation When the circuit is in regulation, the buck switch is turned on each cycle for a time determined by R3 and VIN according to Equation 1: TON = 10 -10 x R2 VIN (1) The on-time of this evaluation board ranges from 2.17 µs at VIN = 12 V to 477 ns at VIN = 48 V. The ontime varies inversely with input voltage. At the end of each on-time, the buck switch is off for at least 144 ns. In normal operation, the off-time is much longer. During the off-time, the load current is supplied by the output capacitor (C6). When the output voltage falls sufficiently that the voltage at FB is below 1.225 V, the regulation comparator initiates a new on-time period. For stable, fixed frequency operation, a minimum of 25 mV of ripple is required at FB to switch the regulation comparator. For a more detailed block diagram and a complete description of the various functional blocks, see the LM25018 48V, 325mA Constant On-Time Synchronous Buck Regulator Data Sheet (SNVS953). 3 UVLO The UVLO resistors (R1, R7) are selected using Equation 2: VIN(HYS) = IHYSR1 (2) and Equation 3: VIN (UVLO,rising) = 1.225V x § R1 © R7 +1· ¹ (3) On this evaluation board, R1 = 127 kΩ and R7 = 14.0 kΩ, resulting in UVLO rising threshold at VIN = 12 V and a hysteresis of 2.5 V. 4 Board Connection and Start-up The input connections are made to J2. The load is connected to J1. Ensure the wires are adequately sized for the intended load current. Before start-up, a voltmeter should be connected to the input and output terminals. The load current should be monitored with an ammeter or a current probe. It is recommended that the input voltage be increased gradually to 12 V, at which time the output voltage should be 10 V. If the output voltage is correct, increase the input voltage as desired and proceed with evaluating the circuit. DO NOT EXCEED 48 V AT VIN (J2). 2 LM25018 Evaluation Board SNVU227 – April 2013 Submit Documentation Feedback Copyright © 2013, Texas Instruments Incorporated Bill of Materials (BOM) www.ti.com 5 Bill of Materials (BOM) Designator Value Description C1 0.01uF CAP, CERM, 0.01uF, 16V, +/-10%, X7R, 0603 C2 1uF CAP, CERM, 1uF, 50V, +10/%, X7R, 1206 C3 0.1uF C4 Package Reference Part Number Manufacturer GRM188R71C103KA01D MuRata 1206 GRM31MR71H105KA88L MuRata CAP, CERM, 0.1uF, 50V, +5/%, X7R, 0805 0805 C0805C104J5RACTU Kemet 3300pF CAP, CERM, 3300pF, 50V, +10/%, X7R, 0603 0603 C0603C332K5RACTU Kemet C5 1uF CAP, CERM, 1uF, 25V, +10/%, X7R, 0603 0603 GRM188R71E105KA12D MuRata C6 4.7uF CAP, CERM, 4.7uF, 25V, +10/%, X7R, 1206 1206 GRM31CR71E475KA88L MuRata C7 0.1uF CAP, CERM, 0.1uF, 100V, +10/%, X7R, 0603 0603 GRM188R72A104KA35D MuRata D1 0.65V Diode, Schottky, 40V, 0.35A, SOD-523 SOD-523 ZHCS350TA Diodes Inc. L1 220uH INDUCTOR SHIELD PWR 220UH SMD 7.60mm x 7.60mm DR74-221-R Cooper Bussman Alternate Inductor 220uH INDUCTOR SHIELD PWR 220UH SMD 7.3mm x 4.5mm 744777222 Wurth R1 127k RES, 127k ohm, 1%, 0.1W, 0603 0603 CRCW0603127KFKEA Vishay-Dale R2 237k RES, 237k ohm, 1%, 0.1W, 0603 0603 CRCW0603237KFKEA Vishay-Dale R3 6.98k RES, 6.98k ohm, 1%, 0.1W, 0603 0603 CRCW06036K98FKEA Vishay-Dale R4 0 RES, 0 ohm, 5%, 0.125W, 0805 0805 CRCW08050000Z0EA Vishay-Dale R5 46.4k RES, 46.4k ohm, 1%, 0.1W, 0603 0603 CRCW060346K4FKEA Vishay-Dale R6 1.00k RES, 1.00k ohm, 1%, 0.1W, 0603 0603 CRCW06031K00FKEA Vishay-Dale R7 14.0k RES, 14.0k ohm, 1%, 0.1W, 0603 0603 CRCW060314K0FKEA Vishay-Dale 100V, 325mA Constant On-Time Synchronous Buck Regulator SO-8 PowerPAD LM25018MRE/NOPB Texas instruments U1 SNVU227 – April 2013 Submit Documentation Feedback LM25018 Evaluation Board Copyright © 2013, Texas Instruments Incorporated 3 Ripple Configuration 6 www.ti.com Ripple Configuration The LM25018 is a COT buck and requires adequate ripple at feedback (FB) node. Three commonly used ripple generation methods are shown in Table 1. LM25018 evaluation board has been supplied with minimum ripple configuration (Type 3), but can be configured to Type 1 or Type 2 with modifications as suggested in Table 1. Table 1. Ripple Configuration Type 1 Lowest Cost Configuration Type 2 Reduced Ripple Configuration Type 3 Minimum Ripple Configuration VOUT VOUT VOUT L1 L1 L1 Cac R FB2 R FB2 RC To FB GND 4 C OUT C OUT To FB R FB1 GND R5 open, C4 = 0 Ω. Select R4 and C7: R5, C4, C7 open. Select R4: 25 mV VOUT x ûIL(MIN) VREF R FB2 GND R FB1 R FB1 Cr Cac To FB C OUT R4 > Rr RC 5 gSW (R3 || R6) 25 mV R4 > ûIL R4 = 0 Ω. Select R5, C4, and C7: C7 > (4) (5) LM25018 Evaluation Board C4 = 3300 pF C7 = 100 nF (V ± VOUT)TON R5 x C4 < IN(MIN) 25 mV (6) SNVU227 – April 2013 Submit Documentation Feedback Copyright © 2013, Texas Instruments Incorporated Performance Curves www.ti.com 7 Performance Curves 100 95 90 Efficiency (%) 85 80 75 70 65 VIN=15V 60 VIN=24V 55 VIN=36V 50 0 0.05 0.1 0.15 0.2 0.25 0.3 IOUT(A) 0.35 C001 Figure 2. Efficiency vs Load Current 550 500 Frequency (kHz) 450 400 350 300 250 200 150 100 50 10 15 20 25 30 35 40 Input Voltage (V) 45 50 C002 Figure 3. Frequency vs Input Voltage (IOUT = 100 mA) Figure 4. Typical Switching Waveform (VIN = 24 V, Iout = 100 mA) SNVU227 – April 2013 Submit Documentation Feedback LM25018 Evaluation Board Copyright © 2013, Texas Instruments Incorporated 5 PC Board Layout 8 www.ti.com PC Board Layout Figure 5. Top Silk Figure 6. Top Copper Figure 7. Bottom Copper 6 LM25018 Evaluation Board SNVU227 – April 2013 Submit Documentation Feedback Copyright © 2013, Texas Instruments Incorporated IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms and conditions of sale of semiconductor products. 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