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MP2234S Scheda tecnica(PDF) 14 Page - Monolithic Power Systems |
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MP2234S Scheda tecnica(HTML) 14 Page - Monolithic Power Systems |
14 / 19 page MP2234S – SYNCHRONOUS STEP-DOWN CONVERTER MP2234S Rev.1.0 www.MonolithicPower.com 14 4/3/2015 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 40kΩ for VOUT>1.2V, R2 is then given by: OUT R1 R2 V 1 0.804V = − The T-Type network is recommended highly (see Figure 7) Figure 7. T-Type Network Table 1 lists the recommended resistors and compensation values for common output voltages. Table 1: Resistor Selection for Common Output Voltages (8) VOUT (V) R1 (kΩ) R2 (kΩ) Rt (kΩ) 1 20.5 84.5 34 1.2 30.1 61.9 24 1.8 40.2 32.4 15 2.5 40.2 19.1 6.8 3.3 40.2 13 5.6 5 40.2 7.68 2 Notes: 8) The recommended parameters are based on a 800kHz 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 Use a 1µH-to-22µH inductor with a DC current rating of at least 25% percent higher than the maximum load current for most applications. For highest efficiency, use an inductor with a DC resistance less than 15mΩ. For most designs, the inductance value can be 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 the inductor ripple current to be approximately 30% of the maximum load current. The maximum inductor peak current is: 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 requires a capacitor is to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best 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 can be 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 with an 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, add a small, high quality ceramic capacitor (e.g. 0.1μF) placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input |
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