LT1912 Linear Technology Corporation, LT1912 Datasheet - Page 10

• Manufacturer Part Number: LT1912
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology Corporation

www.DataSheet4U.com
www.DataSheet4U.com
APPLICATIONS INFORMATION
LT1912
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:
where V
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
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
the ripple current is:
where I
guarantee suffi cient output current, peak inductor current
must be lower than the LT1912’s switch current limit (I
The peak inductor current is:
where I
the maximum output load current, and ΔI
ripple current. The LT1912’s switch current limit (I
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:
Be sure to pick an inductor ripple current that provides
suffi cient maximum output current (I
10
ΔI
I
I
V
L(PEAK)
OUT(MAX)
IN MAX
L
(
= 0.4(I
OUT(MAX)
L(PEAK)
IN(MIN)
) =
= I
= I
OUT(MAX)
OUT(MAX)
f
is the minimum input voltage, and t
SW
is the peak inductor current, I
V
LIM
OUT
is the maximum output load current. To
t
ON MIN
– ΔI
+ V
L
(
)
increases with higher V
+ ΔI
L
D
/2
)
L
– V
/2
D
+ V
SW
OUT(MAX)
L
is the inductor
OUT(MAX)
).
IN
OFF(MIN)
or V
LIM
LIM
OUT
) is
is
).
The largest inductor ripple current occurs at the highest
V
specifi ed maximum, the inductor value should be chosen
according to the following equation:
where V
V
voltage, f
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
effi ciency 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
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
VENDOR
Murata
TDK
Toko
Sumida
IN
IN(MAX)
L =
. To guarantee that the ripple current stays below the
D
V
is the maximum input voltage, V
SW
OUT
f
is the voltage drop of the catch diode (~0.4V),
SW
URL
www.murata.com
www.componenttdk.com
www.toko.com
www.sumida.com
is the switching frequency (set by RT), and
+ V
I
L
D
1–
V
V
OUT
IN(MAX)
+ V
PART SERIES
LQH55D
SLF7045
SLF10145
D62CB
D63CB
D75C
D75F
CR54
CDRH74
CDRH6D38
CR75
D
OUT
is the output
TYPE
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Shielded
Shielded
Open
1912f