LM2696EVAL

User's Guide SNVA129B – October 2005 – Revised April 2013 AN-1410 LM2696 Demonstration Board 1 Introduction The LM2696 is a constant on-time, buck regulator capable of delivering up to 3A into a load. The LM2696 is capable of switching frequencies in the range of 100 kHz to 500 kHz and accepts input voltages from 4.5 V to 24 V. An internal soft-start and power-good flag are also provided to allow for simple sequencing between multiple regulators. The operating conditions for the evaluation board are the following: VIN = 6 V to 24 V VOUT = 2.5 V IOUT = 0A to 3A fSW = 250 kHz LM2696 VPGOOD PGOOD VSD EXTVCC SD CVCC CSD RON CBOOT RON VIN CIN CBY CAVIN CSS CBOOT AVIN SW PVIN GND L VOUT RFB1 DSW SS COUT FB RFB2 Figure 1. Evaluation Board Schematic Table 1. Bill of Materials (BOM) ID Part Number U1 LM2696 L MSS1260-682MX Type Size 3A Constant ontime Regulator HTSSOP-16 Parameters Qty Vendor 1 TI Coilcraft Inductor MSS1260 6.8 µH, 4.9A ISAT 1 CIN EEUFC1V181 Capacitor 8 x 11.5 180 µF, 35 V 1 Sanyo CBY VJ0805Y104KXAM Capacitor 0805 0.1 µF 1 Vishay CSS VJ080JY103KXX Capacitor 0805 0.01 µF 1 Vishay CVCC VJ0805Y105JXACW1BC Capacitor 0805 1 µF 1 Vishay CBOOT VJ0805Y104KXAM Capacitor 0805 0.1 µF 1 Vishay CAVIN VJ0805Y105JXACW1BC Capacitor 0805 1 µF 1 Vishay All trademarks are the property of their respective owners. SNVA129B – October 2005 – Revised April 2013 Submit Documentation Feedback AN-1410 LM2696 Demonstration Board Copyright © 2005–2013, Texas Instruments Incorporated 1 Performance www.ti.com Table 1. Bill of Materials (BOM) (continued) ID 2 Type Size Parameters Qty Vendor COUT Part Number TPSW476M010R0150 Capacitor W 47 µF, 10 V, 150 mΩ 1 AVX CSD VJ0805Y102KXXA Capacitor 0805 1 nF 1 Vishay RFB1 CRCW08051001F Resistor 0805 1 kΩ 1 Vishay RFB2 CRCW08051001F Resistor 0805 1 kΩ 1 Vishay RON CRCW08051433F Resistor 0805 143 kΩ 1 Vishay DSW CMSH3-40M-NST Schottky Diode SMB 40 V @ 3A diode, VF = 0.55 V 1 Central Semiconductor 160-1026-02 -05-00 Solder Terminals 7 Wearnes Terminals for VIN, GND and VOUT Performance Benchmark data has been taken from the evaluation board using the LM2696. Figure 2 shows an efficiency measurement taken with VIN at 12 V. Figure 2. Efficiency with VIN = 12 V The advantage of the evaluation board is the ability to examine performance tradeoffs through substitution of parts. By careful selection of the components used, it is possible to optimize the application circuit for a given parameter. For instance, the inductor footprint has been designed to accommodate DO-3316 and MSS-1278 packages. The inductor selection would then be determined by the design constraints. 3 Frequency Selection The resistor connected to the RON pin sets the switching frequency of the LM2696. This resistor controls the current flowing into the RON pin and is directly related to the on-time pulse. Connecting a resistor from this pin to PVIN allows the switching frequency to remain constant as the input voltage changes. In normal operation this pin is approximately 0.65 V above GND. In shutdown, this pin becomes a high impedance node to prevent current flow. The value of RON may be expressed as: RON = (VIN ± VD) x VOUT kON x fSW x VIN 106 (1) Where RON is in kΩ, fSW is in kHz, and kON is in µA • µs 2 AN-1410 LM2696 Demonstration Board SNVA129B – October 2005 – Revised April 2013 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Inductor Selection www.ti.com Under no condition should a bypass capacitor be connected to the RON pin. Doing so couples any AC perturbations into the pin and prevents proper operation. For this demo board, RON is calculated as : RON = 4 (12V ± 0.65V) x 2.5V 6 66 PA x Ps x 250 kHz x 12V 10 = 143 k: (2) Inductor Selection Typically an inductor is selected such that the maximum peak-to-peak ripple current is less than 30% of the maximum load current. The inductor current ripple (ΔIL) may be expressed as: (VIN - VOUT) D 'IL = L fSW (3) The inductor for this demo board was calculated as shown in Equation 4: L= (12V ± 2.5V) x 0.21V 3 10 = 6.8 PH (40% x 3A) x 250 kHz (4) A standard value of 10 µH may be chosen. The other characteristics of the inductor that should be taken into account are saturation current and core material. A shielded inductor or low profile unshielded inductor is recommended to reduce EMI. Physical orientation of the inductor effects the parts stability. The inductor should be oriented such that the magnetic flux flows down through the center of the inductor and returns through the ground plane. Simply put, the inductor should be oriented such that terminal associated with the dot or label is connected to the switchnode. 