Datasheet

9
LTC3402
3402fb
APPLICATIO S I FOR ATIO
WUUU
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3402 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.
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 and Sanyo POSCAP or Taiyo-
Yuden ceramic X5R or X7R type capacitors are recom-
mended. 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. In some layouts it may
be required to place a 1μF low ESR capacitor as close to the
V
OUT
and GND pins as possible.
V
OUT
3402 F01
R
t
MODE
V
IN
SW
GND
SHDN
V
C
FB
V
OUT
POK
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
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 without saturat-
ing. Molded chokes or chip inductors usually do not have
enough core to support the peak inductor currents in the
1A to 2A region. To minimize radiated noise, use a toroid,
pot core or shielded bobbin inductor. See Table 1 for
suggested components and Table 1 for a list of component
suppliers.