This design monitors a 3.3-V rail and flags an undervoltage fault at the RESET output when supply rail falls approximately 12% below the nominal rail voltage. The TLV803E device has an open-drain output topology so an external pull-up resistor is required and is calculated to ensure that V_OL does not exceed max limit given the
I_RESET/RESET spec of ±5 mA is not violated at the expected supply voltage. Section 7.5 table provides 500 µA Isink for 1.7 V VDD, which is the closest voltage to this design example. Using 500 µA of Isink and 300 mV max V_OL, gives us 5.36kΩ for the external pull-up resistor. Any value greater than 5.36kΩ would ensure that V_OL will not exceed 300 mV max specification. If you are using the TLV809E device variant, no pull-up resistor is required because TLV809E has push-pull output topology.