LM2853-1.8EVAL

User's Guide SNVA180A – October 2006 – Revised April 2013 AN-1513 LM2853 Evaluation Board 1 Introduction The LM2853 synchronous SIMPLE SWITCHER® buck regulator is a synchronous switching regulator capable of delivering up to 3A of current into a load. The LM2853 represents the ultimate in ease of use, as internal type-3 compensation minimizes the necessary external components and eases the selection of those components. The LM2853 is capable of accepting an input voltage between 3.0V and 5.5V and delivering an output voltage that is factory programmable from 0.8V to 3.3V in 100mV increments. The nominal switching frequency of the LM2853 is 550 kHz. The LM2853 Evaluation Board was designed to accommodate three standard output voltage options (1.2V/1.8V/3.3V) using the same layout and external components. Just five external components are included on the board, and the entire 3A power supply occupies a minimum amount of space (1.2” by 0.82”) on a two layer PCB without sacrificing efficiency or performance. The input voltage can be varied over the entire operating range of the LM2853 (3.0V to 5.5V) for testing purposes. Also, the board is designed to be stable with all standard LM2853 voltage options, so if another voltage option needs to be tested, the LM2853 IC can be removed and replaced with the desired option. 2 Schematic VIN EN U1 PVIN CIN AVIN EN CBYP SS SNS LM2853 VOUT SW SGND Lo PGND + Co Css 3 Bill Of Materials ID Part Number Type Size Parameters Qty Vendor U1 LM2853 3A Buck HTSSOP-14 x.xV 1 Texas Instruments CIN GRM31CR60J476ME19 Capacitor 1206 47 μF 1 Murata CBYP GRM21BR71C105KA01 Capacitor 0805 1 µF 1 Murata CSS VJ0805Y222KXXA Capacitor 0603 2.2 nF 1 Vishay-Vitramon LO DO3316P-472 Inductor DO3316P 4.7 µH 1 Coilcraft CO TPSD227X06R0050 Capacitor D Case 220 µF (50 mΩ) 1 Vishay-Sprague SIMPLE SWITCHER is a registered trademark of Texas Instruments. All other trademarks are the property of their respective owners. SNVA180A – October 2006 – Revised April 2013 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated AN-1513 LM2853 Evaluation Board 1 Performance 4 www.ti.com Performance Figure 1. Efficiency vs. ILOAD (VIN = 5V) 5 Component Selection 5.1 CIN and CBYP Figure 2. Horizontal Resolution: 200 µs/Div. Trace 1: VOUT (100 mV/Div.) Trace 2: ILOAD (1 A /Div.) The necessary RMS current rating of the input capacitor can be estimated by the following equation: IRMS = ILOAD D(1-D) (1) where the variable D refers to the duty cycle, and can be approximated by: D= VOUT VIN (2) From this equation, it follows that the maximum IRMS will occur at a full 3A load current with the system operating at 50% duty cycle. Under this condition, the maximum IRMS is given by: IRMS = 3A 0.5 x 0.5 = 1.5A (3) Ceramic capacitors feature a very large IRMS rating in a small footprint, making a ceramic capacitor ideal for this application. A 47 µF ceramic capacitor from Murata with a 4.9A IRMS rating provides the necessary input capacitance for the evaluation board. For improved load regulation and transient performance, an extra 1 µF ceramic capacitor is placed near to the AVIN pin from VIN to GND. This small capacitor helps to filter high frequency noise pulses on the supply, and prevent those pulses from disturbing the analog control circuitry of the chip. 5.2 CSS The soft-start capacitor has been chosen to provide a soft-start time of roughly 3 ms. Using the internal soft-start resistance of 450 kΩ and the external soft-start capacitor value of 2.2 nF, the approximate softstart time can be calculated as follows: TSS = 3 × CSS × RSS = 3 × 2.2 nF × 450 kΩ = 2.97 ms ≈ 3 ms (4) A 3 ms soft-start time will allow the LM2853 to start up gracefully without triggering over-current protection, regardless of the operating conditions. 2 AN-1513 LM2853 Evaluation Board SNVA180A – October 2006 – Revised April 2013 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated PCB Layout www.ti.com 5.3 LO and CO The selection of the output filter components LO and CO, are intrinsically linked, as both of these parameters affect the stability of the system, and various characteristics of the output voltage. First, a 4.7 μH inductor is chosen to allow stable operation over the entire input voltage range (as per the datasheet recommendations) from 3.0V to 5.5V. The size of the inductor also directly affects the amplitude of the inductor current ripple. This amplitude can be calculated from the following equation: 'IL = D x (VIN - VOUT) fSW x LO (5) From this, it follows that the maximum inductor current ripple using standard operating conditions of the LM2853 and a 4.7 μH inductor will occur at VIN = 5.5V, and VOUT = 2.5V. Under these conditions the inductor current ripple is given as: § ¨ ¨ © § 2.5V (5.5V ± 2.5V) 'IL = ¨¨ = 0.528A © 5.5V 550 kHz x 4.7 PH (6) This means an inductor must be selected with a saturation current higher than 3.264A to ensure that the inductor will never saturate during normal operating conditions. A Coilcraft DO3316P, 4.7 µH inductor provides the necessary current handling capabilities (ISAT = 5.4A) in a relatively small footprint. The ESR of the output capacitor affects both the ripple voltage at the output and the overall stability of the loop. In order to keep the output voltage ripple manageable under all operating conditions, an ESR value of 50 mΩ is selected. As per the datasheet recommendations, a capacitance of 220 μF will ensure stability regardless of VIN and VOUT when coupled with 4.7 μH inductor and 50 mΩ ESR. An AVX low-ESR 6.3V tantalum capacitor provides the necessary ESR and capacitance to stabilize the loop and control the output voltage ripple, with suitable voltage derating for up to a 3.3V output. 6 PCB Layout The PCB layout of the LM2853 demo board was designed to occupy as little board space as possible, while still following sound layout guidelines and techniques. The input capacitor, CIN is placed as close as possible to the PVIN pins and the PGND pins, to minimize stray resistance and inductance between CIN and the LM2853. Likewise, the AVIN bypass capacitor is placed as close as possible to the AVIN and SGND pins. PGND and SGND are connected to each other and the ground plane at a single point, the exposed pad of the LM2853. Also, in order to help conduct heat to the ground plane and away from the LM2853, an array of vias is used to connect the exposed pad to the ground plane, instead of a single via. Finally, the sense pin trace is intentionally routed away from the SW node to minimize any EMI pickup. SNVA180A – October 2006 – Revised April 2013 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated AN-1513 LM2853 Evaluation Board 3 PCB Layout www.ti.com Figure 3. Top Layer (not to scale) Figure 4. Bottom Layer (not to scale) 4 AN-1513 LM2853 Evaluation Board SNVA180A – October 2006 – Revised April 2013 Submit Documentation Feedback Copyright © 2006–2013, 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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