Setting the Shunt Voltage on an Adjustable Shunt
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The ability to set the shunt voltage, VKA, to any voltage between VREF and the maximum rated voltage for the shunt regulator provides a lot of flexibility. It takes two resistors to set the shunt voltage. In an ideal common anode shunt regulator, the shunt voltage would be VREF x (R1/R2 + 1). Real world shunt regulators have limited gain, non-zero reference input current, and suffer from cathode voltage modulation. This application report derives comprehensive formulas that accurately represent the relationship between the shunt voltage and feedback resistors. It also shows a practical example.