LM2595 datasheet(24/29 Pages) NSC

24 / 29 page

Application Information (Continued)

Because of differences in the operation of the inverting regu-

lator, the standard design procedure is not used to select the

inductor value. In the majority of designs, a 68 µH, 1.5A in-

ductor is the best choice. Capacitor selection can also be

narrowed down to just a few values. Using the values shown

Figure 25 will provide good results in the majority of invert-

ing 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 LM2595 current limit (ap-

prox 1.5A) are needed for at least 2 ms or more, until the out-

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.

DS012565-39

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

DS012565-40

CIN:

— 220 µF/25V Tant. Sprague 595D

120 µF/50V Elec. Panasonic HFQ

COUT:

— 22 µF/20V Tant. Sprague 595D

120 µF/25V Elec. Panasonic HFQ

FIGURE 25. Inverting −5V Regulator with Delayed Startup

DS012565-41

FIGURE 26. Inverting Regulator Typical Load Current

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