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LM317 digital control
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Zero999:

--- Quote from: Maobuff on May 19, 2016, 08:53:54 pm ---
--- Quote from: Hero999 on May 19, 2016, 08:43:08 pm ---That circuit won't go down to 0V. The LM317 keeps the voltage on its output 1.25V above the adjust pin so it will vary from 1.25V to 5V.

--- End quote ---
I know, its not a problem for my supply, and i think if i feed 0-5V to ADJ I will get 1.25-6.25 on the output

--- End quote ---
Yes, you're right, it will go from 1.25V to 6.25, not 5V. Post edited.
StillTrying:
If the ouput of the LM317 is 1.25V with a few mA load, then the LM317 must be working OK.

If you can see the op amp output varying 0 to 5V but the output of the LM317 doesn't change from 1.25V then there's a wiring error such as the LM317's Adj pin incorrectly connected to GND.

The output voltage range of your circuit is going to be only min 2.2V to max 6.25V.
danadak:
Looking at two different datasheets they show output C of 1 uF.

Makes me wonder what the phase margin is if 10 uF like you have.
Note there is a graph of Zo vs f for 1 uF, maybe an estimate can
be done from that. This affects transient response, ringing due to
load changes.

Regards, Dana.
StillTrying:
You might be able to do something like this. The transistor helps to pull the ADJ pin and min load current ( 8 ma?)down to 0V. The 2 2K2s multiply the 0-5V input by 2 to give a max output voltage of 11.25V - depending on V+.
ZeTeX:

dont forget your bypass caps and protection diodes if needed.
R1 and R2 have a gain of 2, your buffer output goes instead of V2.
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