1. Peak Inductor or peak switch current
Application Information (Continued)
2. Minimum load current before the circuit becomes dis-
continuous
3. Output Ripple Voltage = (∆IIND)x(ESR of COUT
)
= 0.62Ax0.1Ω=62 mV p-p
4.
01258333
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
nomographs shown in Figure 4 through 8 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).
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.
These magnetic lines of flux will induce a voltage into any
wire or PC board copper trace that comes within the induc-
tor’s magnetic field. The strength of the magnetic field, the
orientation and location of the PC copper trace to the mag-
netic field, and the distance between the copper trace and
the inductor, determine the amount of voltage generated in
the copper trace. Another way of looking at this inductive
coupling is to consider the PC board copper trace as one
turn of a transformer (secondary) with the inductor winding
as the primary. Many millivolts can be generated in a copper
trace located near an open core inductor which can cause
stability problems or high output ripple voltage problems.
These curves are only correct for continuous mode opera-
tion, and only if the inductor selection guides are used to
select the inductor value
If unstable operation is seen, and an open core inductor is
used, it’s possible that the location of the inductor with
respect to other PC traces may be the problem. To deter-
mine if this is the problem, temporarily raise the inductor
away from the board by several inches and then check
circuit operation. If the circuit now operates correctly, then
the magnetic flux from the open core inductor is causing the
problem. Substituting a closed core inductor such as a tor-
roid or E-core will correct the problem, or re-arranging the
PC layout may be necessary. Magnetic flux cutting the IC
device ground trace, feedback trace, or the positive or nega-
tive traces of the output capacitor should be minimized.
Consider the following example:
VOUT = 5V, maximum load current of 2.5A
VIN = 12V, nominal, varying between 10V and 16V.
The selection guide in Figure 5 shows that the vertical line
for a 2.5A load current, and the horizontal line for the 12V
input voltage intersect approximately midway between the
upper and lower borders of the 33 µH inductance region.
A 33 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 2.5A line approxi-
mately midway into the inductance region, and read the
peak-to-peak inductor ripple current (∆IIND) on the left hand
axis (approximately 620 mA p-p).
Sometimes, locating a trace directly beneath a bobbin in-
ductor will provide good results, provided it is exactly in the
center of the inductor (because the induced voltages cancel
themselves out), but if it is off center one direction or the
other, then problems could arise. If flux problems are
present, even the direction of the inductor winding can make
a difference in some circuits.
As the input voltage increases to 16V, it approaches the
upper 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 2.5A, the
This discussion on open core inductors is not to frighten the
user, but to alert the user on what kind of problems to watch
out for when using them. Open core bobbin or “stick” induc-
tors are an inexpensive, simple way of making a compact
efficient inductor, and they are used by the millions in many
different applications.
peak-to-peak inductor ripple current (∆IIND) is 620 mA with
12V in, and can range from 740 mA at the upper border (16V
in) to 500 mA at the lower border (10V in).
Once the ∆IIND value is known, the following formulas can be
used to calculate additional information about the switching
regulator circuit.
23
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