Datasheet

LT3971A/LT3971A-5
13
3971af
maximum input voltage (V
IN(OP-MAX)
) that guarantees
optimum output voltage ripple for that application can be
found by applying the following equation:
V
IN(OP-MAX)
=
V
OUT
+ V
D
f
SW
•t
ON(MIN)
–V
D
+ V
SW
where t
ON(MIN)
is the minimum switch on-time. Note that
a higher switching frequency will decrease the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve normal operation
at higher input voltages.
The circuit will tolerate inputs above the maximum op-
erating input voltage and up to the Absolute Maximum
Ratings of the V
IN
and BOOST pins, regardless of chosen
switching frequency. However, during such transients
where V
IN
is higher than V
IN(OP-MAX)
, the LT3971A will
enter pulse-skipping operation where some switching
pulses are skipped to maintain output regulation. The
output voltage ripple and inductor current ripple will be
higher than in typical operation. Do not overload when
V
IN
is greater than V
IN(OP-MAX)
.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L =
V
OUT
+ V
D
f
SW
where f
SW
is the switching frequency in MHz, V
OUT
is the
output voltage, V
D
is the catch diode drop (~0.5V) and L
is the inductor value in H.
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 0.1, and the core material should be intended for
high frequency applications. Table 2 lists several vendors
and suitable types.
The inductor value must be sufficient to supply the desired
maximum output current (I
OUT(MAX)
), which is a function
of the switch current limit (I
LIM
) and the ripple current.
I
OUT(MAX)
=I
LIM
–
ΔI
L
2
The LT3971A limits its peak switch current in order to
protect itself and the system from overload faults. The
LT3971A’s switch current limit (I
LIM
) is typically 2.5A at low
duty cycles and decreases linearly to 1.75A at DC = 0.8.
Table 2. 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
D73C
D75F
Shielded
Shielded
Shielded
Open
Coilcraft www.coilcraft.com MSS7341
MSS1038
Shielded
Shielded
Sumida www.sumida.com CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor:
ΔI
L
=
(1−DC) •(V
OUT
+ V
D
)
L•f
SW
APPLICATIONS INFORMATION