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I need a electronically programmable buck converter module.

Because I do not trust my skills to approach high-frequency circuits, I thought I would modify one of these very cheap pre-built modules as a basis:

Mini360 Modules

From the datasheet of the MP2307DN controller, I suspect their circuit is as follows:

MP2307 typical application

Can I remove R1 & R2 from these modules and:

Is there any downside to having the potentiometer in the lower leg of the voltage divider? Is there anything I am overlooking?


Edit 1: Attempt at implementing Andy aka's answer:

enter image description here

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Andy is right about parasitic capacitances.

Changing feedback resistor values will also change stability conditions. The current source solution is nice, but a simple voltage source connected to the feedback node through a resistor of appropriate value will work just as well.

It is also possible to connect the GND pin of R2 to ground via a capacitor, but set its DC voltage with the output of an opamp and another resistor.

If you bought this module pre-made though, you won't know the voltage rating of the output capacitors. If you increase output voltage, caps could need a replacement for properly rated ones.

EDIT:

schematic

simulate this circuit – Schematic created using CircuitLab

Using this simple schematic, you can pull the feedback voltage by using the PWM output from a microcontroller, filtered by a cap. You could also use a DAC for lower ripple.

Now, how to obtain resistor values...

Your chip has a 0.925V reference voltage. This means "FB" node is at 0.925V. On your original schematic, R1 and R2 form a simple voltage divider.

Let's suppose we want Vout max=5V. We pick R1=10k and R2=2.222k, and we get FB=0.925V for Vout=5V. This gives us R1.

Now, we're doing a DC calculation, so we forget about the cap. We set PWM=0V. R3 and R4 are in series, and R3+R4 are in parallel with R2.

For Vout=5V at PWM=0V => R2 // (R3+R4) = 2.22k

This does not give resistor values yet! There is still a choice to make. We have to adjust R2 versus R3+R4 to get the sensitivity we want. For example, if our micro has a PWM output which goes to 3.3V, we might want a 3.3V PWM to correspond to a 1V output. Or maybe another value, it's your choice.

I'm too lazy to do the math right now, but I guess you get the idea!...

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  • \$\begingroup\$ @Thanks for the suggestions. I am working my way through understanding all the answers I received. Regarding your opamp solution: Are you are thinking of a voltage buffer that raises the lower leg of the DC-isolated R2? \$\endgroup\$
    – ARF
    Feb 26 '17 at 13:30
  • \$\begingroup\$ post above edited, added schematic. Note the optocoupler you added will have unspecified temperature drift and DC characteristics unless very expensive, which is not conducive to adequate precision on output voltage... \$\endgroup\$
    – bobflux
    Feb 26 '17 at 14:43
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On the face of it, it might work but also, because of parasitic capacitances in the digipot, it might turn the regulator into an oscillator or it might produce a much degraded load transient response.

A better way might be to make a high side variable constant current source and supply a small additional current into R2. This will make the chip regulate at a lower voltage. In fact, if you put a diode in series with R1 you should be able to regulate down to nearly 0 volts.

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  • \$\begingroup\$ Thank you very much for your answer. I attempted to implement your high-side current source suggestion. Would you mind taking a look at the edit to my question and checking whether I understood your intention correctly? \$\endgroup\$
    – ARF
    Feb 26 '17 at 14:26
  • \$\begingroup\$ More like this electronics.stackexchange.com/questions/82659/… the opto idea is cool but won't be stable with temperature however, if you have an ADC that measures the output voltage you can implement closed loop control. However, you might as well go for a voltage being injected via a resistor into the FB node. With a precision current source, my idea would be to have it fed from the input supply rail i.e. 12 volts. \$\endgroup\$
    – Andy aka
    Feb 26 '17 at 14:59
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Can I remove R1 & R2 from these modules

you can.

But that's too much work to me.

if you only want to output a voltage higher than 3.3v, just parallel the digital pot to R2.

for lower-only voltage, parallel it to R1.

and make sure that the schematic is indeed how your board is wired up.

edit: looks like on your board, the pot is R1 and R2 is a 922(?) resistor.

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