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# Shunt regulator between two power rails

#### pacman22

##### Junior Member level 1

Can you please explain the working of this circuit? What does the voltage shunt regulator do between the two power rails?
VDD_SHV5 = 3.3V
VDD_1V8 = 1.8V

Hi,

(edit: wrong assumption that VDD_SHV5 means 5.0V. --> see post#5)

short and simply speaking:
It generates 1.8V from 5V by dropping 3.2V.
3.2V is the shunt regulator voltage.

Klaus

Last edited:

I must be sleeping but I see two power rails, in and out, with no modification
other than a circuit that uses some of the power rail current ?

And that current is fixed by the Vref 431, but what is the Ic of the transistor ?
Totally dependent on Vbe ? Current thru R423 fixed though.

There's a very precise relationship between emitter current Ie and Vbe:

Ie = Is * exp(Vbe/Vt)

Since Ic is about 99.5% of Ie for most small signal transistors (Ic = Ie * beta/(beta+1)), we often assume that:

Ic = Is * exp(Vbe/Vt)
So its dependent on Is and Vt, great, we can either toast marshmallows on it or
use it to increase entropy in the Galaxy ?

One can see that interior node of circuit is regulated with respect to power rail
VDD_1V8......ok thats wonderful........

Regards, loosing it, Dana.

KlausST has explained circuit operation. OP didn't ask if it's a useful solution (not generally) and what are the conditions for correct operation (obviously some amount of load current).

Hi,

but with one point I was wrong: assuming that VDD_SHV5 means 5V.

Math proved that the drop voltage is 1.5V.
Now it also makes sense why the OP wrote: that VDD_SHV5 = 3.3V

Conclusion:
--> Then it uses the 3.3V supply to generate 1.8V by dropping 1.5V

Klaus

KlausST has explained circuit operation. OP didn't ask if it's a useful solution (not generally) and what are the conditions for correct operation (obviously some amount of load current).
Clearly we have, in a sense, a duty to let OPs know they are trying to work
with circuits that may not work. At least thats my thought of people coming
here for help. At minimum ask the OP what they think circuit does. And if it
does not inform them.

And in confirmation I see forum helpers doing just that in posts as a general rule.

My post was targeted actually at my lack of understanding what circuit does, and
I figured Klaus would educate me.....

Maybe I am missing some deeper meaning of forums designed to help folks. I
am open to behavioural modification, have a wife seeking that often....

Regards, Dana.

FvM

To answer the OP's question, if the 5V power rail gets too high above the 1v8 rail then the 431 acts to shunt current from the top rail to the 1v8 rail - this may of may not work in terms of keeping the 5V rail to 5V - it depends on the rest of the circuit ( not shown ).
--- Updated ---

The divider resistors have been calculated wrongly however - making it even less likely to work

Hi,

My personal opinion:
* everybody has a different way to teach
* and everybody has a different way to learn.
Thus different answers, different styles of teaching, different side informations, different explanations, different points of view, different ways of expressing something ... there´s nothing wrong with it. Variety and diverstity is a good thing and a true benefit for an international forum.

Usually we don´t know the members who ask the questions, so we don´t know what answers they expect.
So it´s a benefit for the member that he/she gets different answers. (It wouldn´t make much sense when multiple posts say exactly the same). He/she can ignore answers and can pick the answer that suits the most.

And for sure this will result in minor conflicts and misunderstandings. Some member is upset by an answer, the other finds the exact answer he/she was looking for.
And for sure there will be situations where one "helper" clarifies or even corrects an other´s post. Nothing wrong with this. Still a benefit for the "asker".

And for me speaking:
When I have a different opinion - even when I write this without sugar coating - this does by far not mean that I generally disrespect the other member. Sometimes it may sound nit-picking - but that´s not the way it is meant to be. It´s not meant personally, it´s focussed on the technical problem. I still see you and the other posters as very valuable members. I don´t want to stop someone from posting, I don´t someone to change his/her style of teaching.

Klaus

FvM