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MP1497S Scheda tecnica(PDF) 11 Page - Monolithic Power Systems |
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MP1497S Scheda tecnica(HTML) 11 Page - Monolithic Power Systems |
11 / 15 page MP1497S – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS MP1497S Rev. 1.01 www.MonolithicPower.com 11 9/7/2015 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). The feedback resistor R1 sets the feedback loop bandwidth. R2 is then given by: OUT R1 R2 V 1 0.807V The T-type network shown in Figure 4 is highly recommended. FB 8 RT R2 R1 VOUT Figure 4: T-Type Network Table 1 lists the recommended T-type resistors value for common output voltages. Table 1: Resistor Values for Common Output Voltages VOUT (V) R1 (kΩ) R2 (kΩ) Rt (kΩ) 1.0 20.5 82 82 1.2 30.1 60.4 82 1.8 40.2 32.4 56 2.5 40.2 19.1 33 3.3 40.2 13 33 5 40.2 7.68 33 Selecting the Inductor Use a 1µH-to-10µH inductor with a DC current rating of at least 25% percent higher than the maximum load current for most applications. For highest efficiency, select an inductor with a DC resistance less than 15mΩ. For most designs, calculate the inductance value with: OUT IN OUT 1 IN L OSC V(V V ) L VI f Where ΔIL is the inductor ripple current. Choose an 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 Use a larger inductance for improved light-load efficiency. Selecting the Input Capacitor The input current to the step-down converter is discontinuous, therefore requires a capacitor supply the AC current to the step-down converter while maintaining the DC input voltage. Use low-ESR capacitors for the best performance, such as ceramic capacitors with X5R or X7R dielectrics that have low ESR and small temperature coefficients. For most applications, use a 22µF capacitor. The input capacitor (C1) requires an adequate ripple current rating because it absorbs the input switching current. Estimate the RMS current in the input capacitor as: 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 the 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, include a small, high-quality, ceramic capacitor—e.g. 0.1μF—as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to prevent excessive input voltage ripple. Estimate the input voltage ripple caused by the capacitance as: LOAD OUT OUT IN IN SIN IV V V1 fC1 V V Selecting the Output Capacitor The output capacitor (C2) maintains the DC output voltage. Use ceramic, tantalum, or low- ESR electrolytic capacitors. Low ESR capacitors are preferred to keep the output |
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