ESR capacitors because they can affect the loop stability,
resulting in oscillation problems. If very low output ripple
voltage is needed (less than 20 mV), a post ripple filter is
recommended. (See Figure 1.) 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.
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
(200 mA and below), the maximum switch current will still be
less than the switch current limit.
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.
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 Typical Perfomance Character-
istics photo titled Discontinuous Mode Switching Wave-
forms) This ringing is normal for discontinuous operation,
and is not caused by feedback loop instabilities. In discon-
tinuous 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 ring-
ing. Normally this ringing is not a problem, unless the ampli-
tude becomes great enough to exceed the input voltage, and
even then, there is very little energy present to cause dam-
age.
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.
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 4.2) will provide all component values for continu-
ous and discontinuous modes of operation.
DS012593-37
FIGURE 21. Peak-to-Peak Inductor
Ripple Current vs Load Current
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
DS012593-36
voltage). For
a given input and output voltage, the
FIGURE 20. Post Ripple Filter Waveform
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.
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.
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-
less how large the inductor value is) will be forced to run
discontinuous if the output is lightly loaded. This is a per-
fectly acceptable mode of operation.
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
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