Datasheet

LT1913
11
1913f
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 25V are acceptable regardless of the
switching frequency. In this mode, the LT1913 may enter
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
LT1913’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 MIN
()
=
V
OUT
+ V
D
1– f
SW
t
OFF 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 suffi cient output current, peak inductor current
must be lower than the LT1913’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 LT1913’s switch current limit (I
LIM
) is
5.5A at low duty cycles and decreases linearly to 4.5A at
DC = 0.8. The maximum output current is a function of
the inductor ripple current:
I
OUT(MAX)
= I
LIM
– ΔI
L
/2
Be sure to pick an inductor ripple current that provides
suffi cient 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
specifi ed maximum, the inductor value should be chosen
according to the following equation:
L =
V
OUT
+ V
D
f
SW
I
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 RT), 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. To keep the effi ciency high, the series
resistance (DCR) should be less than 0.05
, 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 SLF10145 Shielded
Toko www.toko.com D75C
D75F
Shielded
Open
Sumida www.sumida.com CDRH74
CR75
CDRH8D43
Shielded
Open
Shielded
NEC www.nec.com MPLC073
MPBI0755
Shielded
Shielded
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
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