Electronic Components Datasheet Search
 Selected language English  ▼

## 71128 Datasheet(PDF) 1 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

 1 page AN731Vishay SiliconixDocument Number: 7112828-Jan-00www.vishay.comS FaxBack 408-970-56001Simple Solution for Dynamically Programming theOutput Voltage of DC-DC ConvertersINTRODUCTIONFigure 1 shows a typical dc-to-dc converter configuration. As inmany PWM controllers, the non-inverting input of the voltagefeedback error amplifier is internally connected to thereference voltage, Vr. The output voltage of the converter is setby a resistor divider network R1 and R2. This configurationenables a fixed output voltage that is equal to or greater thanthe reference voltage. But in many power conversion designs,it is useful to vary the output to a value lower than the referencevoltage and to dynamically adjust the output voltage. Theop-amp circuit shown in Figure 3 gives designers a simple wayto do this. Scaling the output voltage with respect to a controlvoltage VC is totally flexible. This application note describeshow to use this simple solution and provides a step-by-stepdesign procedure to assist designers in calculating the specificparameters required by their circuits.DESIGN REQUIREMENTReference voltage, VrControl voltage: VC = VC1 to VC2Output voltage: VO = VO1 at VC = VC1, VO = VO2 at VC = VC2,and VO is linear for any value of VC in its range.These design requirements are shown in Figure 2. The outputvoltage is a linear function with respect to the control voltage.The function crosses two end points A = (VC1, VO1) and B =(VC2, VO2).R1Power StagePWMVOUTR2–+FBVrVINPWM ControllerEA–+BVr2LM358VXR3R4VCAOp-Amp CircuitFIGURE 1. Typical DC/DC Converter ConfigurationR1Power StagePWMVOUTR2–+FBVrVINPWM ControllerEAFIGURE 2. Output Voltage vs Control Voltage RequirementVO (V)B (VC2, VO2)VC (V)A (VC1, VO1)FIGURE 3. Op-Amp Circuit Offers Programmable Output Function