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UPDATED: MADE A FEW CHANGES TO GET 2.5V REFERENCE AS INTENDED.
The TL431 is a programmable voltage reference. It’s a 3 pin device available in several packages including a TO92 ‘transistor’ case. It can provide 100mA output, or more with additional circuitry. By adjusting the ratio of the resistors at the reference input, here we have two 10k resistors, the output can be set over a wide range. It can be used effectively as a zener diode for any value (within its spec) - useful if you need something that isn’t a standard zener value.
This is the circuit from the TI data sheet. The real component maintains a constant voltage of 2.5V at its reference, here the junction of the two 10k resistors. I’ve added a 100 Ohm load at the right hand side. The output voltage will be reference voltage x (1 + R1/R2), so here with R1=R2=10k, output is 2 x reference voltage. Data sheet also gives an additional, more precise formula.
You’ll notice that here the reference voltage is 2.06V rather than 2.5V which, assuming TI have not made an error in the data sheet, I presume is either an EC anomaly or because we can’t alter some of the transistor parameters.
You can connect reference pin directly to the output pin to give a 2.5V output (or 2.06 on EC).
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