KIS-3R33S — what resistor gives the desired output voltage?

Last year I bought some KIS-3R33S 3A DC-DC step-down power modu­les based on the MP2307. (Link is to eBay sel­ler Bosity’s cur­rent lis­ting of this item. This is the sel­ler that I orde­red it from. A search on eBay for KIS-3R33S is avai­lable here.) This is a DC to DC power con­ver­ter module that by default con­verts an input vol­tage of 4.75 to 23 V DC to 3.3 V DC, sup­p­ly­ing up to 3 A (con­stant load).

You may need or want an out­put vol­tage other than 3.3 V DC, and you can, by con­necting the Vadj pin through a resis­tor to eit­her ground or Vout. Using Max­ima and infor­ma­tion from  Digole and other pages I came up with for­mu­las for cal­cu­la­ting the pro­per resistance to use for any desi­red out­put vol­tage using an unmo­di­fied KIS-3R33S module.

The out­put vol­tage of the MP2307 is 0.925 * (R1+R2)/R2 (accor­ding to Digole, con­fir­med by datasheet from the manu­factu­rer, Mono­lit­hicPo­wer), where R1 is con­nec­ted to Vout and R2 to GND. The unmo­di­fied KIS-3R33S module has two 51K resistors in paral­lell as R1 and one 10K resis­tor as R2 (giving us a the­o­re­ti­cal actual out­put vol­tage of 3.28375 V DC). The module’s Vadj pin has a 3K3 resis­tor that will be in series with wha­te­ver you choose to con­nect to the Vadj pin.

For an out­put vol­tage hig­her than 3.3 V DC you want to con­nect the module’s Vadj pin through a resis­tor to GND. The out­put vol­tage using added resistance R will be  (2627*R+27539100)/(800*R+2640000). To find the requi­red resistance R nee­ded to achi­eve out­put vol­tage V you would use R=-((2640000*V-27539100)/(800*V-2627)). For an out­put of 5 V you would want an exter­nal resistance of 10443.62709395484, Using stan­dard E12 series resistors a single resis­tor of 10K would be close (the­o­re­ti­cal Vout of 5.057246240601504), and you can get clo­ser using a 10K in series with a 390R (the­o­re­ti­cal Vout of 5.006722972972973, which is actu­ally a much hig­her pre­ci­sion than that of stan­dard 5 % resistors).

For an out­put vol­tage lower than 3.3 V DC you would con­nect the Vadj through a resis­tor to Vout. The out­put vol­tage using added resistance R should be 0.925*((1/((1/51000)+(1/51000)+(1/(R+3300)))+10000)/10000), and the resistance nee­ded for an out­put vol­tage of V should be -((23040000*V-27539100)/(800*V-2627)). For an out­put vol­tage of 1.5 V you would use an exter­nal resistance of 4920.042046250876 con­nec­ted to Vout. Using E12 series resistors that would pro­bably mean a 5K6 for 1.535 V or 4K7 for 1.488 V.

So far I have not tested any of this IRL—caveat emptor!