The 100-μA, cross-current requirement at nominal field-supply voltage (24 V) determines that the total impedance of the resistor divider comprised of R5 and R6 is 240 kΩ. The impedance of the voltage divider is dominated by R5, and therefore R5 is chosen as 237 kΩ.

At a field-supply voltage of 20.4 V, the voltage across R6 must equal the fixed-comparator threshold of 300 mV. This value determines the voltage divider ratio and the ideal value of R6 is calculated as R6 = R5 × 300 mV / (V_TRIP – 300 mV), where V_TRIP equals 20.4 V. The calculated value for R6 is 3.54 kΩ and the closest lower value from the E192 series is 3.52 kΩ.

With R6 and R5 known, the voltage can be calculated that is present at the input of the comparator when the field supply reaches 28.8 V, which is the upper limit of the valid operating range. This voltage is V₂ = 28.8 V × (R6 / (R5 + R6) = 421.5 mV, and determines the value of R1. R1 is the resistor connected to the REF pin of the AMC23C14. R1 is calculated as (V₂ – V_HYS) / I_REF = (421.5 mV – 4 mV) / 100 μA = 4.17 kΩ. The value 4.17 kΩ matches a value in the E192 series. The comparator hysteresis voltage (V_HYS) is subtracted from V₂ because the comparator trips at V_REF + V_HYS, see Figure 7-1.

With R5 = 237 kΩ, R6 = 3.52 kΩ, and R1 = 4.17 kΩ, the resulting rising and falling thresholds are 20.8 V and 20.5 V for undervoltage detection and 28.8 V and 28.5 V for overvoltage detection, see Figure 8-3.

Table 8-4 summarizes the key parameters of the design.