Configure the circuit as shown in Figure 8-1. Connect V+ to a power supply that is compatible with the input logic level of the device connected to the output, and connect V- to ground. Resistors R₁ and R₂ create the over-voltage alert level at 6.5 V and resistors R₃ and R₄ create the over-voltage alert level at 4 V. When the V_MON rises, the resistor divider voltage crosses V_IT+. This causes the comparator output to transition from a logic low level (normal operation), to a logic high level. When V_MON falls back down and the resistor divider voltage crosses V_IT- and signal that the system is approaching normal operating voltage levels once again. Make sure to set V_MON at a value below the absolute maximum voltage of the system in question.

Equation 2.

where

• R₁/R₃ and R₂/R₄ are the resistor values for the resistor divider connected to INx
• V_MON is the voltage source that is being monitored for an over-voltage condition
• V_IT+ is the rising edge threshold where the comparator output changes state from low to high

Rearranging Equation 2 and solving for R₁ yields Equation 3. Set R₂/R₄ to a fixed value.

Equation 3.

Using the nearest 1% resistors and the equation above, R₁ = 300 kΩ, R₂ =1.33 MΩ, R₃ = 953 kΩ, and R₄ = 407 kΩ. To get the trip point as close as possible to rising threshold, V_IT+, V_MON are adjusted so that V_MON1 = 6.49 V and V_MON2 = 3.99 V. Using equation Equation 4 will determine when the output will fall low (crossing V_IT-). The over-voltage signal will go low when V_MON1 = 6.16 V and V_MON2 = 3.79 V.

Equation 4.

where

• V_MON is the voltage at which the resistor divider crosses the falling threshold, V_IT-

Choose R_TOTAL (equal to R1 + R2 & R3 + R4) so that the current through the divider is approximately 100 times higher than the input current at the INx pins. The resistors can have high values to minimize current consumption as a result of low input bias current without adding significant error to the resistive divider. For details on sizing input resistors, see the Optimizing Resistor Dividers at a Comparator Input application report (SLVA450), available for download from www.ti.com.