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## 71128 Datasheet(PDF) 3 Page - Vishay Siliconix

 Part No. 71128 Description Simple Solution for Dynamically Programming the Output Voltage of DC-DC Converters Download 7 Pages Scroll/Zoom 100% Maker VISHAY [Vishay Siliconix] Homepage http://www.vishay.com Logo

 3 page AN731Vishay SiliconixDocument Number: 7112828-Jan-00www.vishay.comS FaxBack 408-970-56003To limit the common mode range of VX, the following equationscan be used:1. To keep VX equal or greater than a minimum value, VXm=1 V:Vr2wVxm(Vo2–Vr–aVc2)) aVc2VrVo2) a(Vr–Vxm)–Vr+1(3.4–1.3–1.22.7)) 1.22.71.33.4) 1.2(1.3–1)–1.3+ 1.249(12)2. To keep VX equal or less than a maximum value, VXm =3 V:Vr2wVxM(Vo2–Vr–aVc2)) aVc1VrVo2) a(Vr–VxM–Vc2 ) Vc1)–Vr+2(3.4–1.3–1.22.7)) 1.20.21.33.4) 1.2(1.3–2–2.7 ) 0.2)–1.3+ 1.13 V(13)So, Vr2 can be any value between 1.249 V and 1.3 V. ChooseVr2 = 1.25 V.Calculate m1 and then R2:m1+Vr–Vr2Vo2) a(Vr2–Vc2)–Vr+1.3–1.253.4) 1.2(1.25–2.7)–1.3+ 0.139m1+ R2R1å R2 + m1R1 + 0.13922.1 k+ 3.07 kW(14)Choose R2 = 3.01 kW as a practical valueCalculate R3 and R4:a+ m2m1 +R3m1R4å R3 + am1R4(15)Choose R4, then calculate R3. The value of R4 should be largeenough such that the current going through it will not be solarge as to cause excessive power dissipation under extremeconditions. On the other hand, R4 should be small enough thatits current will not be overly sensitive to noise and op-amp biascurrent.If R4 is set at 22.1k, then R3 = 3.68 k, and we can choose R3= 3.60 k as a practical value.Express VO as a function of VC, using practical values of R’s:a+ m2m1 +R1R3R2R4+ 22.1kx3.68k3.01kx22.1k+ 1.223b+ 1m1 )1 Vr– 1m1 )a Vr2+R1R2) 1 Vr– R1R2) a Vr2b+ 22.1k3.01k) 1 1.3– 22.1k3.01k) 1.223 1.25 + 0.1389(16)The final result: VO = aVC + b = 1.223 x VC + 0.1389EXPERIMENTAL RESULTSA circuit was built and tested (see Figure 5). The result istabulated in Table 1 and plotted in Figure 4:TABLE 1VVXVOVCMeasuredCalculatedMeasuredRequired0.11.430.260.320.21.420.380.420.400.41.380.630.680.640.81.321.121.161.121.21.251.611.661.601.61.192.102.122.082.01.112.582.682.562.41.053.073.113.042.61.023.323.333.282.71.003.443.463.402.80.993.563.583.00.963.813.7800.51.01.52.02.53.03.54.000.51.01.52.02.53.0Control Voltage—VC (v)FIGURE 4.VX MeasuredVOThe measured values are very much in agreement with thecalculated and required values. A negligible error results fromthe difference between an ideal op-amp and the actual circuitwith its finite offset voltage and bias current.