nomographs shown in Figure 4 through Figure 7 are used to
select an inductor value, the peak-to-peak inductor ripple
current can immediately be determined. The curve shown in
Figure 18 shows the range of (∆IIND) that can be expected
for different load currents. The curve also shows how the
peak-to-peak inductor ripple current (∆IIND) changes as you
go from the lower border to the upper border (for a given load
current) within an inductance region. The upper border rep-
resents a higher input voltage, while the lower border repre-
sents a lower input voltage (see Inductor Selection Guides).
Application Information (Continued)
reduction. The ESR of this capacitor may be as low as de-
sired, because it is out of the regulator feedback loop. The
photo shown in Figure 17 shows a typical output ripple volt-
age, 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.
These curves are only correct for continuous mode opera-
tion, and only if the inductor selection guides are used to se-
lect the inductor value
Consider the following example:
=
VOUT 5V, maximum load current of 300 mA
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 evalu-
ate these transients, all contribute to the amplitude of these
spikes.
=
VIN 15V, nominal, varying between 11V and 20V.
The selection guide in Figure 5 shows that the vertical line
for a 0.3A load current, and the horizontal line for the 15V in-
put voltage intersect approximately midway between the up-
per and lower borders of the 150 µH inductance region. A
150 µH inductor will allow a peak-to-peak inductor current
(∆IIND) to flow that will be a percentage of the maximum load
current. Referring to Figure 18, follow the 0.3A line approxi-
mately midway into the inductance region, and read the
peak-to-peak inductor ripple current (∆IIND) on the left hand
axis (approximately 150 mA p-p).
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
As the input voltage increases to 20V, it approaches the up-
per border of the inductance region, and the inductor ripple
current increases. Referring to the curve in Figure 18, it can
be seen that for a load current of 0.3A, the peak-to-peak in-
ductor ripple current (∆IIND) is 150 mA with 15V in, and can
range from 175 mA at the upper border (20V in) to 120 mA at
the lower border (11V in).
voltage). For
a given input and output voltage, the
peak-to-peak amplitude of this inductor current waveform re-
mains 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 discon-
tinuous mode of operation. Most switcher designs (irregard-
less how large the inductor value is) will be forced to run dis-
continuous if the output is lightly loaded. This is a perfectly
acceptable mode of operation.
Once the ∆IIND value is known, the following formulas can be
used to calculate additional information about the switching
regulator circuit.
1. Peak Inductor or peak switch current
2. Minimum load current before the circuit becomes dis-
continuous
3. Output Ripple Voltage
=
=
(∆IIND)x(ESR of COUT
)
=
0.150Ax0.240Ω 36 mV p-p
or
4. ESR of COUT
DS012439-33
FIGURE 18. Peak-to-Peak Inductor
Ripple Current vs Load Current
In a switching regulator design, knowing the value of the
peak-to-peak inductor ripple current (∆IIND) can be useful for
determining a number of other circuit parameters. Param-
eters such as, peak inductor or peak switch current, mini-
mum load current before the circuit becomes discontinuous,
output ripple voltage and output capacitor ESR can all be
calculated from the peak-to-peak ∆IIND. When the inductor
OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or
unstable operation is from an open core inductor. Ferrite
bobbin or stick inductors have magnetic lines of flux flowing
through the air from one end of the bobbin to the other end.
21
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