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ISL6219A Datasheet(PDF) 7 Page  Intersil Corporation 

ISL6219A Datasheet(HTML) 7 Page  Intersil Corporation 
7 / 17 page 7 Operation MultiPhase Power Conversion Multiphase power conversion provides the most cost effective power solution when load currents are no longer easily supported by singlephase converters. Although its greater complexity presents additional technical challenges, the multiphase approach offers costsaving advantages with improved response time, superior ripple cancellation, and excellent thermal distribution. INTERLEAVING The switching of each channel in a multiphase converter is timed to be symmetrically out of phase with each of the other channels. In a 3phase converter, each channel switches 1/3 cycle after the previous channel and 1/3 cycle before the following channel. As a result, the threephase converter has a combined ripple frequency three times greater than the ripple frequency of any one phase. In addition, the peakto 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 perchannel inductance and lower total output capacitance for any performance specification. Figure 2 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 peaktopeak current waveforms 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 multiphase circuit, examine the equation representing an individual channel’s peaktopeak inductor current. In Equation 1, VIN and VOUT are the input and output voltages respectively, L is the singlechannel inductor value, and fS is the switching frequency. The output capacitors conduct the ripple component of the inductor current. In the case of multiphase 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 peaktopeak current after the summation of N symmetrically phaseshifted inductor currents in Equa tion 2. Peaktopeak ripple current decreases by an amount proportional to the number of channels. Outputvoltage ripple is a function of capacitance, capacitor equivalent series resis tance (ESR), and inductor ripple current. Reducing the induc tor 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. Multiphase 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 3 illustrates input currents from a threephase converter combining to reduce the total input ripple current. The converter depicted in Figure 3 delivers 36A to a 1.5V load from a 12V input. The rms input capacitor current is 5.9A. Compare this to a singlephase converter also down 12V to 1.5V at 36A. The singlephase converter has 11.9A rms input capacitor current. The singlephase converter FIGURE 2. PWM AND INDUCTORCURRENT WAVEFORMS FOR 3PHASE CONVERTER 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 I LPP , V IN V OUT – () V OUT Lf S VIN  = (EQ. 1) I PP V IN NV OUT – () V OUT Lf S VIN  = (EQ. 2) FIGURE 3. CHANNEL INPUT CURRENTS AND INPUT CAPACITOR RMS CURRENT FOR 3PHASE CONVERTER Channel 1 input current 10A/DIV Channel 2 input current 10A/DIV Channel 3 input current 10A/DIV Inputcapacitor current, 10A/DIV 1 µs/div ISL6219A 
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