LTC3533 Datasheet, PDF (11/16 Pages) Linear Technology – 2A Wide Input Voltage Synchronous Buck-Boost DC/DC Converter

LTC3533
APPLICATIONS INFORMATION
COMPONENT SELECTION
1 R_T
2 BURST
3 SGND
4 SW1
5 PGND
6 PGND
7 SW2
V_C 14
FB 13
RUN/SS 12
PV_IN 11
V_IN 10
PV_OUT 9
V_OUT 8
V_IN
V_OUT
GND
MULTIPLE VIAS
3533 F04
Figure 4. 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.
PV_OUT and PV_IN Ceramic Capacitors Close to the IC Pins.
Inductor Selection
The high frequency operation of the LTC3533 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:
L_BOOST
>
V_IN(MIN)2
f•
• (V_OUT − V_IN(MIN))
∆I_L • V_OUT2
H
L_BUCK
>
V_OUT
f
• (V_IN(MAX) − V_OUT
• ∆I_L • V_IN(MAX)
)
H
where f = switching frequency, Hz
∆I_L = maximum allowable inductor ripple current
V_IN(MIN) = minimum input voltage
V_IN(MAX) = maximum input voltage
V_OUT = output voltage
For high efficiency, choose a ferrite inductor with a high
frequency core material to reduce core losses. The induc-
tor 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 4A to 6A 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
=
(V_IN(MAX) − V_OUT ) • 100
8L C_OUT • V_IN(MAX) • f²
%
where C_OUT = output filter capacitor
I_OUT(MAX) = maximum output load current
The output capacitance is usually many times larger than
the minimum value in order to handle the transient response
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
3533f
11