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MP8720 Datasheet(PDF) 18 Page  Monolithic Power Systems 

MP8720 Datasheet(HTML) 18 Page  Monolithic Power Systems 
18 / 23 page MP8720 – 26V, 10A, SYNCHRONOUS BUCK CONVERTER WITH ADJUSTABLE CLM MP8720 Rev. 1.01 www.MonolithicPower.com 18 8/1/2017 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2017 MPS. All Rights Reserved. The worstcase condition occurs at VIN = 2VOUT, shown in Equation (7): OUT CIN I I 2 (7) For simplification, choose an input capacitor with an RMS current rating greater than half of the maximum load current. The input capacitance value determines the input voltage ripple of the converter. If there is an input voltage ripple requirement in the system, choose an input capacitor that meets the specification. The input voltage ripple can be estimated with Equation (8): OUT OUT OUT IN SW IN IN IN IV V V(1 ) FC V V (8) The worstcase condition occurs at VIN = 2VOUT, shown in Equation (9): OUT IN SW IN I 1 V 4F C (9) Selecting the Output Capacitor The output capacitor is required to maintain the DC output voltage. Ceramic or POSCAP capacitors are recommended. The output voltage ripple can be estimated using Equation (10): OUT OUT OUT ESR SW IN SW OUT VV 1 V(1 ) (R ) FL V 8 F C (10) When using ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance, which mainly causes the output voltage ripple. For simplification, the output voltage ripple can be estimated with Equation (11): OUT OUT OUT 2 SW OUT IN VV V(1 ) 8F L C V (11) The output voltage ripple caused by the ESR is very small. Therefore, an external ramp is needed to stabilize the system. The external ramp can be generated through resistor R4 and capacitor C4. When using POSCAP capacitors, the ESR dominates the impedance at the switching frequency. The ramp voltage generated from the ESR dominates the output ripple. The output ripple can be approximated with Equation (12): OUT OUT OUT ESR SW IN VV V(1 ) R FL V (12) The maximum output capacitor limitation should be considered in the design application. The MP8720 has a softstart time period around 1.6ms. If the output capacitor value is too high, the output voltage cannot reach the design value during the softstart time, causing it to fail to regulate. The maximum output capacitor value (Co_max) can be limited approximately with Equation (13): O_ MAX LIM _ AVG OUT ss OUT C(I I ) T / V (13) Where ILIM_AVG is the average startup current during the softstart period (which can be equivalent to the current limit), and Tss is the softstart time. Selecting the Inductor An inductor is necessary for supplying constant current to the output load while being driven by the switched input voltage. A largervalue inductor results in less ripple current, resulting in a lower output ripple voltage, but also has a larger physical footprint, a higher series resistance, and a lower saturation current. A good rule for determining the inductance value is to design the peaktopeak ripple current in the inductor to be in the range of 30% to 50% of the maximum output current, and the peak inductor current below the maximum switch current limit. The inductance value can be calculated with Equation (14): OUT OUT SW L IN VV L(1 ) FI V (14) Where ∆IL is the peaktopeak inductor ripple current. The inductor should not saturate under the maximum inductor peak current (including a shortcurrent), so ISAT is recommended to be >13A. 
