The supply undervoltage and
overvoltage thresholds are set using the resistors R1, R2 & R3 whose values can
be calculated using Equation 6 and Equation 7:
Equation 6. Equation 7.
is the UVLO rising
threshold and VOV(R)
is the OVLO rising threshold . Because R1, R2 and R3
leak the current from input supply VIN
, these resistors must be selected
based on the acceptable leakage current from input power supply VIN
current drawn by R1, R2 and R3 from the power supply is IR123 = VIN
/ (R1 +
R2 + R3). However, leakage currents due to external active components connected to the
resistor string can add error to these calculations. So, the resistor string current,
IR123 must be chosen to be 20 times greater than the leakage current expected on the
EN/UVLO and OVLO pins.
From the device
electrical specifications, both the EN/UVLO and OVLO leakage currents are 0.1 μA
(max), VOV(R) = 1.2 V and VUVLO(R) = 1.2 V. From design
requirements, VIN(OV) = 13.2 V and VIN(UV) = 10.8 V. To solve
the equation, first choose the value of R1 = 470 kΩ and use the above equations to
solve for R2 = 10.7 kΩ and R3= 48 kΩ.
Using the closest standard 1%
resistor values, we get R1 = 470 kΩ, R2 = 11 kΩ, and R3 = 47 kΩ.