Datasheet

LT1976/LT1976B
16
1976bfg
APPLICATIO S I FOR ATIO
WUUU
1. Choose a value in microhenries from the graph of
maximum load current. Choosing a small inductor with
lighter loads may result in discontinuous mode of
operation, but the LT1976 is designed to work well in
either mode.
Table 4. Inductor Selection Criteria
VENDOR/
PART NUMBER VALUE (μH) I
RMS
(A) DCR (Ω) HEIGHT (mm)
Coiltronics
UP2B-150 15 2.4 0.041 6
UP2B-330 33 1.7 0.062 6
UP2B-470 47 1.4 0.139 6
UP2B-680 68 1.2 0.179 6
UP2B-101 100 0.95 0.271 6
UP3B-150 15 3.9 0.032 6.8
UP3B-330 33 2.4 0.069 6.8
UP3B-470 47 1.9 0.101 6.8
UP3B-680 68 1.6 0.156 6.8
UP3B-101 100 1.4 0.205 6.8
Sumida
CDRH8D28-150M 15 2.2 0.053 3
CDRH124-150M 15 3.2 0.05 4.5
CDRH127-150M 15 4.5 0.02 8
CDRH8D28-330M 33 1.4 0.122 3
CDRH124-330M 33 2.7 0.97 4.5
CDRH127-330M 33 3.0 0.048 8
CDRH8D28-470M 47 1.25 0.150 3
CDRH125-470M 47 1.8 0.058 6
CDRH127-470M 47 2.5 0.076 8
CDRH124-680M 68 1.5 0.228 4.5
CDRH127-680M 68 2.1 0.1 8
CDRH124-101M 100 1.2 0.30 4.5
CDRH127-101M 100 1.7 0.17 8
Coilcraft
DT3308P-153 15 2.0 0.1 3
DT3308P-333 33 1.4 0.3 3
DT3308P-473 47 1 0.47 3
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 2A overload condition.
For applications with a duty cycle above 50%, the
inductor value should be chosen to obtain an inductor
ripple current of less than 40% of the peak switch
current.
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, especially
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 continuous mode of opera-
tion, but it errs only slightly on the high side for discon-
tinuous mode, so it can be used for all conditions.
II
VVV
fLV
PEAK OUT
OUT IN OUT
IN
=+
()
()()( )
–
2
V
IN
= maximum input voltage
f = switching frequency, 200kHz
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. 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 radiation will be a problem.
4. After making an initial choice, consider the secondary
things like output voltage ripple, second sourcing, etc.
Use the experts in the Linear Technology’s applications
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.