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MPQ8634B Datasheet(HTML) 18 Page - Monolithic Power Systems |
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MPQ8634B Datasheet(HTML) 18 Page - Monolithic Power Systems |
![]() MPQ8634B – 16V, 20A, SYNC, STEP-DOWN CONVERTER W/ ADJUSTABLE CURRENT LIMIT, PROGRAMMBALE FREQUENCY, AND VOLTAGE TRACKING MPQ8634B Rev. 1.02 www.MonolithicPower.com 18 11/16/2018 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2018 MPS. All Rights Reserved. APPLICATION INFORMATION Selecting the Input Capacitor The input current to the step-down converter is discontinuous and therefore requires a capacitor to supply AC current to the step-down converter while maintaining the DC input voltage. Use ceramic capacitors for the best performance. During layout, place the input capacitors as close to VIN as possible. The capacitance can vary significantly with the temperature. Capacitors with X5R and X7R ceramic dielectrics are recommended because they are fairly stable over a wide temperature range and offer very low ESR. The capacitors must have a ripple current rating that exceeds the converter’s maximum input ripple current. Estimate the input ripple current with Equation (9): ) V V 1 ( V V I I IN OUT IN OUT OUT CIN (9) The worst-case condition occurs at VIN = 2VOUT, shown in Equation (10): 2 I I OUT CIN (10) For simplification, choose an input capacitor with an RMS current rating that exceeds half the maximum load current. The input capacitor value determines the converter input voltage ripple. If there is an input voltage ripple requirement in the system, select an input capacitor that meets the specification. Estimate the input voltage ripple with Equation (11): ) V V 1 ( V V C F I V IN OUT IN OUT IN SW OUT IN (11) The worst-case condition occurs at VIN = 2VOUT, shown in Equation (12): IN SW OUT IN C F I 4 1 V (12) Selecting the Output Capacitor The output capacitor maintains the DC output voltage. Use POSCAP or ceramic capacitors. Estimate the output voltage ripple with Equation (13): ) C F 8 1 R ( ) V V 1 ( L F V V OUT SW ESR IN OUT SW OUT OUT (13) When using ceramic capacitors, the capacitance dominates the impedance at the switching frequency. The capacitance also dominates the output voltage ripple. For simplification, estimate the output voltage ripple with Equation (14): ) V V 1 ( C L F 8 V V IN OUT OUT 2 SW OUT OUT (14) The ESR dominates the switching frequency impedance for the POSCAP capacitors. For simplification, the output ripple can be approximated with Equation (15): ESR IN OUT SW OUT OUT R ) V V 1 ( L F V V (15) Selecting the Inductor The inductor supplies constant current to the output load while being driven by the switching input voltage. A larger value inductor results in less ripple current and a lower output ripple voltage, but also has a larger physical size, higher series resistance, and lower saturation current. Select an inductor value that allows the inductor peak-to-peak ripple current to be 30 - 40% of the maximum switch current limit. Also design for a peak inductor current that is below the maximum switch current limit. Calculate the inductance value using Equation (16): ) V V 1 ( I F V L IN OUT L SW OUT (16) Where ∆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 with Equation (17): ) V V 1 ( L F 2 V I I IN OUT SW OUT OUT LP (17) |
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