LTC3443 Datasheet, PDF (11/16 Pages) Linear Technology – High Current Micropower 600kHz Synchronous Buck-Boost DC/DC Converter

LTC3443
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
1 SHDN/SS
2 GND
3 PGND
4 SW1
5 SW2
6 PGND
FB 12
V_C 11
V_IN 10
PV_IN 9
V_OUT 8
MODE 7
V_IN
V_OUT
GND
3443 F06
MULTIPLE VIAS
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 LTC3443 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)2 • V_OUT – V_IN(MIN) • 100
f • I_OUT(MAX) • %Ripple • V_OUT2
H,
( ) L > VOUT • VIN(MAX) – VOUT • 100 H
f • I_OUT(MAX) • %Ripple • V_IN(MAX)
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 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 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:
[phone] [phone]
[phone]
www.murata.com
Sumida
USA:
[phone] [phone]
Japan:
81(3) 3607-5111 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)
C_OUT • V_OUT2 • f
• 100
%
( ) _{%Ripple _Buck =} ^IOUT(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
sn3443 3443fs
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