LM25575BLDT/NOPB

Application Report SNVA211B – January 2007 – Revised April 2013 AN-1571 Quick Start Guide for a 1.5 Amp Buck Regulator Using the LM5575 and LM25575 ..................................................................................................................................................... ABSTRACT This application note provides an easy to use process to help users design a 1.5 Amp buck regular using the LM5575 and LM25575 switching regulators. 1 2 Contents Introduction .................................................................................................................. 2 Quick Guide Design Worksheet ........................................................................................... 2 List of Figures 1 Application Schematic ...................................................................................................... 2 List of Tables 1 Bill of Materials .............................................................................................................. 5 All trademarks are the property of their respective owners. SNVA211B – January 2007 – Revised April 2013 Submit Documentation Feedback AN-1571 Quick Start Guide for a 1.5 Amp Buck Regulator Using the LM5575 and LM25575 Copyright © 2007–2013, Texas Instruments Incorporated 1 Introduction 1 www.ti.com Introduction The LM5575 and LM25575 switching regulators feature all of the functions necessary to implement an efficient high voltage buck regulator using a minimum of external components. These easy to use regulators include either a 42V (LM25575) or a 75V (LM5575) N-Channel buck switch with an output current capability of 1.5 Amps. The operating frequency is programmable from 50kHz up to 1MHz. Protection features include: current limit, thermal shutdown and remote shutdown capability. The device is available in a power enhanced TSSOP-16 package featuring an exposed die attach pad to aid thermal dissipation. This step-by-step guide provides an easy to use process to quickly select the external components necessary to complete a design. More detailed information including theory of operation, design trade-offs and additional application guidance is available in the device datasheet. Shown below in Figure 1 is a complete schematic for a 1.5 Amp step-down DC-DC converter. Several external component values can be standardized for most applications. The input voltage range, output voltage and desired operating frequency dictate the remaining component values. An Excel based spreadsheet derived from the guide is available on www.ti.com. BST INPUT 3 L1 SW SD C2 4 PRE SYNC 5 COMP IS LM(2)5575 Rcomp R4 PGND AGND Ccomp C5 Cboot C6 VIN 2 Cin C1 16 6 OUT FB RAMP RT 8 7 SS OUTPUT 14 15 Rfb2 R2 D1 C9 13 Cout C8 12 Rfb1 R1 9 11 VCC 10 1 Cbyp C7 CRAMP C3 Rt R3 Css C4 Figure 1. Application Schematic 2 Quick Guide Design Worksheet Step 1. List the basic requirements: Output Current: 0 to 1.5 Amp Output Voltage: Input Voltage Min Spec: Input Voltage Max Spec: V [1] V [2] V [3] Step 2. If the Input Voltage Max Spec [3] is less than 42V use the LM25575. If the Input Voltage Max Spec [3] is greater than 42V but less than 75V use the LM5575. Both devices require the Input Voltage Min Spec [2] to be greater than 6V. Selected Regulator: 2 [4] AN-1571 Quick Start Guide for a 1.5 Amp Buck Regulator Using the LM5575 and LM25575 SNVA211B – January 2007 – Revised April 2013 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated Quick Guide Design Worksheet www.ti.com Step 3. Selection of the operating frequency is a trade-off between the conversion efficiency and solution size. Operating at a high frequency, with a relatively high input voltage will severely impact the efficiency and consequently generate a lot of heat. In some applications, the selection of a high operating frequency will limit the input voltage range. The recommended maximum operating frequency for applications using LM5575 is 500 KHz. When using the LM25575 the operating frequency may be set as high as 1MHz. Select a target operating frequency from 50 kHz to 1MHz (500kHz for the LM5575). Check to see if the Vin(min) limits the selected operating frequency: Fsw(max) = Vin(min)[2] ± (Vout[1] + 0.6) Vin(min)[2] x 5.5 x 10-7 (1) The selected operating frequency must be less than Fsw(max) calculated above, if not reduce the operating frequency. Check to see if the Vin(max) limits the selected operating frequency: (Vout[1] + 0.6) Fsw(max) = Vin(max)[3] x 8 x 10-8 (2) The selected operating frequency must be less than Fsw(max) calculated above, if not reduce the operating frequency. Selected operating frequency Fsw: Hz [5] Step 4. Calculate the value of Rt for the selected operating frequency. Rt = 1 - 580 x 10-9 Fsw[5] 135 x 10-12 (3) Selected value for R1: Ohms [6] Step 5. Calculate