INVERTING REGULATOR SHUTDOWN METHODS
Application Information (Continued)
To use the ON /OFF pin in a standard buck configuration is
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, R1 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 (CIN). For severe start up conditions, the input ca-
pacitor can be made much larger than normal.
@
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
25
www.national.com
NSC [ National Semiconductor ]
