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MP2224 Datasheet(PDF) 13 Page - Monolithic Power Systems |
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MP2224 Datasheet(HTML) 13 Page - Monolithic Power Systems |
13 / 19 page MP2224 –18V/4A SYNCHRONOUS STEP-DOWN CONVERTER MP2224 Rev. 1.0 www.MonolithicPower.com 13 12/18/2015 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). Also, the feedback resistor R1 sets the feedback loop bandwidth with the internal compensation capacitor. First, choose a value for R1, R2 is then given with Equation (1): OUT R1 R2 V 1 0.6V (1) The feedback network —as shown in Figure 5— is highly recommended. R1 R2 VOUT FB 8 Figure 5: Feedback Network Table 1 lists the recommended resistors values for common output voltages. Table 1: Component Selection for Common Output Voltages (9) VOUT (V) R1 (kΩ) R2 (kΩ) 1.0 120 180 1.2 120 120 1.35 100 80.6 1.5 80.6 53.6 1.8 80.6 40.2 2.5 80.6 25.5 3.3 40.2 8.87 5 40.2 5.49 9) The recommended parameters are based on a 500kHz switching frequency. Different output inductors and capacitors affect the recommended values of R1 and R2. For the other components ’ parameters, please refer to the TYPICAL APPLICATION CIRCUITS on pages 16-18. Selecting the Inductor Use a 1µH to 10µH inductor with a DC current rating at least 25% 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 Equation (2). OUT IN OUT 1 IN L OSC V (V V ) L V I f (2) Where ΔI L 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 (3) 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 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 with Equation (4): IN OUT IN OUT LOAD 1 C V V 1 V V I I (4) The worse case condition occurs at VIN = 2VOUT, see Equation (5): 2 I I LOAD 1 C (5) 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 |
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