5
LT1944-1
Choosing an Inductor
Several recommended inductors that work well with the
LT1944-1 are listed in Table 1, although there are many
other manufacturers and devices that can be used. Con-
sult each manufacturer for more detailed information 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 (714) 852-2001
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 LT1944-1
(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
= 100mA
(or 175mA) and t
OFF
= 400ns (or 1.5µs); for designs with
varying V
IN
such as battery powered applications, use the
minimum V
IN
value in the above equation. For most
APPLICATIO S I FOR ATIO
WUU
U
systems with output voltages below 7V, a 10µH inductor
is the best choice, even though the equation above might
specify a smaller value. This is due to the inductor current
overshoot 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 74µH inductor is called for with
the above equation, but a 22µH 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 LT1944-1.
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 LT1944-1,
the power switch is turned off only after the 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
input voltages greater than 5V), the current limit over-
shoot can be quite high. Although it is internally current
