LM2596 datasheet(21/29 Pages) ARTSCHIP

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LM2596

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Application information (Continued)

DISCONTINUOUS MODE OPERATION

The selection guide chooses inductor values suitable for continuous

mode operation, but for low current applications and/or high input

voltages, a discontinuous mode design may be a better choice. It

would use an inductor that would be physically smaller, and would

need only one half to one third the inductance value needed for a

continuous mode design. The peak switch and inductor currents will

be higher in a discontinuous design, but at these low load currents

(1A and below), the maximum switch current will still be less than

the switch current limit.

Discontinuous operation can have voltage waveforms that are

considerable different than a continuous design. The output pin

(switch) waveform can have some damped sinusoidal ringing

present. (See Typical Performance Characteristics photo titled

Discontinuous Mode Switching Waveforms) This ringing is normal

for discontinuous operation, and is not cause by feedback loop

instabilities. In discontinuous operation, there is a period of time

where neither the switch or 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 be

comes 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. The computer aided design software Switchers

Mode Simple (version 4.3) will provide all component values for

continuous and discontinuous modes of operation.

2 µsec /div

DSO12583-32

FIGURE 17.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 20mV), a post ripple filter is

recommended. (See Figure 1.) The inductance required is typically

between 1µH and 5µH, 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 17 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.

If the load current drops to a low enough level, the bottom of the

sawtooth current waveform will reach zero, and the switcher will

smoothly change from a continuous to a discontinuous mode of

operation. Most switcher designs (irregardless how large the

inductor value is) will be forced to run discontinuous if the output is

lightly loaded. This is a perfectly acceptable mode of operation.

FIGURE 18. Peak-to-Peak Inductor Ripple Current vs Load Current

In a switching regulator design, knowing the value of the

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