Datasheet MP2307 (Monolithic Power Systems) - 7

MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER APPLICATIONS INFORMATION COMPONENT SELECTION
The inductance value can be calculated by:
Setting the Output Voltage
V ⎛ V ⎞ The output voltage is set using a resistive L = OUT × OUT ⎜⎜1− ⎟⎟ f × ∆I V voltage divider connected from the output S L ⎝ IN ⎠ voltage to FB. The voltage divider divides the Where VOUT is the output voltage, VIN is the output voltage down to the feedback voltage by input voltage, fS is the switching frequency, and the ratio: ∆IL is the peak-to-peak inductor ripple current. R2 V = V Choose an inductor that will not saturate under FB OUT 1 R + R2 the maximum inductor peak current, calculated Thus the output voltage is: by: 1 R + R2 V ⎛ V ⎞ V = 0.925 × I = I + OUT × 1− OUT OUT LP LOAD ⎜⎜ ⎟⎟ R2 2 × f × S L ⎝ IN V ⎠ R2 can be as high as 100kΩ, but a typical value Where ILOAD is the load current. is 10kΩ. Using the typical value for R2, R1 is determined by: The choice of which style inductor to use mainly depends on the price vs. size requirements and 1 R = 10.81× (V − 0.925) OUT (kΩ) any EMI constraints. For example, for a 3.3V output voltage, R2 is
Optional Schottky Diode
10kΩ, and R1 is 26.1kΩ. Table 1 lists During the transition between the high-side recommended resistance values of R1 and R2 switch and low-side switch, the body diode of for standard output voltages. the low-side power MOSFET conducts the inductor current. The forward voltage of this
Table 1—Recommended Resistance Values
body diode is high. An optional Schottky diode MP2393
VOUT R1 R2
may be paralleled between the SW pin and 1.8V 9.53kΩ 10kΩ GND pin to improve overall efficiency. Table 2 lists example Schottky diodes and their 2.5V 16.9kΩ 10kΩ O Manufacturers. 3.3V 26.1kΩ 10kΩ 5V 44.2kΩ 10kΩ
Table 2—Diode Selection Guide
12V 121kΩ 10kΩ
Voltage/Current Part Number Vendor Rating
RECOMMENDED FOR
Inductor
The inductor is required to supply constant B130 30V, 1A Diodes, Inc. NEW DESIGNS current to the load while being driven by the SK13 30V, 1A Diodes, Inc. switched input voltage. A larger value inductor International MBRS130 30V, 1A will result in less ripple current that will in turn Rectifier result in lower output ripple voltage. However,
Input Capacitor
NOT the larger value inductor will have a larger The input current to the step-down converter is physical size, higher series resistance, and/or discontinuous, therefore a capacitor is required REFER T lower saturation current. A good rule for to supply the AC current while maintaining the determining inductance is to allow the peak-to- DC input voltage. Use low ESR capacitors for peak ripple current to be approximately 30% of the best performance. Ceramic capacitors are the maximum switch current limit. Also, make preferred, but tantalum or low-ESR electrolytic sure that the peak inductor current is below the capacitors will also suffice. Choose X5R or maximum switch current limit. X7R dielectrics when using ceramic capacitors. MP2307 Rev. 1.9 www.MonolithicPower.com
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5/28/2008 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved.