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MP5402M Datasheet(PDF) 14 Page  Monolithic Power Systems 

MP5402M Datasheet(HTML) 14 Page  Monolithic Power Systems 
14 / 17 page MP5402M ? STEPDOWN CONVERTER WITH SMART DUAL USB CHARGING PORTS MP5402M Rev.1.0 www.MonolithicPower.com 14 10/10/2015 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2015 MPS. All Rights Reserved. APPLICATION INFORMATION COMPONENT SELECTION Selecting the Inductor For most applications, an inductor with a DC current rating at least 25% higher than the maximum load current is recommended. Select an inductor with small DC resistance for optimum efficiency. The inductor value for most designs can be derived from Equation (1): OUT IN OUT 1 IN L OSC V(V V ) L VI f ×? = ×? × (1) Where ?IL is the inductor ripple current. Set the inductor ripple current to approximately 30% of the maximum load current. The maximum inductor peak current is shown in Equation (2): 2 I I I L LOAD ) MAX ( L ? + = (2) Typically, 22?H inductance is recommended to improve EMI. Selecting the Buck Input Capacitor The input current to the stepdown converter is discontinuous and therefore requires a capacitor to supply the AC current while maintaining the DC input voltage. Use low ESR capacitors for optimum performance. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small temperature coefficients. For a CLA application, a 100µF electrolytic capacitor and two 10µF ceramic capacitors are recommended. 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 with Equation (3): ? ? ? ? ? ? ? ? × ? × = IN OUT IN OUT LOAD 1 C V V 1 V V I I (3) The worse case condition occurs at VIN = 2VOUT, shown in Equation (4): 2 I I LOAD 1 C = (4) For simplification, choose an input capacitor with an RMS current rating of greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum, or ceramic. When using an electrolytic capacitor, place two additional highquality ceramic capacitors as close to VIN as possible. Estimate the input voltage ripple caused by the capacitance with Equation (5): LOAD OUT OUT IN IN SIN IV V V1 fC1 V V ?? ?= × × ? ?? × ?? (5) Selecting the Buck Output Capacitor The device requires an output capacitor (C2) to maintain the DC output voltage. Estimate the output voltage ripple with Equation (6): OUT OUT OUT ESR S1 IN S VV 1 V1 R fL V 8 f C2 ?? ?? ?= × ? × + ?? ?? ×× × ?? ?? (6) Where L1 is the inductor value and RESR is the equivalent series resistance (ESR) value of the output capacitor. For an electrolytic capacitor, ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated with Equation (7): OUT OUT OUT ESR IN S1 VV ?V1 R fL V ?? =× ? × ?? × ?? (7) The characteristics of the output capacitor affect the stability of the regulatory system. A low ESR electrolytic capacitor is recommended for a low output ripple and good control loop stability. For a CLA application, a 1µF ceramic capacitor and a 270µF polymer/electrolytic capacitor with ~20m? ESR are recommended. PCB Layout Guidelines(8) Efficient PCB layout is critical for achieving stable operation and thermal dissipation. For best results, refer to Figure 5 and follow the guidelines below: 1. Use short, direct, and wide traces to connect OUT. Adding vias under the IC and routing the OUT trace on both PCB layers is highly recommended. 2. Use a large copper plane for PGND. Add multiple vias to improve thermal dissipation. 3. Connect AGND to PGND. 
