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ISL6322CRZ Arkusz danych(PDF) 37 Page - Intersil Corporation |
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ISL6322CRZ Arkusz danych(HTML) 37 Page - Intersil Corporation |
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37 / 41 page  37 FN6328.2 August 2, 2007 The ESR of the bulk capacitors also creates the majority of the output-voltage ripple. As the bulk capacitors sink and source the inductor ac ripple current (see “Interleaving” on page 11 and Equation 2), a voltage develops across the bulk capacitor ESR equal to IC(P-) (ESR). Thus, once the output capacitors are selected, the maximum allowable ripple voltage, V(P-P)(MAX), determines the lower limit on the inductance. Since the capacitors are supplying a decreasing portion of the load current while the regulator recovers from the transient, the capacitor voltage becomes slightly depleted. The output inductors must be capable of assuming the entire load current before the output voltage decreases more than ΔV MAX. This places an upper limit on inductance. Equation 45 gives the upper limit on L for the cases when the trailing edge of the current transient causes a greater output-voltage deviation than the leading edge. Equation 46 addresses the leading edge. Normally, the trailing edge dictates the selection of L because duty cycles are usually less than 50%. Nevertheless, both inequalities should be evaluated, and L should be selected based on the lower of the two results. In each equation: L is the per-channel inductance, C is the total output capacitance, and N is the number of active channels. Switching Frequency There are a number of variables to consider when choosing the switching frequency, as there are considerable effects on the upper MOSFET loss calculation. These effects are outlined in “MOSFETs” on page 31, and they establish the upper limit for the switching frequency. The lower limit is established by the requirement for fast transient response and small output-voltage ripple as outlined in “Compensation without Load-line Regulation” on page 36. Choose the lowest switching frequency that allows the regulator to meet the transient-response requirements. Switching frequency is determined by the selection of the frequency-setting resistor, RT. Figure 25 and Equation 47 are provided to assist in selecting the correct value for RT. Input Capacitor Selection The input capacitors are responsible for sourcing the ac component of the input current flowing into the upper MOSFETs. Their RMS current capacity must be sufficient to handle the ac component of the current drawn by the upper MOSFETs which is related to duty cycle and the number of active phases. For a four-phase design, use Figure 26 to determine the input-capacitor RMS current requirement set by the duty cycle, maximum sustained output current (IO), and the ratio of the peak-to-peak inductor current (IL(P-P)) to IO. Select a bulk capacitor with a ripple current rating that will minimize the total number of input capacitors required to support the RMS current calculated. The voltage rating of the capacitors should also be at least 1.25 times greater than the maximum input voltage. Figures 27 and 28 provide the same input RMS current information for three-phase and two-phase designs respectively. Use the same approach for selecting the bulk capacitor type and number. L ESR V IN NV ⋅ OUT – ⎝⎠ ⎛⎞ V OUT ⋅ f S VIN V PP – () MAX () ⋅⋅ -------------------------------------------------------------------- ⋅ ≥ (EQ. 44) L 2NCVO ⋅⋅⋅ ΔI ()2 --------------------------------- ΔV MAX ΔIESR ⋅ () – ⋅ ≤ (EQ. 45) L 1.25 NC ⋅⋅ ΔI ()2 ----------------------------- ΔV MAX ΔI ESR ⋅ () – V IN V O – ⎝⎠ ⎛⎞ ⋅⋅ ≤ (EQ. 46) R T 10 10.61 1.035 fS () log ⋅ () – [] = (EQ. 47) FIGURE 25. RT vs SWITCHING FREQUENCY SWITCHING FREQUENCY (Hz) 10 100 1000 10k 100k 1000k 10000k FIGURE 26. NORMALIZED INPUT-CAPACITOR RMS CURRENT vs DUTY CYCLE FOR 4-PHASE CONVERTER 00.4 1.0 0.2 0.6 0.8 DUTY CYCLE (VO/VIN) 0.3 0.1 0 0.2 IL(P-P) = 0 IL(P-P) = 0.25 IO IL(P-P) = 0.5 IO IL(P-P) = 0.75 IO ISL6322 |
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