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MP38115 Datasheet(PDF) 6 Page - Monolithic Power Systems |
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MP38115 Datasheet(HTML) 6 Page - Monolithic Power Systems |
6 / 10 page MP38115 – 4A, 5.5V SYNCHRONOUS STEP-DOWN SWITCHING REGULATOR MP38115 Rev. 0.92 www.MonolithicPower.com 6 8/11/2009 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2009 MPS. All Rights Reserved. APPLICATION INFORMATION Output Voltage Setting The external resistor divider sets the output voltage (see Page 1, Schematic Diagram). The feedback resistor R1 also sets the feedback loop bandwidth with the internal compensation (refer to description function). The relation between R1 and feedback loop bandwidth (fC), output capacitance (CO) is as follows: ) F ( C ) KHz ( fc 10 24 . 1 ) K ( 1 R O 6 μ × × = Ω The feedback loop bandwidth (fC) is no higher than 1/10th of switching frequency of MP2107. In the case of ceramic capacitor as CO, it is usually set in the range of 50KHz and 150KHz for optimal transient performance and good phase margin. If an electrolytic capacitor is used, the loop bandwidth is no higher than 1/4 of the ESR zero frequency (fESR). fESR is given by: ESR ESR O 1 f 2R C = π× × For example, choose fC=70KHz with a ceramic capacitor, CO=47μF, R1 is estimated to be 400KΩ. R2 is then given by: OUT R1 R2 = V -1 0.8V Table 1—Resistor Selection vs. Output Voltage Setting Vout R1 R2 L Cout (Ceramic) 1.2V 400kΩ 806kΩ 0.47μH-1μH 47μF 1.5V 400kΩ 453kΩ 0.47μH-1μH 47μF 1.8V 400kΩ 316kΩ 0.47μH-1μH 47μF 2.5V 400kΩ 187kΩ 0.47μH-1μH 47μF 3.3V 400kΩ 127kΩ 0.47μH-1μH 47μF Inductor Selection A 0.47µH to 1µH inductor with DC current rating at least 25% higher than the maximum load current is recommended for most applications. For best efficiency, the inductor DC resistance shall be <10mΩ. See Table 2 for recommended inductors and manufacturers. For most designs, the inductance value can be derived from the following equation: OUT IN OUT IN L OSC Vx(V - V ) L= VxΔIxf where ∆IL is Inductor Ripple Current. Choose inductor ripple current approximately 30% of the maximum load current, 4A. The maximum inductor peak current is: L L(MAX) LOAD ΔI I= I + 2 Under light load conditions, larger inductance is recommended for improved efficiency. Input Capacitor Selection The input capacitor reduces the surge current drawn from the input and the switching noise from the device. The input capacitor impedance at the switching frequency shall be less than input source impedance to prevent high frequency switching current passing to the input source. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small temperature coefficients. For most applications, a 47µF capacitor is sufficient. Output Capacitor Selection The output capacitor keeps output voltage ripple small and ensures a stable regulation loop. The output capacitor impedance shall be low at the switching frequency. Ceramic capacitors with X5R or X7R dielectrics are recommended. The output ripple ∆VOUT is approximately: Δ≤ OUT IN OUT OUT IN OSC OSC 3 Vx(V - V ) 1 V x(ESR + ) V xf xL 8xf xC External Schottky Diode For this part, an external schottky diode is recommended to be placed close to "SW" and "GND" pins, especially when the output current is larger than 2A. With the external schottky diode, the voltage spike and negative kick on "SW" pin can be minimized; moreover, the conversion efficiency can also be improved a little. For the external schottky diode selection, it's noteworthy that the maximum reverse voltage rating of the external diode should be larger than the maximum input voltage. As for the current rating of this diode, 0.5A rating should be sufficient. |
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