LM22670EVAL

User's Guide SNVA361D – September 2008 – Revised April 2013 AN-1885 LM22670 Evaluation Board 1 Introduction The LM22670 evaluation board is designed to demonstrate the capabilities of the LM22670 switching regulator. The LM22670 evaluation board schematic shown in Figure 1 is configured to provide an output voltage of 3.3V up to 3A load current with an input voltage range of 4.5V to 42V. Due to the low RDS(ON) of the integrated N-channel MOSFET and maximum duty-cycle limitations, the minimum input voltage of 4.5V will only provide a 3.3V output voltage for load currents up to 500 mA. For load currents higher than 500 mA, an input voltage of 5.5V or higher is required. The typical operating frequency is 500 kHz. The evaluation board is designed to operate at ambient temperatures up to 50°C. The performance of the evaluation board is as follows: • Input Range: 4.5V to 42V • Output Voltage: 3.3V • Output Current Range: 0A to 3A • Frequency of Operation: 500 kHz • Board Size: 1.5 × 1.5 inches • Package: PSOP-8 At low input voltages between 4.5V and 6V, a minimum load of approximately 5 mA may be required to reach a regulated 3.3V output voltage. For more details, see LM22670/LM22670Q 42V, 3A SIMPLE SWITCHER, Step-Down Voltage Regulator with Features (SNVS584). To aid in the design and evaluation of DC/DC buck converter solutions based on the LM22670 switching regulator, the evaluation board can be re-configured for different output voltages. The evaluation board is designed to highlight applications with a small solution size. This implies that there will be a tradeoff with the area of heat dissipation available. If this evaluation board is operated continuously at a full 3A load, it will get hot. For higher output voltages than the pre-adjusted 3.3V, the total output power as well as the total power conversion losses will increase. It is recommended to use a fan or other source of air flow when evaluating the board at full 3A load or at output voltages greater than 3.3V. Typical evaluation board performance and characteristics curves are shown in Section 7. The PCB layout is shown in Section 8. Test points are provided to enable easy connection and monitoring of critical signals. When performing over load or short circuit tests, refer to the current limit section of the LM22670 datasheet (SNVS584) to determine if the circuit is in safe operating mode. For more information about device function and electrical characteristics, refer to the LM22670 datasheet (SNVS584). WEBENCH is a registered trademark of Texas Instruments. All other trademarks are the property of their respective owners. SNVA361D – September 2008 – Revised April 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated AN-1885 LM22670 Evaluation Board 1 Evaluation Board Setup www.ti.com R1 976: VIN 4.5V to 42V FB VIN EN C2 22 PF + SYNC C1 6.8 PF C3 10 nF LM22670-ADJ EN RT/SYNC BOOT GND R3* R2 1.54 k: L1 8.2 PH SW D1 60V, 5A VOUT 3.3V C4 120 PF + C5* GND GND *component not populated on LM22670EVAL evaluation board Figure 1. Evaluation Board Schematic 2 Evaluation Board Setup Before applying power to the LM22670 evaluation board, all external connections should be verified. The external power supply must be turned off and connected with proper polarity to the VIN and GND posts. A load resistor or electronic load should be connected between the VOUT and GND posts as desired. Both the VIN and VOUT connections should use the closest GND posts respective to VIN or VOUT. The output voltage can be monitored with a multi-meter or oscilloscope at the VOUT post. Once all connections to the evaluation board have been verified, input power can be applied. A load resistor or electronic load does not require connection during startup. If the EN test point is left floating, the output voltage will ramp up when an input voltage is applied. Make sure that the external power supply (input voltage power source) is capable of providing enough current so that the adjusted output voltage can be obtained. Keep in mind that the startup current will be greater than the steady state current. 3 Synchronization and Adjustable Frequency A SYNC test point is provided to allow synchronization of the LM22670 to an external clock signal. Space for a frequency adjust resistor, R3, is also provided. Refer to the LM22670 datasheet (SNVS584) for more details about the synchronizing and adjustable frequency features. 4 Precision Enable The EN test point provided on the LM22670 evaluation board can be used to shut down the LM22670 when connecting this test point to ground. 2 AN-1885 LM22670 Evaluation Board SNVA361D – September 2008 – Revised April 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated Component Selection www.ti.com 5 Component Selection Before changing the default components, refer to LM22670/LM22670Q 42V, 3A SIMPLE SWITCHER, Step-Down Voltage Regulator with Features (SNVS584) for information regarding component selection. The WEBENCH® designer online circuit simulation tool is also available on the TI website. The output voltage is adjustable with resistors R1 and R2, shown in Figure 1. Any changes to these evaluation board feedback resistors may require changes to the inductor and output capacitor values. It is especially important to change the output capacitor, C4, if the output voltages are adjusted higher than 5V. The Schottky diode, D1, has a voltage rating of 60V to allow for a 42V maximum input voltage. If the input voltage is below 38V, a 40V Schottky diode may be used with a lower forward voltage to improve efficiency. For improved heat dissipation, a Schottky diode in a D2PAK package may be selected for higher efficiency. The input capacitor, C2, is not always required. This capacitor is placed on the evaluation board to make the application robust and minimize input voltage ringing if power is suddenly applied. Capacitor C2 also helps stabilize the transfer function of the converter loop. The input capacitor C1 provides the main contribution to the switch current. Both capacitors are selected with appropriate values intended only for the evaluation of the LM22670. For production designs, the impedance of the power source as well as the ripple current rating of the selected input capacitors need to be taken into consideration and modified accordingly. See the LM22670 datasheet (SNVS584) for more information. Output capacitor C5 is not populated but space is provided to add a second output capacitor. This second output capacitor may be used to further reduce output voltage ripple. 6 Bill of Materials Table 1. LM22670EVAL Bill of Materials Ref # Value Supplier Part Number C1 6.8 µF 50V ceramic TDK C4532X7R1H685M C2 22 µF 63V electrolytic Panasonic EEEFK1J220XP C3 10 nF 50V ceramic TDK C1608X7R1H103K C4 120 µF 6.3V 24mohm Nippon Chemi-Con APXE6R3ARA121ME61G C5 Not populated - C6 Not populated; not applicable for LM22670 - D1 60V 5A CMSH5-60 Central Semiconductor CMSH5-60 L1 8.2 µH 6.25A WE-PD L Wurth 744771008 Coilcraft MSS1260-822MX R1 976Ω 1% - CRCW0603976RFKEA R2 1.54 kΩ 1% - CRCW06031K54FKEA R3 Not populated U1 Texas Instruments SNVA361D – September 2008 – Revised April 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated LM22670 AN-1885 LM22670 Evaluation Board 3 Performance Characteristics 7 www.ti.com Performance Characteristics Unless otherwise specified, VIN = 12V, TA = 25°C. 4 Figure 2. Start-Up Waveforms (Load Resistor = 1.1Ω) Figure 3. Operation at 3A Figure 4. Efficiency vs IOUT Figure 5. Overall Loop Gain and Phase (IOUT = 3A) AN-1885 LM22670 Evaluation Board SNVA361D – September 2008 – Revised April 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated PCB Layout Diagram www.ti.com 8 PCB Layout Diagram Figure 6. Top Layer SNVA361D – September 2008 – Revised April 2013 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated AN-1885 LM22670 Evaluation Board 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. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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