IL2596-xx
Because of differences in the operation of the inverting regulator, the standard design procedure is not used to select the
inductor value. In the majority of designs, a 33 µH, 3.5A inductor is the best choice. Capacitor selection can also be narrowed
down to just a few values. Using the values shown in Figure 7will provide good results in the majority of inverting designs.
This type of inverting regulator can require relatively large amounts of input current when starting up, even with light
loads. Input currents as high as the IL2596 current limit (approx 4.5A) are needed for at least 2 ms or more, until the output
reaches its nominal output voltage. The actual time depends 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 relatively high startup currents required by the inverting topology, the delayed startup feature (C1, R
1
and R
2
) shown in Figure 7 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 input current needed for startup is now supplied by the
input capacitor (C
IN
). For severe start up conditions, the input capacitor 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 9and 10.
Figure 9. Inverting Regulator Ground Referenced Shutdown
Figure 10. Inverting Regulator Ground Referenced Shutdown using Opto Device
Rev. 03 9
IKSEMICON [ IK SEMICON CO., LTD ]
