Equation 7 illustrates an example of how to adjust the voltage threshold with external resistor dividers. The resistors can be calculated depending on the desired voltage threshold and device part number. TI recommends using the 0.8 V voltage threshold device when using an adjustable voltage variant. This variant bypasses the internal resistor ladder.

For example, consider a 12 V rail being monitored V_MON for undervoltage (UV) using channel 1 of the TPS38A010122DSKR variant. Using Equation 7 and shown in Figure 8-10, R₁ is the top resistor of the resistor divider that is between V_MON and V_SENSE2, R₂ is the bottom resistor that is between V_SENSE2 and GND, V_MON is the voltage rail that is being monitored and V_SENSE2 is the input threshold voltage. The monitored UV threshold, denoted as V_MON-, where the device will assert a reset signal occurs when V_SENSE2 = V_IT-(UV) or, for this example, V_MON- = 10.8V which is 90% from 12 V. Using Equation 7 and assuming R₂ = 10kΩ, R₁ can be calculated shown in Equation 8 where I_R1 is represented in Equation 9:

Substituting Equation 9 into Equation 8 and solving for R₁ in Equation 7, R₁ = 125kΩ. The TPS38A010122DSKR is typically meant to monitor a 0.8 V rail with ±2% voltage threshold hysteresis. For the reset signal to become deasserted, V_MON would need to go above V_IT- + V_HYS. For this example,
V_MON = 11.016 V when the reset signal becomes deasserted.

There are inaccuracies that must be taken into consideration while adjusting voltage thresholds. Aside from the tolerance of the resistor divider, there is an internal resistance of the SENSE pin that may affect the accuracy of the resistor divider. Although expected to be very high impedance, users are recommended to calculate the values for the design specifications. The internal SENSE resistance R_SENSE can be calculated by the SENSE voltage V_SENSE divided by the SENSE current I_SENSE as shown in Equation 11. V_SENSE can be calculated using Equation 7 depending on the resistor divider and monitored voltage. I_SENSE can be calculated using Equation 10.

Figure 8-10 Adjustable Voltage Threshold with External Resistor Dividers