LT1945
APPLICATIO S I FOR ATIO
Choosing an Inductor
Several recommended inductors that work well with the
LT1945 are listed in Table 1, although there are many other
manufacturers and devices that can be used. Consult 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 recommenda-
tions 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
)
LQH3C4R7
LQH3C100
LQH3C220
CD43-4R7
CD43-100
CDRH4D18-4R7
CDRH4D18-100
DO1608-472
DO1608-103
DO1608-223
4.7
10
22
4.7
10
4.7
10
4.7
10
22
0.26
0.30
0.92
0.11
0.18
0.16
0.20
0.09
0.16
0.37
VENDOR
Murata
(714) 852-2001
www.murata.com
Sumida
(847) 956-0666
www.sumida.com
Coilcraft
(847) 639-6400
www.coilcraft.com
Inductor Selection—Boost Regulator
The formula below calculates the appropriate inductor
value to be used for a boost regulator using the LT1945 (or
at least provides a good starting point). This value pro-
vides a good tradeoff in inductor size and system perfor-
mance. 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
=
V
OUT
−
V
IN
(
MIN
)
+
V
D
I
LIM
t
OFF
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA
and t
OFF
= 400ns; for designs with varying V
IN
such as
battery powered applications, use the minimum V
IN
value
U
in the above equation. For most regulators 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 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 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 LT1945.
As for the boost inductor selection, a larger or smaller
value can be used.
W
U
U
V
+
V
D
L
=
2
OUT
I
LIM
t
OFF
Inductor Selection—Inverting Regulator
The formula below calculates the appropriate inductor
value to be used for an inverting regulator using the
LT1945 (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
(both inductors should be the same 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 increase 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:
V
OUT
+
V
D
L
=
2
I
LIM
t
OFF
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA
and t
OFF
= 400ns.
1945f
5
LINER [ LINEAR TECHNOLOGY ]
