I have a boost converter TPS61288 which can boost the input upto 18v. Adjustment of the output voltage is done via a potentiometer to the feedback of pin of the converter. 18v is dangerous for my system incase the potentiometer gets accidentally set to the wrong resistance. So i was thinking if it is possible to place a hard limit on the pot.

I want to limit the adjustment to 8V only with and input voltage of 3.7, i want to still be able to adjust between 5-7v with the pot. My plan is to place a fixed resistor before, after, or before and after the potentiometer.


simulate this circuit – Schematic created using CircuitLab

According to the datasheet this is how to get the output voltage:

enter image description here

I can only use a 100k potentiometer. So i can maybe have a setup of skip top resistor and R2 = 10k . then my voltage window is 1.63v (pot at 0 position R1 = 0 R2 = 110k) to 6.6V(pot at 100k position R1 = 100k R2 = 10k). This is very crude as i have to brute force trial and error values till i reach 8V

can anyone help me figure out the formula for the fixed resistors to have a limit at 8V (it would be great if the formula can solve limit not just for 8v). In the mean time i guess ill brute force the calculations

  • \$\begingroup\$ Why "brute force"? Why not high school algebra? You have the governing equation, just plug in values for the potentiometer turned all the way one way, then the potentiometer turned all the way the other, and solve for the values of your fixed resistors with respect to the potentiometer value. \$\endgroup\$
    – TimWescott
    Apr 1 at 15:49

1 Answer 1


You have to write simultaneous equations and solve them

R2min = R2fixed + 0
R2max = R2fixed + 100k
Note that total resistance in the resistor divider is R1fixed + R2fixed + 100k in whichever position of the pot

equation1: voutmax = vrefmax (R1fixed + R2fixed + 100k) / R2min

equation2: voutmin = vrefmin (R1fixed + R2fixed + 100k) / (R2max)

In the above 2 equations, substitute voutmax and voutmin that you want and find the values for R1fixed and R2fixed.


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