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ISL6327CRZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL6327CRZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 30 page  11 FN9276.2 December 20, 2006 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. Figures 19, 20 and 21 in the section titled Input Capacitor Selection can be used to determine the input-capacitor RMS current based on the load current, the duty cycle, and the number of channels. They are provided as aids in determining the optimal input capacitor solution. Figure 22 shows the single phase input-capacitor RMS current for comparison. PWM Modulation Scheme The ISL6327 adopts Intersil's proprietary Active Pulse Positioning (APP) modulation scheme to improve the transient performance. APP control is a unique dual-edge PWM modulation scheme with both PWM leading and trailing edges being independently moved to provide the best response to the transient loads. The PWM frequency, however, is constant and set by the external resistor between the FS pin and GND. To further improve the transient response, the ISL6327 also implements Intersil's proprietary Adaptive Phase Alignment (APA) technique. APA, with sufficiently large load step currents, can turn on all phases together. With both APP and APA control, ISL6327 can achieve excellent transient performance and reduce the demand on the output capacitors. Under the steady state conditions the operation of the ISL6327 PWM modulator appears to be that of a conventional trailing edge modulator. Conventional analysis and design methods can therefore be used for steady state and small signal operation. PWM Operation The timing of each converter is set by the number of active channels. The default channel setting for the ISL6327 is six. The switching cycle is defined as the time between PWM pulse termination signals of each channel. The cycle time of the pulse termination signal is the inverse of the switching frequency set by the resistor between the FS pin and ground. The PWM signals command the MOSFET drivers to turn on/off the channel MOSFETs. In the default 6-phase operation, the PWM2 pulse happens 1/6 of a cycle after PWM1, the PWM3 pulse happens 1/6 of a cycle after PWM2, the PWM4 pulse happens 1/6 of a cycle after PWM3, the PWM5 pulse happens 1/6 of a cycle after PWM4, and the PWM6 pulse happens 1/6 of a cycle after PWM5. The ISL6327 works in 2, 3, 4, 5, or 6 phase configuration. Connecting the PWM6 to VCC selects 5-phase operation and the pulse times are spaced in 1/5 cycle increments. Connecting the PWM5 to VCC selects 4-phase operation and the pulse times are spaced in 1/4 cycle increments. Connecting the PWM4 to VCC selects 3-phase operation and the pulse times are spaced in 1/3 cycle increments. Connecting the PWM3 to VCC selects 2-phase operation and the pulse times are spaced in 1/2 cycle increments. Switching Frequency The switching frequency is determined by the selection of the frequency-setting resistor, RT, which is connected from FS pin to GND (see the figures labelled Typical Applications on pages 4 and 5). Equation 3 is provided to assist in selecting the correct resistor value. where FSW is the switching frequency of each phase. Current Sensing ISL6327 senses the current continuously for fast response. ISL6327 supports inductor DCR sensing, or resistive sensing techniques. The associated channel current sense amplifier uses the ISEN inputs to reproduce a signal proportional to the inductor current, IL. The sensed current, ISEN, is used for the current balance, the load-line regulation, and the overcurrent protection. The internal circuitry, shown in Figures 3 and 4, represents one channel of an N-channel converter. This circuitry is repeated for each channel in the converter, but may not be active depending on the status of the PWM3, PWM4, PWM5, and PWM6 pins, as described in the PWM Operation section. INDUCTOR DCR SENSING An inductor’s winding is characteristic of a distributed resistance as measured by the DCR (Direct Current Resistance) parameter. Consider the inductor DCR as a separate lumped quantity, as shown in Figure 3. The channel current IL, flowing through the inductor, will also pass through the DCR. Equation 4 shows the s-domain equivalent voltage across the inductor VL. A simple R-C network across the inductor extracts the DCR voltage, as shown in Figure 3. The voltage on the capacitor VC, can be shown to be proportional to the channel current IL, see Equation 5. (EQ. 3) R T 2.5X10 10 F SW -------------------------- 600 – = V L I L sL DCR + ⋅ () ⋅ = (EQ. 4) V C s L DCR ------------- ⋅ 1 + ⎝⎠ ⎛⎞ DCR I L ⋅ () ⋅ sRC 1 + ⋅ () --------------------------------------------------------------------- = (EQ. 5) ISL6327 |
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