9.2.2 Detailed Design Procedure

1. Determine the minimum total resistance of the resistor network necessary to achieve the current consumption specification by using Equation 1. For this example, the current flow through the resistor network was chosen to be 13 µA; a lower current can be selected, however, care should be taken to avoid leakage currents that are artifacts of the manufacturing process. Leakage currents significantly impact the accuracy if they are greater than 1% of the resistor network current.
Equation 5.
where
• V_MON(OV) is the target voltage at which an overvoltage condition is detected as V_MON rises.
• I is the current flowing through the resistor network.
2. After R_TOTAL is determined, R₃ can be calculated using Equation 6. Select the nearest 1% resistor value for R₃. In this case, 30.9 kΩ is the closest value.
Equation 6.
3. Use Equation 7 to calculate R2. Select the nearest 1% resistor value for R₂. In this case, 6.81 kΩ is the closest value.
Equation 7.
4. Use Equation 8 to calculate R1. Select the nearest 1% resistor value for R₁. In this case, 2 MΩ is the closest value.
Equation 8.
5. The worst-case tolerance can be calculated by referring to Equation 13 in application report Optimizing Resistor Dividers at a Comparator Input (SLVA450). An example of the rising threshold error, V_MON(OV), is given in Equation 9:
Equation 9.
where
• % TOL(V_IT+(INB)) is the tolerance of the INB positive threshold.
• % ACC is the total tolerance of the V_MON(OV) voltage.
• % TOL_R is the tolerance of the resistors selected.
6. When the outputs switch to the high-Z state, the rise time of the OUTA or OUTB node depends on the pullup resistance and the capacitance on the node. Choose pullup resistors that satisfy the downstream timing requirements; 100-kΩ resistors are a good choice for low-capacitive loads.