Power Sources and Supplies: World Class Designs

Resistors R5 and R6 set the under-voltage lockout value. The voltage at this pin must be

1.28 V before the IC will operate. A reasonable input voltage is 95 V for this pin to be

active. The value of R5 is very large, so the bias current of the UVLO pin will affect the

value needed for R6. We can consider V

IN

and R5 to be a constant current source, so we

need to subtract the bias current from the current supplied by R5 when calculating R6.

The data sheet also gives equations for calculating these resistors.

6.5 Non-isolated Flyback Example

Our next example shows the advantage of a non-isolated fl yback design for automotive

use. An automoti ...