How to replace a potentiometer with a digital solution in a buck feedback loop?

Question

I have a simple pre-made buck converter that looks something like this:

The output voltage can be easily adjusted, by turning the R2 trimpot, which adjusts the voltage the feedback pin is getting.

What I would like to do, is to take this mechanical way of setting the output voltage and change it to a digital one, where I can set the output voltage I want with something like an Arduino.

My first idea was to simply rip out the R2 trimpot and replace it with a digital potentiometer, but this turned out to be a no-go, since the digital pots I have (and most cheap digi pots I have seen) have voltage limits of around 0 - 5V, and I will be outputting voltages between 1.2 - 23V (The power source feeding the buck is 24V)

My second idea would be to somehow use an op amp to replace the R2 trimpot, but I have not been able to figure out how to go about this.

What would be some simple way to achieve my goal?

Answer 1

Put the digipot in R1's position. The max normal voltage seen at the FB pin will only be +1.25 volts so a 5 volt device will be fine. However, there might be fault circumstances when this might rise above 5 volts so put a zener diode across that point.

The other end of the digipot will be at 0 volts so this is another advantage. However, a digipot will introduce capacitance that may cause ringing in the regulator's output voltage and this might require you to have a small capacitor across R2 (about 47 pF).

You should also consider that using a digipot as a rheostat has two disadvantages: -

• The end-to-end resistance of the pot isn't accurately defined so there will be a larger error than when using a conventional fixed resistor
• Digipots are much better in terms of temperature stability when operated as a potentiometer rather than a rheostat.

Answer 2

You can dynamically change the output voltage changing the gain of the network (Vfb/Vout) injecting/extracting a small amount of current in the feedback network. If you inject current to the feedback node the output voltage decreases. You can use a DAC and a current source (transistor) to accomplish this.

Answer 3

The circuit below allows Vout control using either PWM or an analog voltage.

R#/C1 form a low pass filter to convert PWM to DC.
To get low ripple it is usually desirable to have the filter corner frequency much lower than the PWM frequency. For more rapid response I often use a Bessel filter in this role. A 3 pole Bessel can be implemented with 1 opamp section and a 5 pole with 2 opamps. If a quad opamp is used the end result is cheap and compact.

IC1 acts as a comparator, comparing the user provided control voltage with voltage from the divided output voltage. An opamp can typically be used in this role without problems. IC1 here provides high/low output and this action is usually acceptable to SMPS ICs. If desired IC1 could be configured as a finite gain amplifier, allowing analog control of the FB pin, but this level of sophistication is not usually required.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 4

try this circuit. It worked for me after 3 weeks of testing and burning out components. It works fine from 4.1V to 23.5V. You need a 10K trimpot (DS1804) and an op amp (LM311). I tried with other op amps, but without success. Good luck.