op-amp voltage regulator design

Regulator with a comparator, similar to hysteretic :



With some cap ESR :

Tony Stewart said:

They call it "negative rail headroom" in the datasheet.
That means it is almost rail to rail but not enough to turn off Vbe, unless a diode is added. in series or R-divider or a Darlington is used or a different Op Amp.

I suspect this soft limiting speeds up the pulse recovery time for other applications by reducing Cout.

Click to expand...

yes the power supply should be Vcc>Vout+Vbe+op-amp swing > 5+0.7+12 =17.7V approx.

danadak said:

Of course we have OpAmp possibly acting more as a comparator, slew rate
dominated in 324 358 types of parts.

View attachment 149247

TL072 :

View attachment 149248


Moving Vz and OpAmp supply directly to battery made no appreciable difference.

Click to expand...

yes, the op-amp should be operating as a differential amplifier with op-amp output, Vo=A(Vf-Vz), the op-amp may be operating as comparator, but why.

okbro said:

yes the power supply should be Vcc>Vout+Vbe+op-amp swing > 5+0.7+12 =17.7V approx.


yes, the op-amp should be operating as a differential amplifier with op-amp output, Vo=A(Vf-Vz), the op-amp may be operating as comparator, but why.

Click to expand...

Because you've configured it as a comparator, you have no feedback round the opamp so it works as a comparator - but a comparator is probably what you want anyway.

okbro

Your feedback loop thru NPN back thru divider is compromised by slew
rate of OpAmp. When an OpAmp is in slew rate limiting it essentially breaks
the feedback loop, eg. its highly non linear. Basically operating open-loop.
This will also break the virtual ground. Of course these conditions dont
last forever, and OpAmp either regains control or you have driven inputs/
outputs out of linear operation, like outputs into the rails.

Frederiksen has a good treatment of this, and there are several IEEE papers
on loop dynamics and slewing.

I have Frederiksen’s book. A must-have for anyone interested in opamps. It should have been called “All you wanted to know about opamps but were afraid to ask”.
And it tackles complex topics with only the essential mathematical explanation.