KIS-3R33S — what resistor gives the desired output voltage?Posted on August 1st, 2012 2 comments
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!
I’m just working with the KIS-module and Google send me here.
Just a hint: There’s a 5,6V Zener limiting Vout. So if sombody trying to get more than 5,6V out of the module by following the resistor calculations above, the KIS will fade away.
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[…] man das Verhältnis einfach durch Einlöten eines Weiteren anpassen. Hierzu habe ich mich an diese Anleitung gehalten. Ergo reichte das Verbinden von Vadj über einen 10kΩ mit GND, um die gewünschten […]
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