LM2597 datasheet(25/34 Pages) NSC

25 / 34 page

Application Information (Continued)

The inductor manufacturers data sheets include current and

energy limits to avoid inductor saturation.

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

(200 mA 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 sinu-

soidal ringing present. (See photo titled; Discontinuous

Mode Switching Waveforms) This ringing is normal for dis-

continuous operation, and is not caused 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 prob-

lem, 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 differ-

ent amounts of this characteristic ringing. Ferrite core induc-

tors 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 continu-

ous and discontinuous modes of operation.

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 saw-

tooth 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, how-

ever, 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 12.) 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 ca-

pacitor is also required to assure good dynamic load re-

sponse 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 20 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, the diode, and the parasitic inductance of

the output filter capacitor, and its associated wiring. To mini-

mize 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 triangu-

lar 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 de-

creases, 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 discon-

tinuous mode of operation. Most switcher designs (irregard-

DS012440-39

FIGURE 20. Post Ripple Filter Waveform

DS012440-40

FIGURE 21. Peak-to-Peak Inductor

Ripple Current vs Load Current

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