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Application Information (Continued)
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 H,
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 25 will 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 LM2596 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 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
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.
01258339
This circuit has hysteresis
Regulator starts switching at VIN = 13V
Regulator stops switching at VIN =8V
FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator
01258340
CIN
— 68 F/25V Tant. Sprague 595D
470 F/50V Elec. Panasonic HFQ
COUT
— 47 F/20V Tant. Sprague 595D
220 F/25V Elec. Panasonic HFQ
FIGURE 25. Inverting 5V Regulator with Delayed Startup
01258341
FIGURE 26. Inverting Regulator Typical Load Current
LM2596
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26
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