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ISL6556BCBZ-T Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL6556BCBZ-T Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 24 page  10 FN9097.4 December 28, 2004 Operation Multi-Phase Power Conversion Microprocessor load current profiles have changed to the point that the advantages of multi-phase power conversion are impossible to ignore. The technical challenges associated with producing a single-phase converter which is both cost- effective and thermally viable have forced a change to the cost-saving approach of multi-phase. The ISL6556B controller helps simplifying the implementation by integrating vital functions and requiring minimal output components. The block diagrams on pages 2 and 3 provide top level views of multi- phase power conversion using the ISL65556ACB and ISL6556BCR controllers. Interleaving The switching of each channel in a multi-phase converter is timed to be symmetrically out of phase with each of the other channels. In a 3-phase converter, each channel switches 1/3 cycle after the previous channel and 1/3 cycle before the following channel. As a result, the three-phase converter has a combined ripple frequency three times greater than the ripple frequency of any one phase. In addition, the peak-to- peak amplitude of the combined inductor currents is reduced in proportion to the number of phases (Equations 1 and 2). Increased ripple frequency and lower ripple amplitude mean that the designer can use less per-channel inductance and lower total output capacitance for any performance specification. Figure 1 illustrates the multiplicative effect on output ripple frequency. The three channel currents (IL1, IL2, and IL3) combine to form the AC ripple current and the DC load current. The ripple component has three times the ripple frequency of each individual channel current. Each PWM pulse is terminated 1/3 of a cycle after the PWM pulse of the previous phase. The peak-to-peak current for each phase is about 7A, and the dc components of the inductor currents combine to feed the load. To understand the reduction of ripple current amplitude in the multi-phase circuit, examine the equation representing an individual channel’s peak-to-peak inductor current. In Equation 1, VIN and VOUT are the input and output voltages respectively, L is the single-channel inductor value, and fS is the switching frequency. The output capacitors conduct the ripple component of the inductor current. In the case of multi-phase converters, the capacitor current is the sum of the ripple currents from each of the individual channels. Compare Equation 1 to the expression for the peak-to-peak current after the summation of N symmetrically phase-shifted inductor currents in Equation 2. Peak-to-peak ripple current decreases by an amount proportional to the number of channels. Output- voltage ripple is a function of capacitance, capacitor equivalent series resistance (ESR), and inductor ripple current. Reducing the inductor ripple current allows the designer to use fewer or less costly output capacitors. Another benefit of interleaving is to reduce input ripple current. Input capacitance is determined in part by the maximum input ripple current. Multi-phase topologies can improve overall system cost and size by lowering input ripple current and allowing the designer to reduce the cost of input capacitance. The example in Figure 2 illustrates input currents from a three-phase converter combining to reduce the total input ripple current. The converter depicted in Figure 2 delivers 36A to a 1.5V load from a 12V input. The RMS input capacitor current is 5.9A. Compare this to a single-phase converter also stepping down 12V to 1.5V at 36A. The single-phase converter has 11.9A RMS input capacitor current. The single-phase converter must use an input capacitor bank with twice the RMS current capacity as the equivalent three-phase converter. FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS 1 µs/DIV PWM2, 5V/DIV PWM1, 5V/DIV IL2, 7A/DIV IL1, 7A/DIV IL1 + IL2 + IL3, 7A/DIV IL3, 7A/DIV PWM3, 5V/DIV IPP VIN VOUT – () V OUT LfS VIN ------------------------------------------------------ = (EQ. 1) I L PP , VIN NVOUT – () V OUT LfS VIN ------------------------------------------------------------ = (EQ. 2) FIGURE 2. CHANNEL INPUT CURRENTS AND INPUT- CAPACITOR RMS CURRENT FOR 3-PHASE CONVERTER Channel 1 input current 10A/DIV Channel 2 input current 10A/DIV CHANNEL 3 INPUT CURRENT 10A/DIV INPUT-CAPACITOR CURRENT, 10A/DIV 1 µs/DIV CHANNEL 2 INPUT CURRENT 10A/DIV CHANNEL 1 INPUT CURRENT 10A/DIV ISL6556B |
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