LM2594 datasheet(25/28 Pages) NSC

25 / 28 page

Application Information (Continued)

put reaches its nominal output voltage. The actual time de-

pends on the output voltage and the size of the output

capacitor. Input power sources that are current limited or

sources that can not deliver these currents without getting

loaded down, may not work correctly. Because of the rela-

tively high startup currents required by the inverting topology,

the delayed startup feature (C1, R

1 and R2) shown in Figure

25 is recommended. By delaying the regulator startup, the

input capacitor is allowed to charge up to a higher voltage

before the switcher begins operating. A portion of the high in-

put current needed for startup is now supplied by the input

capacitor (C

IN). For severe start up conditions, the input ca-

pacitor can be made much larger than normal.

INVERTING REGULATOR SHUTDOWN METHODS

To use the ON /OFF pin in a standard buck configuration is

simple, pull it below 1.3V (@25˚C, referenced to ground) to

turn regulator ON, pull it above 1.3V to shut the regulator

OFF. With the inverting configuration, some level shifting is

required, because the ground pin of the regulator is no

longer at ground, but is now setting at the negative output

voltage level. Two different shutdown methods for inverting

regulators are shown in

Figure 27 and Figure 28.

DS012439-42

FIGURE 27. Inverting Regulator Ground Referenced Shutdown

DS012439-43

FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device

LM2594 Datasheet(PDF) 25 Page - National Semiconductor (TI)