8.2.2.15 R1, R3, C21

A voltage divider can be connected to the UVLO pin to set a minimum operating voltage VIN_(UVLO) for the regulator. If this feature is required, the easiest approach to select the divider resistor values is to choose a value for R1 between 10 kΩ and 100 kΩ, while observing the minimum value of R1 necessary to allow the UVLO switch to pull the UVLO pin low. This value is:

R1 ≥ 1000 × V_IN(MAX)

R1 ≥ 75 k

R3 is then calculated from:

Equation 44.

Because VIN_(MIN) for our example is 5 V, set VIN_(UVLO) to 4 V for some margin in component tolerances and input ripple.

R1 = 75 k is chosen because it is a standard value

R3 = 29.332 k is calculated from Equation 44. 29.4 k was used because it is a standard value

Capacitor C21 provides filtering for the divider and the off time of the hiccup duty cycle during current limit. The voltage at the UVLO pin should never exceed 15 V when using an external set-point divider. It may be necessary to clamp the UVLO pin at high input voltages.

Knowing the desired off time during hiccup current limit, the value of C21 is given by:

Equation 45.

Notice that t_OFF varies with V_IN

In this example, C21 was chosen to be 0.1 µF. This will set the t_OFF time to 723 µs with VIN = 12 V.