8
LT1372/LT1377
Choosing the Inductor
For most applications the inductor will fall in the range of
2.2µH to 22µH. Lower values are chosen to reduce physi-
cal size of the inductor. Higher values allow more output
current because they reduce peak current seen by the
power switch, which has a 1.5A limit. Higher values also
reduce input ripple voltage and reduce core loss.
When choosing an inductor you might have to consider
maximum load current, core and copper losses, allowable
component height, output voltage ripple, EMI, fault
current in the inductor, saturation, and of course, cost.
The following procedure is suggested as a way of handling
these somewhat complicated and conflicting requirements.
1. Assume that the average inductor current for a boost
converter is equal to load current times V
OUT
/V
IN
and
decide whether or not the inductor must withstand
continuous overload conditions. If average inductor
current at maximum load current is 0.5A, for instance,
a 0.5A inductor may not survive a continuous 1.5A
overload condition. Also be aware that boost convert-
ers are not short circuit protected, and that under
output short conditions, inductor current is limited only
by the available current of the input supply.
2. Calculate peak inductor current at full load current to
ensure that the inductor will not saturate. Peak current
can be significantly higher than output current, espe-
cially with smaller inductors and lighter loads, so don’t
omit this step. Powdered iron cores are forgiving be-
cause they saturate softly, whereas ferrite cores satu-
rate abruptly. Other core materials fall in between
somewhere. The following formula assumes continu-
ous mode operation but it errors only slightly on the
high side for discontinuous mode, so it can be used for
all conditions.
I
PEAK
= I
OUT
×
V
IN
= Minimum Input Voltage
f = 500kHz Switching Frequency (LT1372) or
1MHz Switching Frequency (LT1377)
+
V
OUT
V
IN
V
IN
(V
OUT
–
V
IN
)
2(f)(L)(V
OUT
)
3. Decide if the design can tolerate an “open” core geom-
etry like a rod or barrel, which have high magnetic field
radiation, or whether it needs a closed core like a toroid
to prevent EMI problems. One would not want an open
core next to a magnetic storage media for instance!
This is a tough decision because the rods or barrels are
temptingly cheap and small, and there are no helpful
guidelines to calculate when the magnetic field radia-
tion will be a problem.
4. Start shopping for an inductor which meets the re-
quirements of core shape, peak current (to avoid
saturation), average current (to limit heating) and fault
current. If the inductor gets too hot, wire insulation will
melt and cause turn-to-turn shorts. Keep in mind that
all good things like high efficiency, low profile and high
temperature operation will increase cost, sometimes
dramatically.
5. After making an initial choice, consider the secondary
things like output voltage ripple, second sourcing, etc.
Use the experts in the Linear Technology application
department if you feel uncertain about the final choice.
They have experience with a wide range of inductor
types and can tell you about the latest developments in
low profile, surface mounting, etc.
Output Capacitor
The output capacitor is normally chosen by its effective
series resistance, (ESR), because this is what determines
output ripple voltage. At 500kHz, any polarized capacitor
is essentially resistive. To get low ESR takes
volume
, so
physically smaller capacitors have high ESR. The ESR
range for typical LT1372 and LT1377 applications is
0.05Ω to 0.5Ω. A typical output capacitor is an AVX type
TPS, 22µF at 25V, with a guaranteed ESR less than 0.2Ω.
This is a “D” size surface mount solid tantalum capacitor.
TPS capacitors are specially constructed and tested for
low ESR, so they give the lowest ESR for a given volume.
To further reduce ESR, multiple output capacitors can be
used in parallel. The value in microfarads is not particu-
larly critical, and values from 22µF to greater than 500µF
work well, but you cannot cheat mother nature on ESR.
If you find a tiny 22µF solid tantalum capacitor, it will have
high ESR, and output ripple voltage will be terrible. Table
1 shows some typical solid tantalum surface mount
capacitors.
APPLICATIO S I FOR ATIO
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