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MP2351DQ Datasheet(PDF) 5 Page - Monolithic Power Systems |
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MP2351DQ Datasheet(HTML) 5 Page - Monolithic Power Systems |
5 / 8 page MP2351 – 1.5A, 23V, 1.4MHz STEP-DOWN CONVERTER MP2351 Rev. 1.5 www.MonolithicPower.com 5 1/6/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. TM APPLICATION INFORMATION COMPONENT SELECTION Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage divider divides the output voltage down to the feedback voltage by the ratio: 2 R 1 R 2 R V V OUT FB + = Where VFB is the feedback voltage and VOUT is the output voltage. Thus the output voltage is: 2 R 2 R 1 R 23 . 1 VOUT + × = R2 can be as high as 100kΩ, but a typical value is 10kΩ. Using that value, R1 is determined by: ) 23 . 1 V ( 18 . 8 1 R OUT − × = For example, for a 3.3V output voltage, R2 is 10kΩ, and R1 is 17kΩ. Inductor The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × × = IN OUT L S OUT V V 1 ∆I f V L Where VIN is the input voltage, fS is the switching frequency and ∆IL is the peak-to-peak inductor ripple current. Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × × × + = IN OUT S OUT LOAD LP V V 1 L f 2 V I I Where, ILOAD is the load current. Output Rectifier Diode The output rectifier diode supplies the current to the inductor when the high-side switch is off. To reduce losses due to the diode forward voltage and recovery times, use a Schottky diode. Choose a diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating is greater than the maximum load current. Input Capacitor The input current to the step-down converter is discontinuous, therefore a capacitor is required 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 are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Since the input capacitor (C1) absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by: ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ × − × = 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 whose RMS current rating greater than half of the maximum load current. |
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