LTC3421_15 Datasheet, PDF (11/16 Pages) Linear Technology – 3A, 3MHz Micropower Synchronous Boost Converter with Output Disconnect

U
OPERATIO
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
LTC3421
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
COMPONENT SELECTION
24
V_C
1 FB
23 22 21 20 19
LBI LBO V_IN V_OUT V_OUT
V_OUTS 18
2 SHDN
V_OUT 17
3 V_REF
SW 16
4 ENB
SW 15
5 RT
SW 14
6 SS
PGND 13
SYNC ^ILIM BURST GND PGND PGND
7 8 9 10 11 12
V_OUT
V_IN
GND
MULTIPLE VIAS
TO GROUND
PLANE
3421 F01
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
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 >} 3 _{and L >} ^VIN(MIN) ^{• V}OUT(MAX) ^{– V}IN(MIN)
f
f • Ripple • V_OUT(MAX)
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 I²R 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
3421f
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