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ISL6313 Datasheet(PDF) 18 Page - Intersil Corporation |
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ISL6313 Datasheet(HTML) 18 Page - Intersil Corporation |
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18 / 33 page  18 FN6448.0 March 5, 2007 In other cases, the designer may determine that a more cost-effective solution can be achieved by adding droop. Droop can help to reduce the output-voltage spike that results from fast load-current demand changes. The magnitude of the spike is dictated by the ESR and ESL of the output capacitors selected. By positioning the no-load voltage level near the upper specification limit, a larger negative spike can be sustained without crossing the lower limit. By adding a well controlled output impedance, the output voltage under load can effectively be level shifted down so that a larger positive spike can be sustained without crossing the upper specification limit. As shown in Figure 7, a current proportional to the average current of all active channels, IAVG, flows from FB through a load-line regulation resistor RFB. The resulting voltage drop across RFB is proportional to the output current, effectively creating an output voltage droop with a steady-state value defined as: The regulated output voltage is reduced by the droop voltage VDROOP. The output voltage as a function of load current is derived by combining Equations 6, 8, and 9. In Equation 10, VREF is the reference voltage, VOFS is the programmed offset voltage, IOUT is the total output current of the converter, RISEN is the internal sense resistor connected to the ISEN+ pin, RFB is the feedback resistor, N is the active channel number, and DCR is the Inductor DCR value. Therefore the equivalent loadline impedance, i.e. droop impedance, is equal to: Output-Voltage Offset Programming The ISL6313 allows the designer to accurately adjust the offset voltage by connecting a resistor, ROFS, from the OFS pin to VCC or GND. When ROFS is connected between OFS and VCC, the voltage across it is regulated to 1.6V. This causes a proportional current (IOFS) to flow into the OFS pin and out of the FB pin, providing a negative offset. If ROFS is connected to ground, the voltage across it is regulated to 0.3V, and IOFS flows into the FB pin and out of the OFS pin, providing a positive offset. The offset current flowing through the resistor between VSEN and FB will generate the desired offset voltage which is equal to the product (IOFS x RFB). These functions are shown in Figures 8 and 9. Once the desired output offset voltage has been determined, use the following formulas to set ROFS: For Negative Offset (connect ROFS to VCC): For Positive Offset (connect ROFS to GND): (EQ. 9) V DROOP I AVG R FB ⋅ = (EQ. 10) V OUT V REF V OFS – I OUT N ------------- DCR R ISEN ------------------ R FB ⋅⋅ ⎝⎠ ⎜⎟ ⎛⎞ – = R LL R FB N ------------ DCR R ISEN ------------------ ⋅ = (EQ. 11) (EQ. 12) R OFS 1.6 R FB ⋅ V OFFSET -------------------------- = (EQ. 13) R OFS 0.3 R FB ⋅ V OFFSET -------------------------- = E/A FB OFS VCC GND + - + - 0.3V 1.6V GND ROFS RFB VSEN ISL6313 FIGURE 8. POSITIVE OFFSET OUTPUT VOLTAGE PROGRAMMING VREF VOFS + - IOFS E/A FB OFS VCC GND + - + - 0.3V 1.6V VCC ROFS RFB VSEN ISL6313 FIGURE 9. NEGATIVE OFFSET OUTPUT VOLTAGE PROGRAMMING VREF VOFS + - IOFS ISL6313 |
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