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LM2595

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SNVS122C – MAY 1999 – REVISED MAY 2016

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Feature Description (continued)

Figure 19. Inverting Regulator Typical Load Current

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 68-μH, 1.5-A inductor is the best choice. Capacitor

selection is narrowed down to just a few values. Using the values shown in Figure 18 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 LM2595 current limit (approximately 1.5 A) are required 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 start-up currents

recommended. By delaying the regulator start-up, the input capacitor is allowed to charge up to a higher voltage

before the switcher begins operating. A portion of the high input current required for start-up is now supplied by

normal.

8.3.4 Inverting Regulator Shutdown Methods

Using the ON/OFF pin in a standard buck configuration is simple. To turn the regulator ON, pull the ON/OFF pin

below 1.3 V (at 25°C referenced to ground). To shut the regulator OFF, pull the ON/OFF pin above 1.3 V. 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 20 and Figure 21.

Figure 20. Inverting Regulator Ground Referenced Shutdown