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MIC21LV32 Datasheet(PDF) 34 Page - Microchip Technology |
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MIC21LV32 Datasheet(HTML) 34 Page - Microchip Technology |
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34 / 50 page  MIC21LV32 DS20006513A-page 34 2021 Microchip Technology Inc. The capacitance of the input capacitor can be determined in Equation 5-17: EQUATION 5-17: The ESR of the total input capacitance can be determined in Equation 5-18: EQUATION 5-18: The input capacitor must be rated for the input current ripple. The rated RMS value of the input capacitor cur- rent is determined at the maximum output current. Assuming the peak-to-peak inductor current ripple is low, the RMS current rating of the input capacitor can be estimated from Equation 5-19: EQUATION 5-19: The graph in Figure 5-2 shows the normalized RMS input ripple current vs. duty cycle for both single-phase and two-phase buck converter operation. Data are normalized to the output current. FIGURE 5-2: Normalized RMS Input Ripple Current vs. Duty Cycle. For a two-phase buck converter operating at duty cycle D, the input RMS current can also be determined from the graph in Figure 5-2, together with Equation 5-20 below. EQUATION 5-20: The power dissipated in the input capacitor can then be computed from Equation 5-21: EQUATION 5-21: The voltage rating of the input capacitor must be high enough to withstand the high input voltage. The recommended voltage rating is at least 1.25 times the maximum input voltage. 5.4 Switch Power MOSFET Selection The following parameters are important for MOSFET selection: • Voltage rating • Current rating • On-resistance • Total gate charge The voltage rating for both the high-side and low-side MOSFETs in the buck converter is essentially equal to the power stage input voltage, VIN. A safety factor of 30% should be added to the VIN(MAX), while selecting the voltage rating of the MOSFETs to account for voltage spikes due to circuit parasitic elements, as shown in Equation 5-22. EQUATION 5-22: The peak switch current for both the high-side and low-side MOSFET in the buck converter is the same, and is equal to the peak inductor current in each phase, as shown in Equation 5-23 on the next page. Where: k = 0,1,2,3,.... for D > k/n and k < n n = Total Number of Phases CIN ≥ k n fSW × CIN IOUT × D –× 1 n [] – D – k n ESRCIN ≤ VIN × n IOUT Where: IOUT(MAX) = Maximum Output Current k = 0,1,2,3,.... for D > k/n and k < n n = Total Number of Phases ICIN(RMS) ≈ IOUT(MAX) × k n D –× 1 n [] – D – k n √ ICIN(RMS) = ICINRMS(NORM) × IOUT(MAX) Where: ICINRMS(NORM) = Normalized Input Ripple Current at given Duty Cycle for Two-Phase Buck Converter from Figure 5-2 PDISS(CIN) = ICIN(RMS)2 × ESRCIN VDS(RATING) ≥ VIN(MAX) × 1.3 |
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