The supply undervoltage and overvoltage
thresholds are set using the resistors R1, R2, and R3, whose values can be calculated using
Equation 10 and Equation 11:
Equation 10.
Equation 11.
Where V
UVLO(R) is the UVLO rising threshold and
V
OV(R) is the OVLO rising threshold. Because R1, R2, and R3 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, and R3 from the
power supply is IR123 = V
IN / (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 (maximum), 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.