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SS24 Arkusz danych(PDF) 5 Page - Monolithic Power Systems |
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SS24 Arkusz danych(HTML) 5 Page - Monolithic Power Systems |
5 / 11 page MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER MP1591 Rev. 2.3 www.MonolithicPower.com 5 9/27/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. APPLICATION INFORMATION COMPONENT SELECTION Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB. The voltage divider divides the output voltage down by the ratio: ) 2 R 1 R ( 2 R V V OUT FB + × = Where VFB is the feedback voltage and VOUT is the output voltage. Thus the output voltage is: 2 R ) 2 R 1 R ( 230 . 1 VOUT + × = A typical value for R2 can be as high as 100kΩ, but 10kΩ is recommended. Using that value, R1 is determined by: ) 230 . 1 V ( 18 . 8 1 R OUT − × ≅ For example, for a 3.3V output voltage, R2 is 10kΩ, and R1 is 17kΩ. Inductor (L1) The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor results in less ripple current that results in lower output ripple voltage. However, the larger value inductor has a larger physical size, higher series resistance, and/or lower saturation current. Choose an inductor that does not saturate under the worst-case load conditions. A good rule to use for determining the inductance is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum load current that the IC can provide. Also, make sure that the peak inductor current (the load current plus half the peak-to-peak inductor ripple current) is below the 2.3A minimum current limit. The inductance value can be calculated by the equation: ) I f V ( ) V V ( V 1 L IN OUT IN OUT ∆ × × − × = Where VIN is the input voltage, f is the switching frequency and ∆I is the peak-to-peak inductor ripple current. Table 1 lists a number of suitable inductors from various manufacturers. Table 1—Inductor Selection Guide Package Dimensions (mm) Vendor/ Model Core Type Core Material W L H Sumida CR75 Open Ferrite 7.0 7.8 5.5 CDH74 Open Ferrite 7.3 8.0 5.2 CDRH5D28 Shielded Ferrite 5.5 5.7 5.5 CDRH5D28 Shielded Ferrite 5.5 5.7 5.5 CDRH6D28 Shielded Ferrite 6.7 6.7 3.0 CDRH104R Shielded Ferrite 10.1 10.0 3.0 Toko D53LC Type A Shielded Ferrite 5.0 5.0 3.0 D75C Shielded Ferrite 7.6 7.6 5.1 D104C Shielded Ferrite 10.0 10.0 4.3 D10FL Open Ferrite 9.7 1.5 4.0 Coilcraft DO3308 Open Ferrite 9.4 13.0 3.0 DO3316 Open Ferrite 9.4 13.0 5.1 Input Capacitor (C1) The input current to the step-down converter is discontinuous, and so a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. A low ESR capacitor is required to keep the noise at the IC to a minimum. Ceramic capacitors are preferred, but tantalum or low ESR electrolytic capacitors may also suffice. |
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