Before

Ripple

Filter

5 mV/div

Alter

Ripple

Filter

2 µsec/div

14

LM2595

SNVS122C – MAY 1999 – REVISED MAY 2016

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Copyright © 1999–2016, Texas Instruments Incorporated

Feature Description (continued)

Figure 21. Inverting Regulator Ground Referenced Shutdown Using Opto Device

8.4 Device Functional Modes

8.4.1 Discontinuous Mode Operation

The selection guide chooses inductor values suitable for continuous mode operation, but for low current

applications or high input voltages, a discontinuous mode design may be a better choice. Discontinuous mode

would use an inductor that would be physically smaller, and would need only one half to one third the inductance

value required for a continuous mode design. The peak switch and inductor currents will be higher in a

discontinuous design, but at these low load currents (400 mA and below), the maximum switch current will still be

less than the switch current limit.

Discontinuous operation can have voltage waveforms that are considerably different than a continuous design.

The output pin (switch) waveform can have some damped sinusoidal ringing present (see Typical

Characteristics). This ringing is normal for discontinuous operation, and is not caused by feedback loop

instabilities. In discontinuous operation, there is a period of time where neither the switch nor the diode are

conducting, and the inductor current has dropped to zero. During this time, a small amount of energy can

circulate between the inductor and the switch/diode parasitic capacitance causing this characteristic ringing.

Normally this ringing is not a problem, unless the amplitude becomes great enough to exceed the input voltage,

and even then, there is very little energy present to cause damage.

Different inductor types and/or core materials produce different amounts of this characteristic ringing. Ferrite core

inductors have very little core loss and therefore produce the most ringing. The higher core loss of powdered iron

inductors produce less ringing. If desired, a series RC could be placed in parallel with the inductor to dampen the

ringing.

Figure 22. Post Ripple Filter Waveform