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MP2229 Datasheet(PDF) 16 Page - Monolithic Power Systems |
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MP2229 Datasheet(HTML) 16 Page - Monolithic Power Systems |
16 / 22 page MP2229 – 21V, 6A SYNCHRONOUS STEP-DOWN CONVERTER MP2229 Rev. 1.0 www.MonolithicPower.com 16 2/12/2015 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2015 MPS. All Rights Reserved. Generally, choose R6 to be around 100kΩ, then R7 is: LPM CC LPM VR6 R7 VV × = − Refer to Figure 8 when setting the LPM voltage. 0.2 0.24 0.28 0.32 0.36 0.4 02468 10 FIGURE 8. Recommended LPM Selection for Common Output Voltages (VIN=12V, FS=500kHz) Selecting the Input Capacitor The input current to the step-down converter is discontinuous, and therefore requires a capacitor to supply the AC current while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors with X5R or X7R dielectrics are recommended highly because of their low ESR and small temperature coefficients. For most applications, a 22µF and a 10µF capacitor are sufficient. 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: LOAD C1 I I 2 = For simplification, choose an input capacitor with a RMS current rating greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum, or ceramic. When using electrolytic or tantalum capacitors, place a small, high-quality ceramic capacitor (e.g. 0.1μF) as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge in order to prevent excessive voltage ripple at the input. The input- voltage ripple caused by the capacitance can be estimated by: LOAD OUT OUT IN IN SIN IV V V1 fC1 V V ⎛⎞ Δ= × × − ⎜⎟ × ⎝⎠ Selecting the Output Capacitor The device requires an output capacitor (C2) to maintain the DC output voltage. Use ceramic, tantalum, or low ESR electrolytic capacitors. Use low ESR capacitors to limit the output- voltage ripple. Estimate the output-voltage ripple with: OUT OUT OUT ESR S1 IN S VV 1 V1 R fL V 8 f C2 ⎛⎞ ⎛⎞ Δ= × − × + ⎜⎟ ⎜⎟ ×× × ⎝⎠ ⎝⎠ Where L1 is the inductor value and RESR is the equivalent series resistance (ESR) value of the output capacitor. For ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output-voltage ripple is caused mainly by the capacitance. For simplification, the output-voltage ripple can be estimated by: OUT OUT OUT 2 IN S1 VV ΔV1 V 8f L C2 ⎛⎞ =× − ⎜⎟ ×× × ⎝⎠ For 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 ΔV1 R fL V ⎛⎞ =× − × ⎜⎟ × ⎝⎠ The characteristics of the output capacitor affect the stability of the regulatory system. The MP2229 can be optimized for a wide range of capacitance and ESR values. |
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