Datasheet

LTC3441
10
3441fa
COMPONENT SELECTION
APPLICATIO S I FOR ATIO
WUUU
handle the peak inductor current without saturating. 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 2 for a list of component suppliers.
Table 1. Inductor Vendor Information
SUPPLIER PHONE FAX WEB SITE
Coilcraft [phone] [phone] www.coilcraft.com
Coiltronics [phone] [phone] www.coiltronics.com
Murata USA: USA: www.murata.com
[phone] [phone]
[phone]
Sumida USA: www.japanlink.com/
[phone] [phone] sumida
Japan:
81(3) 3607-5111 81(3) 3607-5144
Output Capacitor Selection
The bulk value of the capacitor is set to reduce the ripple
due to charge into the capacitor each cycle. The steady
state ripple due to charge is given by:
%_
•– •
••
%
%_
•–•
•••
%
() ()
() ()
()
Ripple Boost
IVV
CV f
Ripple Buck
IVV
CV Vf
OUT MAX OUT IN MIN
OUT OUT
OUT MAX IN MAX OUT
OUT IN MAX OUT
=
()
=
()
100
100
2
where C
OUT
= output filter capacitor, F
The output capacitance is usually many times larger in
order to handle the transient response of the converter.
For a rule of thumb, the ratio of the operating frequency to
the unity-gain bandwidth of the converter is the amount
the output capacitance will have to increase from the
above calculations in order to maintain the desired tran-
sient response.
The other component of ripple is due to the ESR (equiva-
lent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden ce-
ramic capacitors, AVX TPS series tantalum capacitors or
Sanyo POSCAP are recommended.
12
11
10
9
8
7
1
2
3
4
5
6
FB
V
C
V
IN
PV
IN
V
OUT
MODE
SHDN/SS
GND
PGND
SW1
SW2
PGND
V
IN
V
OUT
GND
MULTIPLE VIAS
3441 F06
Figure 6. Recommended Component Placement. Traces Carrying
High Current are Direct. Trace Area at FB and V
C
Pins are Kept
Low. Lead Length to Battery Should be Kept Short. V
OUT
and V
IN
Ceramic Capacitors Close to the IC Pins
Inductor Selection
The high frequency operation of the LTC3441 allows the
use of small surface mount inductors. The inductor cur-
rent ripple is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
L
VVV
f I Ripple V
H
L
VV V
f I Ripple V
H
IN MIN OUT IN MIN
OUT MAX OUT
OUT IN MAX OUT
OUT MAX IN MAX
>
()
>
()
() ()
()
()
() ()
•– •
••%•
,
•–•
••%•
100
100
where f = operating frequency, Hz
%Ripple = allowable inductor current ripple, %
V
IN(MIN)
= minimum input voltage, V
V
IN(MAX)
= maximum input voltage, V
V
OUT
= output voltage, V
I
OUT(MAX)
= maximum output load current
For high efficiency, choose an inductor with a high fre-
quency core material, such as ferrite, to reduce core loses.
The inductor should have low ESR (equivalent series
resistance) to reduce the I
2
R losses, and must be able to