Datasheet

9
LTC3401
3401fb
APPLICATIO S I FOR ATIO
WUUU
Burst Mode operation is user controlled by driving the
MODE/SYNC pin high to enable and low to disable. It is
recommended that Burst Mode operation be entered after
the part has started up.
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3401 allows the
use of small surface mount inductors. The minimum
inductance value is proportional to the operating fre-
quency and is limited by the following constraints:
L
f
H and L
VV V
f Ripple V
H
IN MIN OUT MAX IN MIN
OUT MAX
>μ >
()
3
() ( ) ()
()
•–
••
where
f = Operating Frequency (Hz)
Ripple = Allowable Inductor Current Ripple (A)
V
IN(MIN)
= Minimum Input Voltage (V)
V
OUT(MAX)
= Maximum Output Voltage (V)
The inductor current ripple is typically set to 20% to 40%
of the maximum inductor current.
For high efficiency, choose an inductor with a high fre-
quency core material, such as ferrite, to reduce core
losses. The inductor should have low ESR (equivalent
series resistance) to reduce the I
2
R losses and must be
able to handle the peak inductor current at full load without
saturating. Molded chokes or chip inductors usually do
not have enough core to support the peak inductor cur-
rents in the 1A to 2A region. To minimize radiated noise,
use a toroid, pot core or shielded bobbin inductor. See
Table 1 for a list of component suppliers.
V
OUT
3401 F01
R
t
MODE
V
IN
SW
GND
SHDN
V
C
FB
V
OUT
PGOOD
Figure 1. Recommended Component Placement. Traces
Carrying High Current Are Direct. Trace Area FB and V
C
Pins
Are Kept Low. Lead Length to Battery Should be Kept Short
Output Capacitor Selection
The output voltage ripple has several components. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The max ripple due
to charge is given by:
VR
IV
CVf
V
BULK
PIN
OUT OUT
=
•
••
where
I
P
= Peak Inductor Current
The ESR can be a significant factor for ripple in most power
converters. The ripple due to capacitor ESR is simply given
by:
VR
CESR
= I
P
• R
ESR
V
where
R
ESR
= Capacitor Series Resistance
Low ESR capacitors should be used to minimize output
voltage ripple. For surface mount applications, AVX TPS
series tantalum capacitors, Sanyo POSCAP, or Taiyo-
Yuden ceramic capacitors are recommended. For through-
hole applications Sanyo OS-CON capacitors offer low ESR
in a small package size. See Table 2 for a list of component
suppliers.
Table 1. Inductor Vendor Information
SUPPLIER PHONE FAX WEBSITE
Coilcraft [phone] [phone] www.coilcraft.com
Coiltronics [phone] [phone] www.coiltronics.com
Murata [phone] [phone] www.murata.com
[phone]
Sumida
USA: [phone] [phone] www.japanlink.com
Japan: 81-3-3607-5111 81-3-3607-5144 sumida
Table 2. Capacitor Vendor Information
SUPPLIER PHONE FAX WEBSITE
AVX [phone] [phone] www.avxcorp.com
Sanyo [phone] [phone] www.sanyovideo.com
Taiyo Yuden [phone] [phone] www.t-yuden.com