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I am designing an adjustable high current buck converter.

The input voltage will be around 12V, but the output needs to be adjustable in the range of 0.2 to 10V.

I have decided to use the LM27402 (datasheet).
The output will be adjustable through I2C, using a digital potentiometer as part of the feedback voltage divider.

However, the reference voltage on the feedback comparator is 0.6V.
Normally this means that even if I connect FB directly to the output, I cannot get a voltage output lower than 0.6V.

(This is true for practically all such regulator controllers, so going for another IC does not seem to be a solution.)

I am thinking of using an opamp with a gain of 3, to increase the FB voltage.
This way, an output of 0.2V will reach the IC feedback pin as 0.6V.

Is this a correct solution?
Is there any other better / recommended solution for this problem?

The opamp solution seems pretty straight-forward, but I am concerned about the stability of the regulator (e.g. causing oscillation of the error amplifier etc). If this is a valid concern, can it be calculated in advance, or is there anything that I can do to my circuit to mitigate this issue?


EDIT
I am adding a schematic of my idea:

enter image description here

+BATT is around 12V. +3.3V is also available from another regulator.

I also need a way to control the voltage, and I am thinking of using a digital potentiometer:

  • In place of R6 (e.g. MCP45HV51T-503E/ST)
  • In place of R7 (e.g. ISL23415TFUZ)
  • Or possibly to control the gain of the OPAMP

This is only a draft schematic of the output stage of the regulator. Unfortunately no "proper" schematic exists yet, as the idea is still researched.

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  • \$\begingroup\$ Not possible as described \$\endgroup\$ Commented Mar 9, 2022 at 21:31
  • \$\begingroup\$ I added a schematic to describe my thoughts better. @TonyStewartEE75 can you please comment on the "fatal flaw" of the idea? \$\endgroup\$ Commented Mar 10, 2022 at 10:26
  • \$\begingroup\$ R ratio's won't create a -400mV fixed offset with variable R for gain from 0.6V. to adjust 0.2 to 10V. Perhaps a diode in series with feedback might but then still not very stable. \$\endgroup\$ Commented Mar 10, 2022 at 19:35

2 Answers 2

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Is this a correct solution? Is there any other better / recommended solution for this problem?

A safer way of doing this is to add 0.4 volts to the feedback signal. This usually avoids any problems with creating too much loop gain and turning what would be a nice regulator circuit into an uncontrollable oscillator. So, I would tend to add 0.4 volt rather than multiply by three.

I'm not 100% saying having a gain of three stage won't work of course.


Idea for adding 0.4 volts to the feedback signal: -

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Interesting. Can you please provide a sample/draft implementation of this "voltage addition"? Maybe by a current source connected to the low resistor? \$\endgroup\$ Commented Mar 9, 2022 at 19:47
  • \$\begingroup\$ You first - show the op-amp gain of three circuit and indicate the power rails for the op-amp so I understand what circuit resources you have @FotisPanagiotopoulos \$\endgroup\$
    – Andy aka
    Commented Mar 9, 2022 at 20:17
  • \$\begingroup\$ I added a schematic. I hope it is clearer now. \$\endgroup\$ Commented Mar 10, 2022 at 10:22
  • \$\begingroup\$ @FotisPanagiotopoulos ditto \$\endgroup\$
    – Andy aka
    Commented Mar 10, 2022 at 10:46
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    \$\begingroup\$ I really like the simplicity of your idea, and I may end up using it. It may also make it easier to interface the digipot. Thank you! \$\endgroup\$ Commented Mar 10, 2022 at 13:34
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The way you'd typically do that is by dumping current from something like an LM334 or a Howland current pump. For either current source, observe its slew rate. For LM334, the slew rate scales with output current, the details are in the datasheet. You'll want to first measure the slew rate the switching converter can achieve on its output, as seen from the feedback node, when the feedback gain is changed in a stepwise fashion. Then set the current source current high enough that the source can develop at least 2x the slew rate needed. Then choose the lower resistor for the feedback divider such that the current source produces the offset you want (i.e. 0.4V). Then choose the upper feedback divider.

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