From: Harold Hallikainen <harold@mai.hallikainen.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Fri, 6 Mar 2020 08:36:24 -0800
Subject: Re: [OT] Adjustable regulator down to 0V
Thread-Topic: [OT] Adjustable regulator down to 0V
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> --------------------------------------------------------------------
> I'm thinking I could use an opamp to sum the bias and sampled output
> voltage and drive the control node with that.  I could apply the feedback
> signal to the noninverting input and a small negative adjustable offset
> voltage on the inverting input hence isolating the feedback and offset
> signals from each other.  Doubtless I'd want a low pass filter for the
> feedback input and possibly an RC network between the opamp output and th=
e
> regulators feedback terminal..

I think this is getting complicated compared with the LT part that has a
reference current. But, if you have a regulated negative supply (from an
LDO or something), and are using something like an LM317 for the positive
regulator, just connect the bottom of the feedback voltage divider to the
regulated negative supply. The output of an LM317 is always 1.25V above
the control pin. If the negative supply is -1.25V, the LM317 output would
be 0V when the voltage divider is "all the way down." If the negative
supply is larger than -1.25V, the voltage divider would be adjusted
appropriately so that the lowest output voltage is -1.25V.

Also, be careful about adding a low pass filter in the feedback. This adds
phase shift and could result in instability.

Good luck!

Harold



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