LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 14

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3690
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
where V
ƒ
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
Murata
TDK
Toko
Coilcraft
Sumida
Vishay
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.
14
SW
L = V
is in MHz, and L is in μH. The inductor’s RMS current
(
LS
OUT
is the voltage drop of the low side switch (0.12V),
URL
www.murata.com
www.tdk.com
www.toko.com
www.coilcraft.com
www.sumida.com
www.vishay.com
+ V
LS
)
•
0.67MHz
ƒ
SW
PART SERIES
LQH6P
CLF10040T
SLF10165T
DEM8045C
FDVE1040
MSS1048
CDRH8D43
CDRH105R
IHLP-2525EZ
Finally, for duty cycles greater than 50% (V
a minimum inductance is required to avoid sub-harmonic
oscillations:
where V
at maximum load), ƒ
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:
where ƒ
is the value of the inductor. The peak inductor and switch
current is:
To maintain output regulation, this peak current must be
less than the LT3690’s switch current limit I
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.
I
L
ΔI
SW(PK)
MIN
L
=
SW
LS
= V
(
is the voltage drop of the low side switch (0.12V
(
1− DC
is the switching frequency of the LT3690 and L
= I
OUT
L(PK)
(
L • ƒ
)
+ V
(
V
= I
LS
OUT
SW
SW
OUT
)
•
is in MHz, and L
)
+ V
0.42MHz
+
LS
ΔI
ƒ
2
SW
)
L
MIN
OUT
LIM
is in μH.
/V
. See the
IN
> 0.5),
3690f