Adjustable Step Down Module - Swap Potentiometer with Resistors?

Hi, I am using a DC-DC Buck Converter that has a MP1584EN Regulator (Amazon Link) and am taking 12v to 5v for Vin on a micro controller.

This module is pretty tiny, and the Pot for adjusting is also super fragile and sensitive. I'd like to pull it and replace with resistors to lock in my 5v out so it doesn't get adjusted inadvertently.

I could use some help on determining how to swap in the resistors, I've traced the 3 posts on the Pot to the Regulator and found one goes to the Feedback Pin 4 on the MP1584EN (#3 on attached photo) and the other two are connected to Vout+ and the Switch Node Pin 1 on the MP1584EN (#1 & 2 on attached photo).

Looking for instructions on replacing the Pot with fixed resistors somehow. Thank you!

Sure! Set the pot to the value you want. Then lock it using a bit of hot glue. Pull the pot. Measure the resistance from each end terminal to the wiper terminal. get resistors for those two values and solder them in so they replace the pot.

Paul

Remove the pot and solder a 43K 0805 smd resistor between terminals 2 and 3. That should provide 5V as R2 in most of these modules is 8K2. With R1 at 43K and R2 at 8K2, vOut is 0.8 * (43+8.2) /8.2 where 0.8 is the Vref of the MP1584.

https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP1584/document_id/204

Thanks WattsThat. I determined ~43k for the value from experimenting as well, good to have a confirmation and formula to back it. However, I posted the resistor between 1&3 - which seemed to work on my experimenting to get ~5v out. Since 1&2 are both connected to SW Pin 1 I assume that is fine?

However, I built up my proto board this weekend and now am seeing some unusual behavior. I am not clear with how the MP1584EN Regulator works, but I am seeing some odd values for resistance. I will try to explain. I put together my breadboard build on a double sided PCB, was pretty detailed and careful to replicate. But for some reason now I get ~2.5v out when testing the regulator (with the 43k added, as in my breadboard test). Upon further testing, I was seeing a lot of "floating" values on the resistance through the step down. On my first tests I was seeing a negative resistance value (which I believe means there is voltage still in the circuit?). I realized my 12v DC adapter was still plugged in to my board (not power) so assumed it was causing that issue. Then I unplugged and I would see a rising value when probing across pin 1 & 3, where if I had my positive probe for my meter on pin 1 /negative on pin 3 it would start at ~32k Ohms and float up into the higher 30's. If I reversed polarity it would start ~8.5k Ohms and rise slower to just over 9. My assumption is I have voltage in the circuit in some capacitance, but not sure.

As you can tell, I am not versed enough to know why these values would float like that, why I get different values with polarity (I always assumed polarity with resistance didn't matter) and why my output voltage changed. I assume I did something wrong when building up my board, but any pointers or education on how to understand the readings I saw would be very appreciated. Thanks.

I did another test. I have 3 resistors in series on my board 20k + 20k +3.3k and lead wires to pin 1 & 3. I desoldered the leads and hooked them up to 3 other resistors in series on a breadboard and got ~5v out. I tested the resistance of the pcb resistors and things look as expected - total of ~43k when checked on my meter. Reconnected the leads again and same issue happened (lower voltage). I'm stumped.

I ended up pulling the resistors off and soldered on new ones. Each resistor tested fine after removing them, so I am not sure what I did wrong. In any case, its working with the new resistors. Thanks for the help.