Datasheet

LT3641
14
3641fa
APPLICATIONS INFORMATION
where V
OUT1
is high voltage channel output voltage, V
D
is
the forward voltage drop of the catch diode, and f
S
is the
switching frequency. For example, 3.3μH is a reasonable
inductance for a 3.3V output with 2MHz switching
frequency.
Once the inductance is selected, the inductor current ripple
and peak current can be calculated:
ΔI
L1
=
(V
OUT1
+ V
D
)
L1• f
S
•1–
V
OUT1
+ V
D
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
I
L(PEAK)
= I
OUT(MAX)
+
ΔI
L
2
To guarantee suffi cient output current, peak inductor
current must be lower than the switch current limit (I
LIM
).
The largest inductor current ripple occurs at the highest
V
IN
. To guarantee current capacity, use V
IN(MAX)
in the
above formula.
The inductance for the low voltage channel is:
L2 = 1.5
V
OUT2
f
S
For a selected inductance, the inductor current ripple can
be calculated:
ΔI
L2
=
V
OUT2
L2 • f
S
•1–
V
OUT2
V
IN2
⎛
⎝
⎜
⎞
⎠
⎟
For robust operation in fault conditions, the inductor
saturation current should be higher than the upper limit
of the corresponding top switch current limit.
To keep the effi ciency high, the inductor series resistance
(DCR) should be as small as possible (must be < 0.1Ω),
and the core material should be intended for the chosen
operation frequency. High effi ciency converters generally
cannot afford the core loss found in low cost powdered
iron cores; instead use ferrite, molypermalloy or Kool Mμ
cores. Table 2 lists several vendors and suitable inductor
series.
Table 2. Inductor Vendors
VENDOR WEBSITE
Murata www.murata.com
TDK www.tdk.com
TOKO www.toko.com
Sumida www.sumida.com
Cooper/Coiltronics www.cooperindustries.com
Coilcraft www.coilcraft.com
Vishay www.vishay.com
NIC www.niccomp.com
Würth Elektronik www.we-online.com
Of course, such a simple design guide will not always
result in the optimum inductors for the applications. A
larger value inductor provides a slightly higher maximum
load current and will reduce the output voltage ripple. A
larger value inductor also results in higher effi ciency in the
condition of same DCR and same magnetic loss. However,
for a same series of inductors, a larger value inductor has
higher DCR. The trade-off between inductance and DCR
is not always obvious. Use experiments to fi nd optimum
inductors.
Low inductance may result in discontinuous mode oper-
ation, which is okay, but reduces maximum load current.
For details of maximum output current and discontinuous
mode operation, see the Linear Technology Application Note
44. For duty cycles greater than 50%, there is a minimum
inductance required to avoid subharmonic oscillations.
See the Linear Technology Application Note 19.
Input Capacitor
Bypass the V
IN
pin of the LT3641 with a ceramic capacitor
of X7R (–55°C to 125°C) or X5R (–55°C to 85°C) type.
Buck converters draw pulse current from the input supply.
The input capacitor is required to reduce the resulting
voltage ripple. Use a ceramic capacitor with:
C
IN
≥
10μF
f
S
where f
S
in the switching frequency in MHz.