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## ISL6277 Datasheet(PDF) 31 Page - Renesas Technology Corp

 Part No. ISL6277 Description Multiphase PWM Regulator for AMD Fusionâ¢ Mobile CPUs Using SVI 2.0 Download 37 Pages Scroll/Zoom 100% Manufacturer RENESAS [Renesas Technology Corp] Direct Link http://www.renesas.com Logo ## ISL6277 Datasheet(HTML) 31 Page - Renesas Technology Corp           31 / 37 page ISL6277FN8270 Rev 1.00Page 31 of 37Mar 8, 2012Load Line SlopeSee Figure 14 for load-line implementation.For inductor DCR sensing, substitution of Equation 27 intoEquation 2 gives the load-line slope expression:For resistor sensing, substitution of Equation 31 into Equation 2gives the load line slope expression:Substitution of Equation 28 and rewriting Equation 34, orsubstitution of Equation 32 and rewriting Equation 35, gives thesame result as in Equation 36:One can use the full-load condition to calculate Rdroop. Forexample, given Iomax = 65A, Idroopmax = 45µA and LL = 2.1mΩ,Equation 36 gives Rdroop = 3.03kΩ.It is recommended to start with the Rdroop value calculated byEquation 36 and fine-tune it on the actual board to get accurateload-line slope. One should record the output voltage readings atno load and at full load for load-line slope calculation. Readingthe output voltage at lighter load instead of full load will increasethe measurement error.CompensatorFigure 23 shows the desired load transient response waveforms.Figure 29 shows the equivalent circuit of a voltage regulator (VR)with the droop function. A VR is equivalent to a voltage source(= VID) and output impedance Zout(s). If Zout(s) is equal to theload-line slope LL, i.e., a constant output impedance, then in theentire frequency range, Vo will have a square response when Iohas a square change.Intersil provides a Microsoft Excel-based spreadsheet to helpdesign the compensator and the current sensing network so thatVR achieves constant output impedance as a stable system.A VR with active droop function is a dual-loop system consisting ofa voltage loop and a droop loop, which is a current loop. However,neither loop alone is sufficient to describe the entire system. Thespreadsheet shows two loop gain transfer functions, T1(s) andT2(s), that describe the entire system. Figure 30 conceptuallyshows T1(s) measurement set-up, and Figure 31 conceptuallyshows T2(s) measurement set-up. The VR senses the inductorcurrent, multiplies it by a gain of the load-line slope, adds it on topof the sensed output voltage, and then feeds it to thecompensator. T1 is measured after the summing node, and T2 ismeasured in the voltage loop before the summing node. Thespreadsheet gives both T1(s) and T2(s) plots. However, only T2(s)can actually be measured on an ISL6277 regulator.T1(s) is the total loop gain of the voltage loop and the droop loop.It always has a higher crossover frequency than T2(s), thereforehas a higher impact on system stability.T2(s) is the voltage loop gain with closed droop loop, thus havinga higher impact on output voltage response.Design the compensator to get stable T1(s) and T2(s) with sufficientphase margin and an output impedance equal to or smaller thanthe load-line slope.Current BalancingRefer to Figures 15 through 22 for information on currentbalancing. The ISL6277 achieves current balancing throughmatching the ISEN pin voltages. Risen and Cisen form filters toLLVdroopIo-------------------RdroopRi-------------------RntcnetRntcnetRsumN---------------+------------------------------------------ DCRN-------------==(EQ. 34)LLVdroopIo-------------------Rsen RdroopNRi---------------------------------------==(EQ. 35)RdroopIoIdroop---------------- LL=(EQ. 36)FIGURE 29. VOLTAGE REGULATOR EQUIVALENT CIRCUIToiVoVIDZout(s) = LLLOADVRFIGURE 30. LOOP GAIN T1(s) MEASUREMENT SET-UPQ2Q1LiOCOUTVOVINGATEDRIVERCOMPMOD.LOAD LINE SLOPEEAVIDCHANNEL BCHANNEL AEXCITATION OUTPUTISOLATIONTRANSFORMER20LOOP GAIN =CHANNEL BCHANNEL ANETWORKANALYZER+++-FIGURE 31. LOOP GAIN T2(s) MEASUREMENT SET-UPQ2Q1LIOCOVOVINGATEDRIVERCOMPMOD.LOADLINESLOPEEAVIDCHANNEL BCHANNEL AEXCITATION OUTPUTISOLATIONTRANSFORMER20LOOP GAIN =CHANNEL BCHANNEL ANETWORKANALYZER+++-

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