5 Output Capacitor The output capacitor size and ESR have a direct affect on the stability of the loop. This is because the constant on-time control scheme works by sensing the output voltage ripple and switching appropriately. The ripple voltage necessary at the feedback pin may be estimated using the following relationship: ΔVFB ≥ 0.057 x fSW + 35 (5) Where fSW is in kHz and ΔVFB is in mV. This minimum ripple voltage is necessary in order for the comparator to initiate switching. The ripple at the output may be calculated by multiplying the feedback ripple voltage by the gain seen through the feedback resistors. This gain H may be expressed as: VOUT H= VFB VOUT = 1.25V (6) For this demo board, the ripple necessary at the feedback pin is calculated as: ΔVFB 21 mV ≥ 0.057 x 250 kHz + 35 (7) Therefore, the ripple at the output is: 'VOUT = 42 mV = 21 mV x 2.5V 1.25V (8) Since the ripple current is calculated as 798 mA, the output capacitor must have an ESR not less than: ESR = 36 m: = Ripple_Voltage Ripple_Current = 42 mV 1200 mA (9) Typically the best performance is obtained using POSCAPs, SP CAPs, tantalum, Niobium Oxide, or similar chemistry type capacitors. Low ESR ceramic capacitors may be used in conjunction with the RC feed forward scheme; however, the feed forward voltage at the feedback pin must greater than 30 mV. For more information, see Section 6. SNVA129B – October 2005 – Revised April 2013 Submit Documentation Feedback AN-1410 LM2696 Demonstration Board Copyright © 2005–2013, Texas Instruments Incorporated 3 Ripple Feed Forward 6 www.ti.com Ripple Feed Forward An RC network may be used to eliminate the need for high ESR capacitors. Such a network is connected as shown in Figure 3. L SW VOUT Rff RFB1 FB COUT Cff RFB2 Figure 3. RC Feed Forward Network The value of Rff should be large in order to prevent any potential offset in VOUT. Typically the value of Rff is on the order of 1MΩ and the value of RFB1 should be less than 10kΩ. The large difference in resistor values minimizes output voltage offset errors in DCM. The value of the capacitor may be selected using the following relationship: (VIN_MIN - VFB) TON_MIN Cff_MAX = 0.03V Rff (10) Where the on-time (TON_MIN) is in µs, and the resistance (Rff) is in MΩ. If a ceramic output capacitor is used with this demo board, Cff_MAX is calculated as: Cff_MAX = (6V ± 1.25V) x 0.42 Ps 0.03V x 1 M: = 67 pF (11) A standard value of 270 pF may be chosen. 7 Feedback Resistors In order to reduce noise at the feedback pin, RFB2 is typically on the order of 1kΩ. To calculate the value of RFB1, one may use the relationship: RFB1 = RFB2 § VOUT ¨ VFB © · ¹ - 1¸ (12) Where VFB is the internal reference voltage (1.255 V typical). The output voltage value can be set in a precise manner by taking into account the fact that the reference voltage is regulating the bottom of the output ripple as opposed to the average value. This relationship is shown in Figure 4. 4 AN-1410 LM2696 Demonstration Board SNVA129B – October 2005 – Revised April 2013 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Soft-Start Capacitor www.ti.com VOUT VOUT_AVG 'VOUT VREF Time Figure 4. Average and Ripple Output Voltages One should note that for high output voltages (>5 V), a load of approximately 15mA may be required for the output voltage to reach the desired value. The resistors for this demo board were selected as: RFB2 = 1kΩ RFB1 = 1 k: 8 § 2.5V - 1· = 1 k: ©1.25V ¹ (13) Soft-Start Capacitor The SS capacitor is used to slowly ramp the reference from 0 V to its final value of 1.25 V. The startup time may be calculated using Equation 14: tSS = 1.25V x CSS ISS x 103 (14) or conversely, capacitance as a function of startup time: CSS = ISS tSS 1.25V x 10-3 (15) Where ISS is the soft-start pin source current (1µA typical) in µA, CSS is in µF, and tSS is in ms. The soft-start capacitor was selected such that the soft start time would be approximately 12.5 ms. The capacitor value was calculated as: CSS = 0.01 PF = 1 PA 9 12.5 ms x 10-3 1.25V (16) Shutdown The state of the shutdown pin enables the device or places it in a sleep state. This pin has an internal pullup and may be left floating or connected to a high logic level. Connecting this pin to GND will shutdown the part. This pin must be bypassed with a 1nF ceramic capacitor to ensure proper logic thresholds. 