Datasheet

LT3991/LT3991-3.3/LT3991-5
12
3991fa
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 μH.
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 2.8A. 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 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
–
Δ
I
L
2
The LT3991 limits its peak switch current in order to protect
itself and the system from overload faults. The LT3991’s
switch current limit (I
LIM
) is at least 2.33A at low duty
cycles and decreases linearly to 1.8A 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
where f
SW
is the switching frequency of the LT3991, DC is
the duty cycle and L is the value of the inductor. Therefore,
the maximum output current that the LT3991 will deliver
depends on the switch current limit, the inductor value,
and the input and output voltages. The inductor value may
have to be increased if the inductor ripple current does
not allow sufficient maximum output current (I
OUT(MAX)
)
given the switching frequency, and maximum input voltage
used in the desired application.
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A larger
value inductor provides a higher maximum load current and
reduces the output voltage ripple. If your load is lower than
the maximum load current, than you can relax the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one with
a lower DCR resulting in higher efficiency. Be aware that if
the inductance differs from the simple rule above, then the
maximum load current will depend on the input voltage. In
addition, low inductance may result in discontinuous mode
operation, which further reduces maximum load current.
For details of maximum output current and discontinuous
operation, see Linear Technology’s Application Note 44.
Finally, for duty cycles greater than 50% (V
OUT
/V
IN
>0.5),
a minimum inductance is required to avoid sub-harmonic
oscillations. See Application Note 19.
One approach to choosing the inductor is to start with
the simple rule given above, look at the available induc
-
tors, and choose one to meet cost or space goals. Then
use the equations above to check that the L
T3991 will be
able to deliver the required output current. Note again
that these equations assume that the inductor current is
continuous. Discontinuous operation occurs when I
OUT
is less than ΔI
L
/2.
applicaTions inForMaTion