# resistor selection for TPS3700DDCR undervoltage circuit

by Aubrey Champagne   Last Updated June 30, 2020 04:25 AM

I am designing an undervoltage circuit for a lipo battery project. I found this tutorial here which lead me to the TPS3700. I am wanting the undervoltage condition to be 3 volts. The part that I am hung up on is choosing the right resistors for the under and over voltage conditions. At the bottom of page 16 of the datasheet, it says `Rt=R1+R2+R3` and `Select a value for RT such that the current through the divider is approximately 100 times higher than the input current at the INA+ and INB– terminals.` I'm having trouble figuring out this value. The max current that would be going through the power rail is 650 milliAmps. Also the equations for `R3` and `R2` have a term `Vit+` which is defined as `Positive-going input threshold voltage`. Texas Instruments also says it's `The input threshold voltage when the input voltage is rising.`, but this doesn't really give me much detail on how to calculate it. I calculated `Vit+` from the tutorial and found this value to be around `.4001` I used this value along with their `Rt` for my resistors and assembled the circuit below: boost gate just leads to a mosfet that controls a 3.7 to 5V boost circuit. Will these resistors cause the undervoltage condition to be 3V? The specific values for the battery I am using is 3.7V 800mAh.

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