lt3641 Linear Technology Corporation, lt3641 Datasheet - Page 14

• Manufacturer Part Number: lt3641
• Description: Dual Monolithic Buck Regulator With Power-on Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3641
where V
the forward voltage drop of the catch diode, and f
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:
To guarantee suffi cient output current, peak inductor
current must be lower than the switch current limit (I
The largest inductor current ripple occurs at the highest
V
above formula.
The inductance for the low voltage channel is:
For a selected inductance, the inductor current ripple can
be calculated:
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.
14
IN
I
L2 = 1.5
ΔI
ΔI
. To guarantee current capacity, use V
L(PEAK)
L1
L2
OUT1
=
=
(V
L2 • f
V
= I
OUT2
V
OUT1
is high voltage channel output voltage, V
OUT2
L1• f
OUT(MAX)
f
S
S
+ V
• 1–
S
⎛
⎝ ⎜
D
)
+
• 1–
V
⎛
⎝ ⎜
V
OUT2
ΔI
IN2
2
L
V
⎞
⎠ ⎟
OUT1
V
IN
+ V
D
⎞
⎠ ⎟
IN(MAX)
S
in the
is the
LIM
D
is
).
Table 2. Inductor Vendors
VENDOR
Murata
TDK
TOKO
Sumida
Cooper/Coiltronics
Coilcraft
Vishay
NIC
Würth Elektronik
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
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:
where f
C
IN
≥
S
in the switching frequency in MHz.
10μF
f
S
IN
pin of the LT3641 with a ceramic capacitor
www.murata.com
www.tdk.com
www.toko.com
www.sumida.com
www.cooperindustries.com
www.coilcraft.com
www.vishay.com
www.niccomp.com
www.we-online.com
WEBSITE
3641f