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ISL6563IRZ Datasheet(PDF) 8 Page - Renesas Technology Corp |
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ISL6563IRZ Datasheet(HTML) 8 Page - Renesas Technology Corp |
8 / 20 page ISL6563 FN9126 Rev 8.00 Page 8 of 20 Jun 10, 2010 voltage droop characteristic. Internal average channel current is fed into the FB pin; the voltage thus developed across R1 is equal to the droop voltage. Assuming identical power switch selection on the two channels, Equation 4 determines the current fed out the FB pin for output voltage droop generation: where, rDS(ON) - lower MOSFET/s’ ON resistance (@5V) IPHASE - average phase current Multiphase Power Conversion Multiphase power conversion provides a cost-effective power solution when load currents are no longer easily supported by single-phase converters. Although its greater complexity presents additional technical challenges, the multiphase approach offers cost-saving advantages with improved response time, superior ripple cancellation, and thermal distribution. INTERLEAVING The switching of each channel in an ISL6563-based converter is timed to be symmetrically out of phase with the other channel. As a result, the two-phase converter has a combined ripple frequency twice the frequency of one of its phases. In addition, the peak-to-peak amplitude of the combined inductor currents is proportionately reduced. Increased ripple frequency and lower ripple amplitude generally translate to lower per-channel inductance and lower total output capacitance for a given set of performance specifications. Figure 2 illustrates the additive effect on output ripple frequency. The two channel currents (IL1 and IL2), combine to form the AC ripple current and the DC load current. The ripple component has two times the ripple frequency of each individual channel current. To understand the reduction of ripple current amplitude in the multiphase circuit, examine Equation 5, which represents an individual channel’s peak-to-peak inductor current. 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 multiphase converters, the capacitor current is the sum of the ripple currents from each of the individual channels. Peak-to-peak ripple current, IPP, 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 (should output ripple be an important design parameter). Another benefit of interleaving is the reduction of input ripple current. Input capacitance is determined in a large 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 two-phase converter combining to reduce the total input ripple current. Figure 11, part of “Input Capacitor Selection” on page 18, can be used to determine the input-capacitor RMS current based on load current and duty cycle. The figure is provided as an aid in determining the optimal input capacitor solution. IFB rDS ON IPHASE RISEN ------------------------------------------------ = (EQ. 4) FIGURE 2. PWM AND INDUCTOR-CURRENT WAVEFORMS FOR 2-PHASE CONVERTER PWM2 PWM1 IL2 IL1 IL1 + IL2 ILPP VIN VOUT – V OUT LfS V IN ---------------------------------------------------------- = (EQ. 5) IPP VIN NVOUT – V OUT LfS V IN -------------------------------------------------------------------- = (EQ. 6) FIGURE 3. INPUT CAPACITOR CURRENT AND INDIVIDUAL CHANNEL CURRENTS IN A 2-PHASE CONVERTER Q1 D-S CURRENT Q3 D-S CURRENT CIN CURRENT |
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