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Below is the circuit for constant voltage. The Value of R1 is 330 ohm and when Vin = 3.6V , the Vka = 1.5V And when Vin changes the value for Vka also changes. it should be stable

Why? The Vka should be equal to 2.5V

enter image description here

MICRO
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    Show the complete circuit, and tell use the voltages you're using. – CL. Mar 21 '17 at 12:22
  • "Test Circuit 4 for VKA = VREF" and VREF is defined to around 2.5V so clearly you are not giving it enough headroom to regulate with your 3.6V – PlasmaHH Mar 21 '17 at 12:32
  • The configuration in your question forces the TL431 to act like a 2.5V zener diode. According to the datasheet, input voltage can be anything between 2.5V and 36V so there's no problem in your circuit. But your bias current is too high (around 3mA but it should be 1mA max), this shouldn't be the cause though. Maybe you can place a higher resistor like (3.6V-2.5V)/0.5mA = 2k. – Rohat Kılıç Mar 21 '17 at 13:39
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    @RohatKılıç: The TL431 needs 1mA minimum, not 1mA max. It won't regulate properly below 1mA. Also, this is not a "zener diode," it's a shunt regulator with much better performance than a zener diode. The configuration as shown is correct, and I believe the values given are all correct, which means the OP is leaving out some important fact that explains the problem. – Warren Young Mar 21 '17 at 13:45
  • Do you have a load on this circuit, or is it being tested unloaded? With a shunt regulator, the proper resistor value depends on the load. – Warren Young Mar 21 '17 at 13:49
  • @WarrenYoung I know what the TL431 is. But the config above forces it to operate "like" a zener diode. About the bias current, yes, it should be "minimum". Datasheet states that typically 0.4mA to 1mA "minimum" is enough. In most SMPS circuits I designed, I stay between 0.5mA to 1mA with no problems. – Rohat Kılıç Mar 21 '17 at 13:51

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