Datasheet

LM2594, LM2594HV

www.ti.com

SNVS118C –DECEMBER 1999–REVISED APRIL 2013

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. The computer aided design software Switchers Made Simple (version 4.1) will provide all component

values for continuous and discontinuous modes of operation.

Figure 29. Post Ripple Filter Waveform

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply operating in the continuous mode will contain a sawtooth ripple

voltage at the switcher frequency, and may also contain short voltage spikes at the peaks of the sawtooth

waveform.

The output ripple voltage is a function of the inductor sawtooth ripple current and the ESR of the output

capacitor. A typical output ripple voltage can range from approximately 0.5% to 3% of the output voltage. To

obtain low ripple voltage, the ESR of the output capacitor must be low, however, caution must be exercised when

using extremely low ESR capacitors because they can affect the loop stability, resulting in oscillation problems. If

very low output ripple voltage is needed (less than 15 mV), a post ripple filter is recommended. (See Figure 20.)

The inductance required is typically between 1 μH and 5 μH, with low DC resistance, to maintain good load

regulation. A low ESR output filter capacitor is also required to assure good dynamic load response and ripple

reduction. The ESR of this capacitor may be as low as desired, because it is out of the regulator feedback loop.

The photo shown in Figure 29 shows a typical output ripple voltage, with and without a post ripple filter.

When observing output ripple with a scope, it is essential that a short, low inductance scope probe ground

connection be used. Most scope probe manufacturers provide a special probe terminator which is soldered onto

the regulator board, preferable at the output capacitor. This provides a very short scope ground thus eliminating

the problems associated with the 3 inch ground lead normally provided with the probe, and provides a much

cleaner and more accurate picture of the ripple voltage waveform.

The voltage spikes are caused by the fast switching action of the output switch and the diode, and the parasitic

inductance of the output filter capacitor, and its associated wiring. To minimize these voltage spikes, the output

capacitor should be designed for switching regulator applications, and the lead lengths must be kept very short.

Wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all contribute

to the amplitude of these spikes.

When a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a

triangular to a sawtooth type of waveform (depending on the input voltage). For a given input and output voltage,

the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current increases or

decreases, the entire sawtooth current waveform also rises and falls. The average value (or the center) of this

current waveform is equal to the DC load current.

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