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MP1475S Fiches technique(PDF) 14 Page - Monolithic Power Systems |
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MP1475S Fiches technique(HTML) 14 Page - Monolithic Power Systems |
14 / 19 page MP1475S – HIGH-EFFICIENCY, 3A, 16V, 500kHz SYNCHRONOUS STEP-DOWN CONVERTER MP1475S Rev. 1.0 www.MonolithicPower.com 14 1/8/2015 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2015 MPS. All Rights Reserved. APPLICATION INFORMATION Setting the Output Voltage The external resistor divider sets the output voltage (see “Typical Application” on page 1). Choose R1 around 40.2kΩ; R2 is then given by: OUT R1 R2 V 1 0.807V = − The T-type network is recommended highly when VOUT is low (see Figure 7). Figure 7. T-Type Network Table 1 lists the recommended T-type resistor values for common output voltages. Table 1. Resistor Selection for Common Output Voltages (8) VOUT (V) R1 (kΩ) R2 (kΩ) Rt (kΩ) 1.0 20.5 84.5 82 1.2 30.1 61.9 82 1.8 40.2 32.4 33 2.5 40.2 19.1 33 3.3 40.2 13 16 5 40.2 7.68 16 Notes: 8) The recommended parameters are based on a 500kHz switching frequency; a different input voltage, output-inductor value, and output-capacitor value may affect the selection of R1, R2, and Rt. For additional component parameters, please refer to the “Typical Application Circuits” section on pages 17 and 18. Selecting the Inductor For most applications, use a1µH to 22µH inductor with a DC current rating at least 25% higher than the maximum load current. For highest efficiency, use an inductor with a DC resistance less than 15mΩ. For most designs, the inductance value is derived from the following equation: OUT IN OUT 1 IN L OSC V(V V ) L VI f ×− = ×Δ × Where ΔIL is the inductor-ripple current. Choose an inductor-ripple current approximately 30% of the maximum load current. The maximum inductor peak current is calculated by the following equation: 2 I I I L LOAD ) MAX ( L Δ + = Use a larger inductor for improved efficiency under light-load conditions (below 100mA). Selecting the Input Capacitor The input current to the step-down converter is discontinuous, therefore it requires a capacitor to supply the AC current while maintaining the DC input voltage. Use low ESR capacitors for optimum performance. Use ceramic capacitors with X5R or X7R dielectrics for best results because of their low ESR and small temperature coefficients. For most applications, use a 22µF capacitor. Since C1 absorbs the input-switching current, it requires an adequate ripple-current rating. The RMS current in the input capacitor is estimated by: ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ × − × = IN OUT IN OUT LOAD 1 C V V 1 V V I I The worst case condition occurs at VIN = 2VOUT, where: 2 I I LOAD 1 C = For simplification, choose an input capacitor that has a RMS current rating greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum, or ceramic. When using electrolytic or tantalum capacitors, a small, high-quality ceramic capacitor (e.g. 0.1μF) should be placed as close to the IC as possible. When using ceramic capacitors, ensure that they have enough capacitance to provide sufficient charge |
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