Datasheet

LT3690
14
3690fa
applicaTions inForMaTion
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = V
OUT
+ V
LS
( )
•
0.67MHz
ƒ
SW
where V
LS
is the voltage drop of the low side switch
(0.12V), ƒ
SW
is in MHz, and L is in μH. The inductor’s
RMS current rating must be greater than the maximum
load current and its saturation current should be at least
30% higher. For highest efficiency, the series resistance
(DCR) should be less than 0.03Ω. Table 2 lists several
vendors and types that are suitable.
Table 2. Inductor Vendors
VENDOR URL PART SERIES
Murata www.murata.com LQH6P
TDK www.tdk.com CLF10040T
SLF10165T
Toko www.toko.com DEM8045C
FDVE1040
Coilcraft www.coilcraft.com MSS1048
Sumida www.sumida.com CDRH8D43
CDRH105R
Vishay www.vishay.com IHLP-2525EZ
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, then you can relax the
value of the inductor and operate with higher ripple cur-
rent. 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 input
voltage. In addition, low inductance may result in discon-
tinuous mode operation, which further reduces maximum
load current. For details of maximum output current and
discontinuous mode operation, see 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:
L
MIN
= V
OUT
+ V
LS
( )
•
0.42MHz
ƒ
SW
where V
LS
is the voltage drop of the low side switch (0.12V
at maximum load), ƒ
SW
is in MHz, and L
MIN
is in μH.
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3690 limits its switch cur-
rent in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3690 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
When the switch is off, the potential across the inductor
is the output voltage plus the low side switch drop. This
gives the peak-to-peak ripple current in the inductor:
∆I
L
=
1−DC
( )
V
OUT
+ V
LS
( )
L • ƒ
SW
( )
where ƒ
SW
is the switching frequency of the LT3690 and L
is the value of the inductor. The peak inductor and switch
current is:
I
SW(PK)
=I
L(PK)
=I
OUT
+
∆I
L
2
To maintain output regulation, this peak current must be
less than the LT3690’s switch current limit I
LIM
. See the
Typical Performance graphs for the change in current
limit vs duty cycle.
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.