LTC3440
12
3440fd
For more information www.linear.com/LTC3440
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3440 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 >
V
IN(MIN)
• V
OUT
− V
IN(MIN)
f •I
OUT(MAX)
•Ripple • V
OUT
µH
L >
V
OUT
• V
IN(MAX)
− V
OUT
( )
f •I
OUT(MAX)
•Ripple • V
IN(MAX)
µH
where f = operating frequency, MHz
Ripple = allowable inductor current ripple
(e.g., 0.2 = 20%)
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
3440 F06
GND
C2
D2
LTC3440
MULTIPLE
VIAS
L1
R
T
V
C
FB
SHDN/SS
V
IN
V
OUT
MODE/SYNC
SW1
GND
SW2
D1
V
IN
R1 R2
V
OUT
C1
1
2
3
4
5
10
9
8
7
6
applications inForMation
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
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
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 (847) 639-6400 (847) 639-1469 www.coilcraft.com
Coiltronics (561) 241-7876 (561) 241-9339 www.coiltronics.com
Murata USA:
(814) 237-1431
(800) 831-9172
USA:
(814) 238-0490
www.murata.com
Sumida
USA:
(847) 956-0666
Japan:
81(3) 3607-5111
(847) 956-0702
81(3) 3607-5144
www
.japanlink.com/
sumida
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 =
I
OUT(MAX)
• V
OUT
– V
IN(MIN)
• 100
C
OUT
• V
OUT
2
• f
%
%Ripple_Buck =
I
OUT(MAX)
• V
IN(MAX)
– V
OUT
( )
• 100
C
OUT
• V
IN(MAX)
• V
OUT
• f
%
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 transient
response.
