8.1.11 R1, R2, C2

A voltage divider can be connected to the SD pin to set a minimum operating voltage Vin_(min) for the regulator. If this feature is required, the easiest approach to select the divider resistor values is to select a value for R1 (between 10 kΩ and 100 kΩ recommended) then calculate R2 from Equation 12:

Equation 12.

Capacitor C2 provides filtering for the divider. The voltage at the SD pin should never exceed 8 V, when using an external set-point divider it may be necessary to clamp the SD pin at high input voltage conditions. The reference design utilizes the full range of the LM25574 (6 V to 42 V); therefore these components can be omitted. With the SD pin open circuit the LM25574 responds once the Vcc UVLO threshold is satisfied.