I have purchased a small quantity of item #799 Boost Regulator 4-25V and want to replace the trim pot with fixed resistor(s) to have a constant 18V output. Can you provide the process to do this?
I understand you offer this service for high-volume applications but I need it for the ones I currently have.
The regulator is trying to get the feedback voltage to be approximately 1.24V. To fix the voltage, you would remove R3 and replace R1 and R5 with a resistors that give the feedback voltage with the VOUT you desire. I attached images of the relevant part of the schematic and the component locations below.
High volume depends on what you want the price to be and how quickly you want them. We make these 40 to a panel, so the multiple will be something like that (allowing for some failed units), and a little under 120 units would be a good starting point for us if we already stock the resistors you want on there.
Thank you Ryan and jan.
My solution was to remove R3 and replace R5 with a 27K ohm resistor.
Simplified math to find the value for R5 with a desired output voltage of 18V:
Determine voltage ratio: 18/1.24 (feed back voltage) = ~14.52
R1 is 360K ; (1/(14.52 - 1)) * 360,000 = 26,627
Nearest common value 27,000;
Find the current with the feed back voltage and common resistor ; 1.24 / 27000 = 0.0000459
Find the output voltage to achieve the the feed back voltage:
0.0000459 * (360,000 + 27,000) = 17.8v, This works good for my applicaiton and the parts I have on hand.
If you take off R3, you are left with a simple voltage divider with R1 and R5. The tolerance to use depends on your requirements. Here are the pairs of common 1% resistor values that are within 0.1% of the correct divider ratio to achieve a fixed output of 12V and have a combined resistance greater than 10K Ohms and a individual value no more than 360K Ohms:
I used the r1: 41200 r5: 4750 pair and the resulting output was 11.8v, well within the 10% tolerance of the device being powered. Thanks for your help!