AN1071

Information About the LM2650 Evaluation Board Rev. 1 Information About the LM2650 Evaluation Board Rev. 1 INTRODUCTION A printed circuit board (PCB) has been developed. This application note contains information about the board. GENERAL DESCRIPTION The LM2650 evaluation board is provided as a tool for developing DC/DC converters based on the LM2650 IC. It is configured for single-output, step-down DC/DC converters. Figure 1 is a complete schematic of the board which can accommodate up to 28 components including the LM2650. Table 1 is a complete list of pads for placing components. Note: Not all applications will require the placement of all 28 components. The number of components placed depends on the requirements of the application and the use of features like programmable soft-start. The LM2650 evaluation board is intended to be a reusable tool on which many different converters meeting the requirements of many different applications can be built. It is not intended to demonstrate only one application of the LM2650. National Semiconductor AN-1071 Steven Hunt September 1997 For convenience, a sample of the LM2650 and eight other components have been assembled: a 0.1 µF capacitor at each of C1, C2, C6, and CB, a 0.2 µF capacitor at CVDD, a 0.01 µF capacitor at CSS, a 24.9 kΩ resistor at R2, and a 0Ω resistor at R4. Of the eight, the first seven are common to many applications, and the last is simply a jumper grounding the SYNC input. When the synchronization feature is not being used, the SYNC input should be grounded. EXAMPLE CIRCUITS The components contained in Tables 2, 3, and 4 can be used to build typical application circuits. As with the design of any DC/DC converter, the design of these involved tradeoffs between efficiency, size, and cost. The converters detailed in Table 2 were designed with efficiency as the number one criteria. Those detailed in Table 4 trade slightly higher switching losses for a much smaller inductor. AN100011-5 FIGURE 1. The LM2650 Evaluation Board Rev. 1 Schematic THERMAL PERFORMANCE The 24-lead SO package is a molded plastic package with a solid copper lead frame. Most of the heat generated at the die flows through the lead frame into the 3-ounce copper planes on the board. The board then acts as a heat sink. The junction-to-ambient thermal resistance of the packaged IC mounted on the board has been measured to be 38˚C/W, 37 ˚C/W, and 35 ˚C/W for the dissipation of 1.0W, 1.5W, and 2.0W respectively. These measurements were made in still air. The junction-to-ambient thermal resistance of the packaged IC alone in still air is 78˚C/W. The board is 0.063″ thick FR-4 material. ART Figure 2 through Figure 4 show the PCB art work. AN-1071 © 1997 National Semiconductor Corporation AN100011 www.national.com TABLE 1. A Complete List of Pads for Placing Components Label U1 C1, C2, C3, and C4 C5 # 1 4 0 For placing the LM2650. Capacitors placed here filter high-frequency switching noise from the input power rail. These pads should not be labeled with a C. No component is placed here. They can be used to ground the SLEEP LOGIC input. The pad connected to the SLEEP LOGIC pin can be used to pull the input up. A capacitor placed here also filters high-frequency switching noise from the input power rail but at the VIN rail, the rail used by the signal-level circuits inside the IC. Usually no component is placed here. A capacitor might be placed here for loop compensation purposes, but most applications don’t use it. A capacitor is placed here for loop compensation purposes. The bootstrap capacitor is placed here. A capacitor is placed here for loop compensation purposes. This capacitor and an internal 6.5 kΩ resistor create an integrator pole. The bulk input capacitors are placed here. The output filter capacitors are placed here. A capacitor placed here programs the soft-start interval. A capacitor placed here bypasses the output of the VDD regulator. The inductor is placed here. One of the feedback resistors is placed here. The other feedback resistor is placed here. A resistor is placed here for loop compensation purposes. These pads can be used to ground the SYNC input when the synchronization feature is not being used. The pad connected to the SYNC input can be used to connect the synchronization signal. The evaluation board has a 0Ω jumper placed here to ground the SYNC input. A resistor is also placed here for loop compensation purposes. This resistor and the capacitor placed at CC create an integrator zero. A resistor placed here adjusts the switching frequency up from the nominal 90 kHz. No component is placed here for applications switching at 90 kHz. Resistors placed here program the sleep-in threshold. Resistors placed here program the sleep-out threshold. Notes C6 C9 C10 CB CC CIN1 and CIN2 COUT1, COUT2, and COUT3 CSS CVDD L1 R1 R2 R3 R4 1 1 1 1 1 2 3 1 1 1 1 1 1 1 RC RFA RSIA and RSIC RSOA and RSOC 1 1 2 2 TABLE 2. Components for Two Typical 90 kHz Application Circuits Input Voltage Applicable Cell Stacks Output Input Filter Capacitors C1, C2, and C6 Bootstrap Capacitor CB Soft-start Capacitor CSS VDD Bypass Capacitor CVDD Input Bulk Capacitors CIN1 and CIN2 Inductor L1 Output Capacitors COUT1, COUT2, and COUT3 Feedback Resistors R1 and R2 5V, 3A Out 0.1 µF ceramic chip capacitor 0.1 µF ceramic chip capacitor 0.01 µF ceramic chip capacitor 0.2 µF ceramic chip capacitor 22 µF, 35V AVX TPS Series or Sprague 593D Series tantalum chip capacitor 40 µH (See Table 3.) 