brownout on MCU's

[jahonen]

Well, the simulation does not look very promising, "check"-circuit works just fine (just to show that TL431 SPICE macromodel works).

Regards,
Janne

[Zero999]

Why is it not working?

It will work, it's not working because I made a mistake on my schematic, can you guess what it is?

Hint: I missed out a component.

Look at the datasheet for the TL431 and the LM431 for more information.

I'll tell you in a day or so, if no one can figure it out.

[jahonen]

It misses the usual B-E-resistor which is needed to bias the TL431 correctly.

Regards,
Janne

[Zero999]

Yes that's it. The TL431 will pass a small current when the voltage is below 2.495V so without a pull up the transistor will turn on before it should.