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MP2158A Folha de dados(PDF) 11 Page - Monolithic Power Systems |
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MP2158A Folha de dados(HTML) 11 Page - Monolithic Power Systems |
11 / 14 page MP2158A – 1A, 6V, 1.5MHz SYNCHRONOUS STEP-DOWN CONVERTER MP2158A Rev. 1.02 www.MonolithicPower.com 11 6/30/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2016 MPS. All Rights Reserved. APPLICATION INFORMATION COMPONENT SELECTION Setting the Output Voltage The external resistor divider sets the output voltage (see Typical Application on page 1). The feedback resistor R1 cannot be too large or too small, considering the trade-off for a dynamic circuit and stability in the circuit. Set R1 to around 120kΩ to 200kΩ. R2 is then given by Equation (2): out R1 R2 V 1 0.6 (2) The feedback circuit is shown in Figure 2. R1 R2 Vout FB MP2158A Figure 2: Feedback Network Table 1 lists the recommended resistor values for common output voltages. Table 1: Resistor Selection for Common Output Voltages VOUT (V) R1 (kΩ) R2 (kΩ) 1.0 200 (1%) 300 (1%) 1.2 200 (1%) 200 (1%) 1.8 200 (1%) 100 (1%) 2.5 200 (1%) 63.2 (1%) 3.3 200 (1%) 44.2 (1%) Selecting the Inductor A 0.68µH to 2.2µH inductor is recommended for most applications. For the highest efficiency, choose an inductor with a DC resistance of less than 15mΩ. For most designs, the inductance value can be derived from Equation (3): OUT IN OUT 1 IN L OSC V(V V ) L VI f (3) Where ΔIL is the inductor ripple current. Set the inductor current to approximately 30% of the maximum load current. The maximum inductor peak current is calculated in Equation (4): 2 I I I L LOAD ) MAX ( L (4) Selecting the Input Capacitor The input current to the step-down converter is discontinuous and requires a capacitor to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR values and small temperature coefficients. For most applications, a 10µF capacitor is sufficient. For a higher output voltage, a 22μF capacitor may be needed to improve system stability. Since the input capacitor absorbs the input switching current, it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated with Equation (5): IN OUT IN OUT LOAD 1 C V V 1 V V I I (5) The worse-case condition occurs at VIN = 2VOUT, shown in Equation (6): 2 I I LOAD 1 C (6) 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, use a small, high-quality ceramic capacitor (0.1Μf) and place it as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide a sufficient charge to prevent an excessive voltage ripple at the input. The input voltage ripple caused by capacitance can be estimated with Equation (7): LOAD OUT OUT IN IN SIN IV V V1 fC1 V V (7) |
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