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MP9151 Datasheet(PDF) 14 Page - Monolithic Power Systems |
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MP9151 Datasheet(HTML) 14 Page - Monolithic Power Systems |
14 / 19 page MP9151 – 20V, 4A SYNCHRONOUS STEP-DOWN CONVERTER MP9151 Rev. 1.0 www.MonolithicPower.com 14 3/20/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2013 MPS. All Rights Reserved. The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1μF, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input voltage ripple caused by capacitance can be estimated by: LOAD OUT OUT IN IN S IN I V V V1 f C1 V V Selecting the Output Capacitor The output capacitor (C2) is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by: OUT OUT OUT ESR S 1 IN S VV 1 V 1 R f L V 8 f C2 Where L1 is the inductor value and RESR is the equivalent series resistance (ESR) value of the output capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple can be estimated by: OUT OUT OUT 2 IN S1 VV ΔV 1 V 8 f L C2 In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to: OUT OUT OUT ESR IN S1 VV ΔV 1 R f L V The characteristics of the output capacitor also affect the stability of the regulation system. The MP9151 can be optimized for a wide range of capacitance and ESR values. External Bootstrap Diode An external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of external BST diode are: VOUT is 5V or 3.3V; and Duty cycle is high: D= IN OUT V V >65% In these cases, an external BST diode is recommended from the VCC pin to BST pin, as shown in Figure 6. Figure 6: Add Optional External Bootstrap Diode to Enhance Efficiency The recommended external BST diode is IN4148, and the BST cap is 0.1~1 μF. PC Board Layout This PCB board layout is referring to the schematic in Figure 7. Place the high-current paths (GND, IN and SW) very close to the device with short, direct and wide traces. The input decoupling capacitor needs to be placed as close as possible to the IN and GND pins. The VCC decoupling capacitor needs to be placed as close as possible to the VCC pin and multiple VIAs should be used on both the ground side of the VCC decoupling capacitor and the GND pins to connect to the inner and bottom ground plane. Place the external feedback resistors next to the FB pin. Keep the switching node SW short and away from the feedback network. |
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