Datasheet

11
LT1374
1374fd
When choosing an inductor you might have to consider
maximum load current, core and copper losses, allowable
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. Choose a value in microhenries from the graphs of
maximum load current and core loss. Choosing a small
inductor may result in discontinuous mode operation
at lighter loads, but the LT1374 is designed to work
well in either mode. Keep in mind that lower core loss
means higher cost, at least for closed core geometries
like toroids. The core loss graphs show both absolute
loss and percent loss for a 5W output, so actual percent
losses must be calculated for each situation.
Assume that the average inductor current is equal to
load current and decide whether or not the inductor
must withstand continuous fault conditions. If maxi-
mum load current is 0.5A, for instance, a 0.5A inductor
may not survive a continuous 4.5A overload condition.
Dead shorts will actually be more gentle on the induc-
tor because the LT1374 has foldback current limiting.
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
because they saturate softly, whereas ferrite cores
saturate abruptly. Other core materials fall somewhere
in between. The following formula assumes continu-
ous mode of operation, but it errs only slightly on the
high side for discontinuous mode, so it can be used for
all conditions.
V
IN
= Maximum input voltage
f = Switching frequency, 500kHz
3. Decide if the design can tolerate an “open” core geom-
etry like a rod or barrel, with high magnetic field
radiation, or whether it needs a closed core like a toroid
to prevent EMI problems. One would not want an open
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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 (see representative
surface mount units in Table 2) which meets the
requirements of core shape, peak current (to avoid
Table 2
SERIES CORE
VENDOR/ VALUE DC CORE RESIS- MATER- HEIGHT
PART NO. (
µ H) (Amps) TYPE TANCE( Ω ) IAL (mm)
Coiltronics
CTX2-1 2 4.1 Tor 0.011 KMµ 4.2
CTX5-4 5 4.4 Tor 0.019 KMµ 6.4
CTX8-4 8 3.5 Tor 0.020 KMµ 6.4
CTX2-1P 2 3.4 Tor 0.014 52 4.2
CTX2-3P 2 4.6 Tor 0.012 52 4.8
CTX5-4P 5 3.3 Tor 0.027 52 6.4
Sumida
CDRH125 10 4.0 SC 0.025 Fer 6
CDRH125 12 3.5 SC 0.027 Fer 6
CDRH125 15 3.3 SC 0.030 Fer 6
CDRH125 18 3.0 SC 0.034 Fer 6
Coilcraft
DT3316-222 2.2 5 SC 0.035 Fer 5.1
DT3316-332 3.3 5 SC 0.040 Fer 5.1
DT3316-472 4.7 3 SC 0.045 Fer 5.1
Pulse
PE-53650 4 4.8 Tor 0.017 Fer 9.1
PE-53651 5 5.4 Tor 0.018 Fer 9.1
PE-53652 9 5.5 Tor 0.022 Fer 10
PE-53653 16 5.1 Tor 0.032 Fer 10
Dale
IHSM-4825 2.7 5.1 Open 0.034 Fer 5.6
IHSM-4825 4.7 4.0 Open 0.047 Fer 5.6
IHSM-5832 10 4.3 Open 0.053 Fer 7.1
IHSM-5832 15 3.5 Open 0.078 Fer 7.1
IHSM-7832 22 3.8 Open 0.054 Fer 7.1
Tor = Toroid
SC = Semi-closed geometry
Fer = Ferrite core material
52 = Type 52 powdered iron core material
KMµ = Kool Mµ