220 µF, 10V AVX TPS Series or Sprague 593D Series tantalum chip capacitor R1 = 75 kΩ, 1%, R2 = 24.9 kΩ, 1% 7 to 18V IN 8- to 12-Cell NiCd or NiMH, 3- to 4-Cell Li Ion, 8- to 11-Cell Alkaline, 6-Cell Lead Acid 3.3V, 3A Out 0.1 µF ceramic chip capacitor 0.1 µF ceramic chip capacitor 0.01 µF ceramic chip capacitor 0.2 µF ceramic chip capacitor 22 µF, 35V AVX TPS Series or Sprague 593D Series tantalum chip capacitor 33 µH (See Table 3.) 220 µF, 10V AVX TPS Series or Sprague 593D Series tantalum chip capacitor R1 = 41.2 kΩ, 1%, R2 = 24.9 kΩ, 1% www.national.com 2 TABLE 2. Components for Two Typical 90 kHz Application Circuits (Continued) Compensation Components RC, CC, R3, and C10 Sleep Resistors RSIA and RSOA RC = 37.4 kΩ, CC = 4.7 nF, R3= 3.57 kΩ,C10 = 5.6 nF RSIA = 33 kΩ, RSOA = 200 kΩ RC = 23.2 kΩ, CC= 8.2 nF, R3 = 2.0 kΩ, C10 = 10 nF RSIA = 39 kΩ, RSOA = 130 kΩ TABLE 3. Toroidal Inductors Using Cores from MICROMETALS, INC. Core # 15 µH 20 µH 33 µH 40 µH T38 T38 T50 T50(B) Core Material −52 −52 −52 −18 Wire Gauge AWG #23 AWG #23 AWG #21 AWG #21 # of Strands 1 1 1 1 # of Turns 21 25 41 41 TABLE 4. Components for Two Typical 200 kHz Application Circuits Input Voltage Applicable Cell Stacks Output Input Filter Capacitors C1, C2, and C6 Bootstrap Capacitor CB Soft-start Capacitor CSS VDD Bypass Capacitor CVDD Input Bulk Capacitors CIN1 and CIN2 Inductor L1 Output Capacitors COUT1, COUT2, and COUT3 Feedback Resistors R1 and R2 Compensation Components RC, CC, R3, and C10 Sleep Resistors RSIA and RSOA Frequency Adjusting Resistor RFA 5V, 3A Out 0.1 µF ceramic chip capacitor 0.1 µF ceramic chip capacitor 0.01 µF ceramic chip capacitor 0.2 µF ceramic chip capacitor 22 µF, 35V AVX TPS Series or Sprague 593D Series tantalum chip capacitor 20 µH (See Table 3.) 220 µF, 10V AVX TPS Series or Sprague 593D Series tantalum chip capacitor R1 = 75 kΩ, 1%, R2 = 24.9 kΩ, 1% RC = 53.6 kΩ, CC = 2.7 nF, R3= 4.02 kΩ, C10 = 4.7 nF RSIA = 47 kΩ, RSOA = 200 kΩ RFA = 24.9 kΩ 7 to 18V IN 8- to 12-Cell NiCd or NiMH, 3- to 4-Cell Li Ion, 8- to 11-Cell Alkaline, 6-Cell Lead Acid 3.3V, 3A Out 0.1 µF ceramic chip capacitor 0.1 µF ceramic chip capacitor 0.01 µF ceramic chip capacitor 0.2 µF ceramic chip capacitor 22 µF, 35V AVX TPS Series or Sprague 593D Series tantalum chip capacitor 15 µH (See Table 3.) 220 µF, 10V AVX TPS Series or Sprague 593D Series tantalum chip capacitor R1 = 41.2 kΩ, 1%, R2 = 24.9 kΩ, 1% RC = 33.2 kΩ, CC= 3.9 nF, R3 = 3.01 kΩ, C10 = 6.8 nF RSIA = 47 kΩ, RSOA = 91 kΩ RFA = 24.9 kΩ 3 www.national.com AN100011-1 FIGURE 2. LM2650 Evaluation Board Top Silk Screen (Scale 1:1) AN100011-2 FIGURE 3. LM2650 Evaluation Board Bottom Silk Screen (Scale 1:1) www.national.com 4 AN100011-3 FIGURE 4. LM2650 Evaluation Board Component Side (Scale 1:1) AN100011-4 FIGURE 5. LM2650 Evaluation Board Solder Side (Scale 1:1) 5 www.national.com Information About the LM2650 Evaluation Board Rev. 1 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 OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 2. A critical component in any component of a life support 1. Life support devices or systems are devices or device or system whose failure to perform can be systems which, (a) are intended for surgical implant reasonably expected to cause the failure of the life into the body, or (b) support or sustain life, and support device or system, or to affect its safety or whose failure to perform when properly used in effectiveness. accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. AN-1071 National Semiconductor Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: support@nsc.com www.national.com National Semiconductor Europe Fax: +49 (0) 1 80-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 1 80-530 85 85 English Tel: +49 (0) 1 80-532 78 32 Français Tel: +49 (0) 1 80-532 93 58 Italiano Tel: +49 (0) 1 80-534 16 80 National Semiconductor Hong Kong Ltd. 13th Floor, Straight Block, Ocean Centre, 5 Canton Rd. Tsimshatsui, Kowloon Hong Kong Tel: (852) 2737-1600 Fax: (852) 2736-9960 National Semiconductor Japan Ltd. Tel: 81-3-5620-6175 Fax: 81-3-5620-6179 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.