Using digital pots to adjust output voltage of DC-DC boost converter? - Page 1
Forum section: electronics / beginners

author: LinuxHata
author_rank: Frequent Contributor
timestamp: December 11, 2023, 09:40:50 am
content: Hello.I need to control from the microcontroller output voltage of some boost converters (nothing fancy, up to 30V 20mA, XL6007 or similar).Is it possible to use a digital pot, something like CAT5401 or any other, in a feedback divider, to control the output voltage?I tried to find a boost converter with direct digital control over output voltage, but can't find any. There are only power bank solutions available, which, in best case, can adjust output only in 0.2V steps, and can't output 30V at all.

author: voltsandjolts
author_rank: Unknown
timestamp: December 11, 2023, 10:31:09 am
content: Digital pot may cause trouble in the feedback loop. The more conventional approach would be a pwm dac controlling an offset adjustment to the feedback signal.

author: ledtester
author_rank: Super Contributor
timestamp: December 11, 2023, 10:49:06 am
content: Here's a forum discussion from 2013:https://www.eevblog.com/forum/projects/arduino-controlled-power-supply/msg335864/#msg335864

author: mariush
author_rank: Super Contributor
timestamp: December 11, 2023, 11:48:28 am
content: Here's a schematic of an adjustable linear power supply that uses digital potentiometers to adjust things with buttons ... just in case someone wants to inspire from it.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: December 11, 2023, 07:08:34 pm
content: Well, I read that thread. I found there nothing saying that digital pot won't work, except things like "if it was not done 38 year before, you should not be doing it now".

author: voltsandjolts
author_rank: Unknown
timestamp: December 11, 2023, 07:45:21 pm
content: Try it. Then let us know how nice the transient response is on your psu.

author: uer166
author_rank: Super Contributor
timestamp: December 11, 2023, 08:23:52 pm
content: A DAC wired up with resistor to FB pin is a much better behaved solution. It can be a PWM DAC, or a real DAC, or a dedicated MCU DAC.

author: voltsandjolts
author_rank: Unknown
timestamp: December 11, 2023, 09:03:06 pm
content: You might be able to build a pwm dac from your scrap pile, e.g.https://www.edn.com/combine-two-8-bit-outputs-to-make-one-16-bit-dac/https://www.ednasia.com/cancel-pwm-dac-ripple-with-analog-subtraction-revisited/

author: Jwillis
author_rank: Super Contributor
timestamp: December 11, 2023, 09:12:16 pm
content: Quote from: LinuxHata on December 11, 2023, 07:08:34 pmOn the XL6007 the voltage is controlled by the voltage divider R1 and R2 .Changing R2 changes the output voltage . A poteniometer as a rheostat at R2 makes an adjustable voltage based on the formula Vout = 1.25 * (1+R2/R1). The wipers of the CAT5401 have a maximum current of 3mA but very little current goes to The feed back. So theirs no reason a digital potentiometer would not work at R2.

author: voltsandjolts
author_rank: Unknown
timestamp: December 11, 2023, 09:23:54 pm
content: Power supply noise is often measured over a 20MHz BW.The CAT5401 has a specified BW of 400kHz, for the 50K version.

author: Konkedout
author_rank: Frequent Contributor
timestamp: December 12, 2023, 04:24:23 am
content: Don't put the digipot in the fb divider. If it has low impedance output, run that through a resistor and into the FB pin of the regulator/controller IC. In that way, the FB pin works as a summing junction.Seehttps://www.google.com/search?q=op+amp+summing+junction&rlz=1C1RXMK_enUS1029US1029&oq=op+amp+summing+jun&gs_lcrp=EgZjaHJvbWUqCAgBEAAYFhgeMgYIABBFGDkyCAgBEAAYFhgeMg0IAhAAGIYDGIAEGIoFMg0IAxAAGIYDGIAEGIoFMg0IBBAAGIYDGIAEGIoFMg0IBRAAGIYDGIAEGIoF0gEJODAzM2owajE1qAIAsAIA&sourceid=chrome&ie=UTF-8Here is a recent project of mine that does this:

author: Jeroen3
author_rank: Super Contributor
timestamp: December 12, 2023, 07:26:33 am
content: I've tried this, it's very hard to get it stable because it will add filtering into the fb loop the chip designers didn't intend.Maybe a slow switcher at a few khz will work?You can put them in bias circuit as voltsandjolts suggest injecting an offset into fb. I have seen this.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: December 13, 2023, 05:14:34 am
content: You mean that digital pot starts to act as a capacitor?

author: Terry Bites
author_rank: Super Contributor
timestamp: December 13, 2023, 10:16:16 am
content: The man, he say....https://www.analog.com/en/technical-articles/linear-adjustment-of-switched-mode-power-supplies.html

author: LinuxHata
author_rank: Frequent Contributor
timestamp: December 13, 2023, 08:24:47 pm
content: Well, all that is quite understandable and I don't see any significant issues with all that.The proposed IC is nice, but for asking price - way too much.But anyways, I now know the direction to look into, so will raid LCSC, in search of cheaper alternativeThanks!

author: voltsandjolts
author_rank: Unknown
timestamp: December 13, 2023, 08:54:00 pm
content: If you don't want to spend $3 on a fancy AD5141 digipot with 3MHz bandwidth, then why not go with the one of the cheaper alternatives mentioned above. PWM would be circa $0.50. Why the fascination with digipots? Just curious.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: December 14, 2023, 04:33:27 am
content: See, I'm using PIC16xxx series chips, for low budget, and they don't have built-in DAC, so I have to source one externally, so, to reduce BOM and complexity, instead of adding extra chips for DAC, opamp, their compensation and feedback resistors/caps, etc. I believe, that single digipot should be more beneficial. That's all. I don't have any personal affection to digipots