Datasheet

LTC3421
11
3421f
OPERATIO
U
disconnect, there must not be any external Schottky
diodes connected between the SW pins and V
OUT
.
Note: Board layout is extremely critical to minimize volt-
age overshoot on the SW pins due to stray inductance.
Keep the output filter capacitors as close as possible to the
V
OUT
pins and use very low ESR/ESL ceramic capacitors,
tied to a good ground plane. In V
OUT
> 4.3V applications,
a Schottky diode is required from the switch nodes to V
OUT
to limit the peak switch voltage to less than 6V unless
some form of external snubbing is employed. (See 5V
Applications section.)
APPLICATIO S I FOR ATIO
WUUU
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3421 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
and L
VV V
f Ripple V
IN MIN OUT MAX IN MIN
OUT MAX
>>
()
3
() ( ) ()
()
•–
••
24 23 22 21 20 19
7 8 9 10 11 12
13
18
17
16
15
14
6
1
2
3
4
5
FB
V
C
LBI LBO V
IN
V
OUT
V
OUT
V
OUT
SYNC
I
LIM
BURST GND PGND PGND
GND
3421 F01
MULTIPLE VIAS
TO GROUND
PLANE
SHDN
V
REF
ENB
R
T
SS
V
OUTS
V
OUT
SW
SW
SW
PGND
V
IN
Figure 1. Recommended Component Placement. Traces Carrying
High Current are Direct (PGND, SW, V
OUT
). Trace Area at FB and
V
C
are Kept Low. Lead Length to Battery Should be Kept Short.
V
IN
and V
OUT
Ceramic Capacitors Should be as Close to the IC
Pins as Possible
where
f = Operating Frequency in MHz
Ripple = Allowable Inductor Current Ripple (Amps
Peak-Peak)
V
IN(MIN)
= Minimum Input Voltage
V
OUT(MAX)
= Maximum Output Voltage
The inductor current ripple is typically set to 20% to 40%
of the maximum inductor current.
For high efficiency, choose an inductor with 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 peak inductor currents in the 1A to 4A region.
To minimize radiated noise, use a toroidal or shielded
inductor. See Table 1 for suggested inductor suppliers and
Table 2 for a list of capacitor suppliers.
Table 1. Inductor Vendor Information
SUPPLIER PHONE FAX WEB SITE
Coilcraft [phone] [phone] www.coilcraft.com
Coiltronics [phone] [phone]
Murata USA: USA: www.murata.com
[phone] [phone]
[phone]
Sumida USA: USA: www.sumida.com
[phone] [phone]
Japan: Japan:
81-3-3607-5111 81-3-3607-5144
TDK [phone] [phone] www.component.tdk.com
TOKO [phone] [phone] www.toko.com