8
LT1373
component height, output voltage ripple, EMI, fault cur-
rent 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. Powered 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 = 250kHz switching frequency
+
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 require-
ments 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 LT1373 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 particularly 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.
Table 1. Surface Mount Solid Tantalum Capacitor
ESR and Ripple Current
E CASE SIZE ESR (MAX Ω) RIPPLE CURRENT (A)
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
AVX TAJ 0.7 to 0.9 0.4
D CASE SIZE
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
AVX TAJ 0.9 to 2.0 0.36 to 0.24
C CASE SIZE
AVX TPS 0.2 (Typ) 0.5 (Typ)
AVX TAJ 1.8 to 3.0 0.22 to 0.17
B CASE SIZE
AVX TAJ 2.5 to 10 0.16 to 0.08
APPLICATIO S I FOR ATIO
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