10 Layout Guidelines Good layout for DC-DC converters can be implemented by following a few simple design guidelines: 1. Place the power components (catch diode, inductor, and filter capacitors) close together. Make the traces between them as short and wide as possible. 2. Use wide traces between the power components and for power connections to the DC-DC converter circuit. SNVA129B – October 2005 – Revised April 2013 Submit Documentation Feedback AN-1410 LM2696 Demonstration Board Copyright © 2005–2013, Texas Instruments Incorporated 5 PCB Layouts www.ti.com 3. Connect the ground pins of the input and output filter capacitors and catch diode as close as possible using generous component-side copper fill as a pseudo-ground plane. Then, connect this to the ground plane through several vias. 4. Arrange the power components so that the switching loops curl in the same direction. 5. Separate noise sensitive traces, such as the voltage feedback path, from noisy traces associated with the power components. 6. Ensure a low-impedance ground for the converter IC. 7. Place the supporting components for the converter IC, including frequency selection components as close to the converter IC as possible, but away from noisy traces and the power components. Make their connections to the converter IC and its pseudoground plane as short as possible. 8. Place noise sensitive circuitry such as radio or modem blocks away from the DC-DC converter. 11 PCB Layouts Figure 5. Top Layer Figure 6. Bottom Layer 6 AN-1410 LM2696 Demonstration Board SNVA129B – October 2005 – Revised April 2013 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Alternate Application Circuit www.ti.com 12 Alternate Application Circuit LM2696 VPGOOD PGOOD VSD EXTVCC SD CVCC CSD RON CBOOT RON VIN CIN CBY CAVIN CSS CBOOT AVIN SW PVIN GND L VOUT RFB1 DSW SS COUT FB RFB2 Figure 7. 5 V to 2.5 V Voltage Applications Circuit ID Part Number Type Size Parameters Qty Vendor U1 LM2696 3A Constant ontime Regulator HTSSOP-16 1 NSC L MSS1260-103MX Inductor MSS1260 10 µH, 4.0A ISAT 1 Coilcraft CIN EEUFC1V181 Capacitor 10 x 12.5 180 µF, 35 V, 90 mΩ 1 Panasonic CBY VJ0805Y104KXAM Capacitor CSS VJ080JY103KXX Capacitor 0805 0.1 µF 1 Vishay 0805 0.01 µF 1 Vishay CVCC VJ0805Y105JXACW1BC Capacitor 0805 1 µF 1 Vishay CBOOT VJ0805Y104KXAM Capacitor 0805 0.1 µF 1 Vishay CAVIN VJ0805Y105JXACW1BC Capacitor 0805 1 µF 1 Vishay COUT TPSC107M006R0075 Capacitor C 100 µF, 6 V, 75 mΩ 1 AVX CSD VJ0805Y102KXXA Capacitor 0805 1 nF 1 Vishay RFB1 CRCW08051651F Resistor 0805 1.65 kΩ 1 Vishay RFB2 CRCW08051001F Resistor 0805 1 kΩ 1 Vishay RON CRCW08051543F Resistor 0805 154 kΩ 1 Vishay DSW CMSH3-40M-NST Schottky Diode SMB 40 V @ 3A diode, VF = 0.55 V 1 Central Semiconduct or 160-1026-02-0500 Solder Terminals 7 Wearnes SNVA129B – October 2005 – Revised April 2013 Submit Documentation Feedback Terminals for VIN, GND and VOUT AN-1410 LM2696 Demonstration Board Copyright © 2005–2013, Texas Instruments Incorporated 7 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. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily performed. TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional restrictions. Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use of any TI components in safety-critical applications. In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and requirements. Nonetheless, such components are subject to these terms. No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties have executed a special agreement specifically governing such use. Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and regulatory requirements in connection with such use. TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of non-designated products, TI will not be responsible for any failure to meet ISO/TS16949. Products Applications Audio www.ti.com/audio Automotive and Transportation www.ti.com/automotive Amplifiers amplifier.ti.com Communications and Telecom www.ti.com/communications Data Converters dataconverter.ti.com Computers and Peripherals www.ti.com/computers DLP® Products www.dlp.com Consumer Electronics www.ti.com/consumer-apps DSP dsp.ti.com Energy and Lighting www.ti.com/energy Clocks and Timers www.ti.com/clocks Industrial www.ti.com/industrial Interface interface.ti.com Medical www.ti.com/medical Logic logic.ti.com Security www.ti.com/security Power Mgmt power.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video RFID www.ti-rfid.com OMAP Applications Processors www.ti.com/omap TI E2E Community e2e.ti.com Wireless Connectivity www.ti.com/wirelessconnectivity Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265 Copyright © 2013, Texas Instruments Incorporated