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ISL6552 Datasheet(PDF) 13 Page - Renesas Technology Corp |
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ISL6552 Datasheet(HTML) 13 Page - Renesas Technology Corp |
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13 / 18 page  ISL6552 FN4918 Rev 2.00 Page 13 of 18 July 2004 under full load. This droop compensation allows larger transient voltage deviations and thus reduces the size and cost of the output filter components. RIN should be selected to give the desired “droop” voltage at the normal full load current 50 A applied through the RISEN resistor (or at a different full load current if adjusted as outlined in the Over-Current, Selecting RISEN section). RIN = Vdroop/50A For a Vdroop of 80mV, RIN = 1.6k The AC feedback components, RFB and Cc, are scaled in relation to RIN. Current Balancing The detected currents are also used to balance the phase currents. Each phase’s current is compared to the average of all phase currents, and the difference is used to create an offset in that phase’s PWM comparator. The offset is in a direction to reduce the imbalance. The balancing circuit can not make up for a difference in rDS(ON) between synchronous rectifiers. If a FET has a higher rDS(ON), the current through that phase will be reduced. Figures 8 and 9 show the inductor current of a two phase system without and with current balancing. Inductor Current The inductor current in each phase of a multi-phase Buck converter has two components. There is a current equal to the load current divided by the number of phases (ILT / n), and a sawtooth current, (iPK-PK) resulting from switching. The sawtooth component is dependent on the size of the inductors, the switching frequency of each phase, and the values of the input and output voltage. Ignoring secondary effects, such as series resistance, the peak to peak value of the sawtooth current can be described by: iPK-PK = (VIN x VCORE - VCORE2)/(L x FSW x VIN) Where: VCORE = DC value of the output or VID voltage VIN = DC value of the input or supply voltage L = value of the inductor FSW = switching frequency Example: For VCORE= 1.6V, VIN = 12V, L = 1.3 H, FSW = 250kHz, Then iPK-PK = 4.3A The inductor, or load current, flows alternately from VIN through Q1 and from ground through Q2. The ISL6552 samples the on-state voltage drop across each Q2 transistor to indicate the inductor current in that phase. The voltage drop is sampled 1/3 of a switching period, i/FSW, after Q1 is turned OFF and Q2 is turned on. Because of the sawtooth current component, the sampled current is different from the average current per phase. Neglecting secondary effects, the sampled current (ISAMPLE) can be related to the load current (ILT) by: ISAMPLE = ILT / n + (VINVCORE -3VCORE2)/(6L x FSW x VIN) Where: ILT = total load current n = the number of channels Example: Using the previously given conditions, and For ILT = 100A, n = 4 Then ISAMPLE = 25.49A As discussed previously, the voltage drop across each Q2 transistor at the point in time when current is sampled is rDSON (Q2) x ISAMPLE. The voltage at Q2’s drain, the PHASE node, is applied through the RISEN resistor to the ISL6552 ISEN pin. This pin is held at virtual ground, so the current into ISEN is: ISENSE = ISAMPLE x rDS(ON) (Q2)/RISEN. RIsen = ISAMPLE x rDS(ON) (Q2)/50A 0 5 10 15 20 25 FIGURE 8. TWO CHANNEL MULTI-PHASE SYSTEM WITH CURRENT BALANCING DISABLED 0 5 10 15 20 25 FIGURE 9. TWO CHANNEL MULTI-PHASE SYSTEM WITH CURRENT BALANCING ENABLED |
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