The supply overvoltage threshold is set using the resistors, R1 and R2, whose values can be calculated as:
Equation 8.
Where V
OV(R) is the OVLO rising threshold. Because R1, R2 leak the current from input supply V
IN, these resistors must be selected based on the acceptable leakage current from input power supply V
IN. The current drawn by R1, R2 from the power supply is IR12 = V
IN / (R1 + R2). However, leakage currents due to external active components connected to the resistor string can add error to these calculations. So, the resistor string current, IR12, must be chosen to be 20 times greater than the leakage current expected on the OVLO pin.
From the device electrical specifications, OVLO leakage current is 0.1 μA (maximum), VOV(R) = 1.2 V. From design requirements, VIN(OV) = 22 V. To solve the equation, first choose the value of R1 = 470 kΩ and use the above equation to solve for R2 = 27.11 kΩ.
Using the closest standard 1% resistor values, we get R1 = 470 kΩ, R2 = 26.7 kΩ.