LTC3442 Datasheet, PDF (13/20 Pages) Linear Integrated Systems – Micropower Synchronous Buck-Boost DC/DC Converter with Automatic Burst Mode Operation

LTC3442
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
V_IN
1 SHDN/SS
2 RT
3 SGND
4 SW1
5 PGND
6 SW2
FB 12
V_C 11
R_LIM 10
V_IN 9
V_OUT 8
BURST 7
V_IN
V_OUT
MULTIPLE
GND ^VIAS
R_T
3442 F06
Figure 6. Recommended Component Placement. Traces Carrying
High Current Should be Short and Wide. 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 LTC3442 allows the
use of small surface mount inductors. The inductor ripple
current is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
2
L_BOOST
>
V_IN(MIN)
f•
• (V_OUT – V_IN(MIN))H
∆I_L • V_OUT2
L_BUCK
>
V_OUT
f
•
(V_IN(MAX)
–
V_OUT
)
H
• ∆I_L • V_IN(MAX)
where f = operating frequency, Hz
∆I_L = maximum allowable inductor ripple current, A
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 a ferrite inductor with a high
frequency core material 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
the peak inductor currents in the 1A to 2A region. To
minimize radiated noise, use a shielded inductor. See
Table 1 for a suggested list of inductor suppliers.
Output Capacitor Selection
The bulk value of the output filter 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)
C_OUT • V_OUT2 • f
)
•100
%
% 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 in Farads and
f = switching frequency in Hz.
Table 1. Inductor Vendor Information
SUPPLIER
PHONE
Coilcraft
[phone]
CoEv Magnetics
[phone]
Murata
[phone]
[phone]
Sumida
USA: [phone]
Japan: 81(3) 3607-5111
TDK
[phone]
TOKO
[phone]
FAX
[phone]
[phone]
[phone]
USA: [phone]
Japan: 81(3) 3607-5144
[phone]
[phone]
WEB SITE
www.coilcraft.com
www.circuitprotection.com/magnetics.asp
www.murata.com
www.sumida.com
www.component.tdk.com
www.tokoam.com
3442f
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