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ISL6322 Datasheet(PDF) 13 Page - Intersil Corporation |
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ISL6322 Datasheet(HTML) 13 Page - Intersil Corporation |
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13 / 41 page 
13 FN6328.0 August 21, 2006 Channel-Current Balance One important benefit of multiphase operation is the thermal advantage gained by distributing the dissipated heat over multiple devices and greater area. By doing this the designer avoids the complexity of driving parallel MOSFETs and the expense of using expensive heat sinks and exotic magnetic materials. In order to realize the thermal advantage, it is important that each channel in a multiphase converter be controlled to carry equal amounts of current at any load level. To achieve this, the currents through each channel must be sampled every switching cycle. The sampled currents, In, from each active channel are summed together and divided by the number of active channels. The resulting cycle average current, IAVG, provides a measure of the total load-current demand on the converter during each switching cycle. Channel-current balance is achieved by comparing the sampled current of each channel to the cycle average current, and making the proper adjustment to each channel pulse width based on the error. Intersil’s patented current- balance method is illustrated in Figure 3, with error correction for channel 1 represented. In the figure, the cycle average current, IAVG, is compared with the channel 1 sample, I1, to create an error signal IER. The filtered error signal modifies the pulse width commanded by VCOMP to correct any unbalance and force IER toward zero. The same method for error signal correction is applied to each active channel. Continuous Current Sampling In order to realize proper current-balance, the currents in each channel are sensed continuously every switching cycle. During this time the current-sense amplifier uses the ISEN inputs to reproduce a signal proportional to the inductor current, IL. This sensed current, ISEN, is simply a scaled version of the inductor current. The ISL6322 supports inductor DCR current sensing to continuously sense each channel’s current for channel-current balance. The internal circuitry, shown in Figure 5 represents channel n of an N-channel converter. This circuitry is repeated for each channel in the converter, but may not be active depending on how many channels are operating. Inductor windings have a characteristic distributed resistance or DCR (Direct Current Resistance). For simplicity, the inductor DCR is considered as a separate lumped quantity, as shown in Figure 5. The channel current IL, flowing through the inductor, passes through the DCR. Equation 3 shows the s-domain equivalent voltage, VL, across the inductor. A simple R-C network across the inductor (R1 and C) extracts the DCR voltage, as shown in Figure 5. The voltage across the sense capacitor, VC, can be shown to be proportional to the channel current IL, shown in Equation 4. In some cases it may be necessary to use a resistor divider R-C network to sense the current through the inductor. This can be accomplished by placing a second resistor, R2, across the sense capacitor. In these cases the voltage FIGURE 3. CHANNEL-1 PWM FUNCTION AND CURRENT-BALANCE ADJUSTMENT ÷ N IAVG I3 I2 Σ - + + - + - f(s) PWM1 I1 VCOMP IER NOTE: CHANNEL 3 AND 4 ARE OPTIONAL. FILTER TO GATE CONTROL LOGIC I4 MODULATOR RAMP WAVEFORM FIGURE 4. CONTINUOUS CURRENT SAMPLING TIME PWM IL ISEN SWITCHING PERIOD V L s () I L sL DCR + ⋅ () ⋅ = (EQ. 3) V C s () sL ⋅ DCR ------------- 1 + ⎝⎠ ⎛⎞ sR 1 C ⋅⋅ 1 + () -------------------------------------- DCR I L ⋅ ⋅ = (EQ. 4) ISL6322 |
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