Datasheet

LTC3533
12
3533fc
For more information www.linear.com/LTC3533
applications inForMation
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)
• (V
OUT
−
V
IN(MIN)
)
f • ΔI
L
• V
OUT
H
L
BUCK
>
V
OUT
• (V
IN(MAX)
− V
OUT
)
f • ΔI
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 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 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)
) • 100
C
OUT
• V
OUT
2
• f
%
%Ripple_Buck =
(V
IN(MAX)
− V
OUT
) • 100
8LC
OUT
• V
IN(MAX)
• f
2
%
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
COMPONENT SELECTION
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.
Table 1. Inductor Vendor Information
SUPPLIER PHONE FAX WEB SITE
Coilcraft [phone] [phone] www.coilcraft.com
CoEv Magnetics [phone] [phone] www.circuitprotection.com/magnetics.asp
Murata [phone]
[phone]
[phone] www.murata.com
Sumida USA: [phone]
Japan: 81(3) 3607-5111
USA: [phone]
Japan: 81(3) 3607-5144
www.sumida.com
TDK [phone] [phone] www.component.tdk.com
TOKO [phone] [phone] www.tokoam.com
14
13
12
11
10
9
8
1
2
3
4
5
6
7
V
C
FB
RUN/SS
PV
IN
V
IN
PV
OUT
V
OUT
R
T
BURST
SGND
SW1
PGND
PGND
SW2
V
IN
V
OUT
3533 F04
MULTIPLE VIAS
GND
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