the value of L1. L1 = Vout[1] x (Vin(max)[3] - Vout[1]) 0.4 x Fsw[6] x Vin(max)[3] (4) Select the nearest standard inductor value. During an overload condition the peak inductor current is limited to 2.1A nominal (2.5A maximum). The selected inductor must be rated for peak current of at least 2.5 Amps. Selected value of L1: Henrys [7] Step 6. Calculate the value of CRAMP: CRAMP = L1[7] x 10-5 (5) Selected value of CRAMP: Farads [8] Step 7. Set Rfb2 to 5kOhms if Vout[1] is less than or equal to 5 Volts. If Vout[1] is greater than 5V set Rfb2 to 10K Ohms. Selected value of Rfb2: Ohms [9] SNVA211B – January 2007 – Revised April 2013 Submit Documentation Feedback AN-1571 Quick Start Guide for a 1.5 Amp Buck Regulator Using the LM5575 and LM25575 Copyright © 2007–2013, Texas Instruments Incorporated 3 Quick Guide Design Worksheet www.ti.com Calculate the value of Rfb1: Rfb1 = 1.225 x Rfb2[9] (Vout[1] ± 1.225) (6) Selected value of Rfb1: Ohms [10] Step 8. Select the re-circulating diode, D1. A Schottky type diode is required for all applications. Ultra-fast diodes are not recommended and may result in damage to the IC due to reverse recovery current transients. The reverse breakdown rating should be greater than the Input Voltage Max Spec[3], plus some safety margin, or worst case design, assume a short circuit load condition. In this case the diode will carry the output current almost continuously. This current can be as high as 2.5A. Assuming a 0.6V drop across the diode, the maximum diode power dissipation can be as high as 1.5W. An SMC or SMB case is recommended. Selected diode part number: [11] Step 9. A good quality input capacitor(s) is necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current during the on-time. The minimum RMS ripple current rating for the input capacitor(s) is 0.75 Amp. A quality ceramic capacitor with a low ESR is recommended. The input capacitor voltage rating should be greater than the Input Voltage Max Spec [3], plus some safety margin. A guide to select the input capacitor(s) value in proportion to the operating frequency is: Cin = 0.7 Fsw[5] (7) Selected value for Cin: Farads [12] Step 10. The output capacitor(s) smooth the inductor ripple current and provide a source of charge for transient loading conditions. A good starting point for the output capacitance is to use a ceramic capacitor (10 µF to 100 µF) An additional low ESR organic or tantalum capacitor (22 µF to 220 µF) could be added in parallel for applications with large load transients. The ceramic capacitor provides ultra low ESR to reduce the output ripple voltage and noise spikes, while the larger bulk capacitor provides a source of charge for transient loading conditions. The output capacitor voltage rating should be greater than the Output Voltage Spec [1], plus some safety margin. An approximation for the output ripple voltage is: 'Vout = 0.4 x ESR + 1 8 x Fsw[5] x Cout Selected value for Cout: (8) Farads [13] Step 11. Ccomp and Rcomp configure the error amplifier gain characteristics to accomplish a stable overall loop gain. One advantage of current mode control is the ability to close the loop with only two feedback components. Calculate the value of Rcomp: Rcomp = 1.2 x 105 x Rfb1[9] x Cout[13] + Selected value of Rcomp: 4 Rfb1[9] Vout[1] (9) Ohms [14] AN-1571 Quick Start Guide for a 1.5 Amp Buck Regulator Using the LM5575 and LM25575 SNVA211B – January 2007 – Revised April 2013 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated Quick Guide Design Worksheet www.ti.com Calculate the value of Ccomp Ccomp = 1 8 x 103 x Rcomp[14] Selected value of Ccomp: (10) Farads [15] Step 12. Shown in Table 1 is the Bill of Materials for your design. Transcribe each value [#] from the worksheet above into the following table. Congratulations, you’re done. 2.1 Bill of Materials Table 1. Bill of Materials SCH REF PART NUMBER DESCRIPTION VALUE WORK SHEET REF C1 (Cin) INPUT CAPACITOR [12] C2 (Cin) OPTIONAL INPUT CAPACITOR [12] C3 (CRAMP) RAMP CAPACITOR C4 (Css) C2012X7R2A103K C5 (Ccomp) CAPACITOR, TDK [8] 0.01µ COMPENSATION CAP -[15] C6 (Cboot) C2012X7R2A223K CAPACITOR, TDK 0.022µ, 100V -- C7 (Cbyp) C2012X7R1C474M CAPACITOR, TDK 0.47µ, 16V -- C8 (Cout) OUTPUT CAPACITOR [13] C9 (Cout) OPTIONAL OUTPUT CAPACITOR [13] D1 SCHOTTKY DIODE [11] L1 INDUCTOR [7] R1 (Rfb1) Feedback RESISTOR [10] R2 (Rfb2) Feedback RESISTOR [9] R3 (Rt) TIMING RESISTOR [6] R4 (Rcomp) COMPENSATION RESISTOR [14] U1 REGULATOR, Texas Instruments [4] SNVA211B – January 2007 – Revised April 2013 Submit Documentation Feedback AN-1571 Quick Start Guide for a 1.5 Amp Buck Regulator Using the LM5575 and LM25575 Copyright © 2007–2013, Texas Instruments Incorporated 5 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. 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