Datasheet

LT1912
10
1912fa
pulse-skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode
the output voltage ripple and inductor current ripple will
be higher than in normal operation.
The minimum input voltage is determined by either the
LT1912’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
V
IN MAX
( )
=
V
OUT
+ V
D
f
SW
t
ON MIN
( )
– V
D
+ V
SW
where V
IN(MIN)
is the minimum input voltage, and t
OFF(MIN)
is the minimum switch off time (150ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔI
L
increases with higher V
IN
or V
OUT
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
the ripple current is:
ΔI
L
= 0.4(I
OUT(MAX)
)
where I
OUT(MAX)
is the maximum output load current. To
guarantee sufficient output current, peak inductor current
must be lower than the LT1912’s switch current limit (I
LIM
).
The peak inductor current is:
I
L(PEAK)
= I
OUT(MAX)
+ ΔI
L
/2
where I
L(PEAK)
is the peak inductor current, I
OUT(MAX)
is
the maximum output load current, and ΔI
L
is the inductor
ripple current. The LT1912’s switch current limit (I
LIM
) is
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a func-
tion of the inductor ripple current:
I
OUT(MAX)
= I
LIM
– ΔI
L
/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (I
OUT(MAX)
).
The largest inductor ripple current occurs at the highest
V
IN
. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
L =
V
OUT
+ V
D
f
SW
ΔI
L
⎛
⎝
⎜
⎞
⎠
⎟
1–
V
OUT
+ V
D
V
IN(MAX)
⎛
⎝
⎜
⎞
⎠
⎟
where V
D
is the voltage drop of the catch diode (~0.4V),
V
IN(MAX)
is the maximum input voltage, V
OUT
is the output
voltage, f
SW
is the switching frequency (set by R
T
), and L
is in the inductor value.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 3.5A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.1Ω, and the core material should be intended for
high frequency applications. Table 1 lists several vendors
and suitable types.
Table 1. Inductor Vendors
VENDOR URL PART SERIES TYPE
Murata www.murata.com LQH55D Open
TDK www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
Toko www.toko.com D62CB
D63CB
D75C
D75F
Shielded
Shielded
Shielded
Open
Sumida www.sumida.com CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 2A, then you can decrease the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one
APPLICATIONS INFORMATION