5
LT1615/LT1615-1
sn16151 16151fas
Choosing an Inductor
Several recommended inductors that work well with the
LT1615 and LT1615-1 are listed in Table 1, although there
are many other manufacturers and devices that can be
used. Consult each manufacturer for more detailed infor-
mation and for their entire selection of related parts. Many
different sizes and shapes are available. Use the equations
and recommendations in the next few sections to find the
correct inductance value for your design.
Table 1. Recommended Inductors
PART VALUE (
µ
H) MAX DCR (
Ω
) VENDOR
LQH3C4R7 4.7 0.26 Murata
LQH3C100 10 0.30 (814) 237-1431
LQH3C220 22 0.92 www.murata.com
CD43-4R7 4.7 0.11 Sumida
CD43-100 10 0.18 (847) 956-0666
CDRH4D18-4R7 4.7 0.16 www.sumida.com
CDRH4D18-100 10 0.20
DO1608-472 4.7 0.09 Coilcraft
DO1608-103 10 0.16 (847) 639-6400
DO1608-223 22 0.37 www.coilcraft.com
Inductor Selection—Boost Regulator
The formula below calculates the appropriate inductor
value to be used for a boost regulator using the LT1615 or
LT1615-1 (or at least provides a good starting point). This
value provides a good tradeoff in inductor size and system
performance. Pick a standard inductor close to this value.
A larger value can be used to slightly increase the available
output current, but limit it to around twice the value
calculated below, as too large of an inductance will in-
crease the output voltage ripple without providing much
additional output current. A smaller value can be used
(especially for systems with output voltages greater than
12V) to give a smaller physical size. Inductance can be
calculated as:
L
VV V
I
t
OUT
IN MIN
D
LIM
OFF
=
−+
()
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA or
100mA, and t
OFF
= 400ns; for designs with varying V
IN
such as battery powered applications, use the minimum
APPLICATIO S I FOR ATIO
WUU
U
V
IN
value in the above equation. For most systems with
output voltages below 7V, a 4.7µH inductor is the best
choice, even though the equation above might specify a
smaller value. This is due to the inductor current over-
shoot that occurs when very small inductor values are
used (see Current Limit Overshoot section).
For higher output voltages, the formula above will give
large inductance values. For a 2V to 20V converter (typical
LCD Bias application), a 21µH inductor is called for with
the above equation, but a 10µH inductor could be used
without excessive reduction in maximum output current.
Inductor Selection—SEPIC Regulator
The formula below calculates the approximate inductor
value to be used for a SEPIC regulator using the LT1615.
As for the boost inductor selection, a larger or smaller
value can be used.
L
VV
I
t
OUT D
LIM
OFF
=
+
2
Current Limit Overshoot
For the constant off-time control scheme of the LT1615,
the power switch is turned off only after the 350mA (or
100mA) current limit is reached. There is a 100ns delay
between the time when the current limit is reached and
when the switch actually turns off. During this delay, the
inductor current exceeds the current limit by a small
amount. The peak inductor current can be calculated by:
II
VV
L
ns
PEAK LIM
IN MAX SAT
=+
−
()
100
Where V
SAT
= 0.25V (switch saturation voltage). The
current overshoot will be most evident for systems with
high input voltages and for systems where smaller induc-
tor values are used. This overshoot can be beneficial as it
helps increase the amount of available output current for
smaller inductor values. This will be the peak current seen
by the inductor (and the diode) during normal operation.
For designs using small inductance values (especially at
