Datasheet

15
LT3437
3437fc
CHOOSING THE INDUCTOR
For most applications the output inductor will fall in the
range of 68µH to 220µH. Lower values are chosen to
reduce physical size of the inductor. Higher values allow
more output current because they reduce peak current
seen by the LT3437 switch, which has a 0.5A limit. Higher
values also reduce output ripple voltage and reduce core
loss.
When choosing an inductor you might have to consider
maximum load current, core and copper losses, allow-
able 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 han-
dling these somewhat complicated and conflicting
requirements.
1. Choose a value in microhenries such that the maximum
load current plus half of the inductor ripple current is
less than the minimum peak switch current (I
PK
).
Choosing a small inductor with lighter loads may result
in discontinuous mode of operation, but the LT3437 is
designed to work well in either mode.
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 0.25A, for instance, a 0.25A
inductor may not survive a continuous minimum peak
switch current 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 do not 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 has 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 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.
Table 3. Inductor Selection Criteria
VENDOR/ VALUE I
DC(MAX)
DCR HEIGHT
PART NO. (
µ H) (mA) (Ohms) (mm)
Coiltronics
UP1B-101 100 530 1.11 5.0
UP1B-151 150 460 1.61 5.0
UP2B-221 220 380 1.96 5.0
Coilcraft
D01605T-473MX 47 450 1.1 1.8
D01605T-104MX 100 300 2.3 1.8
D03308P-154 150 600 0.94 3.0
D03308P-224 220 500 1.6 3.0
Sumida
CDRH4D28-470 47 480 0.387 3.0
CDRH4D28-101 100 290 1.02 3.0
CDRH5D28-101 100 420 0.520 3.0
APPLICATIO S I FOR ATIO
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