Datasheet

LTC3530
10
3530fb
Soft-Start
The soft-start function is combined with shutdown.
When the SHDN/SS pin is brought above 1V typical, the
IC is enabled but the EA duty cycle is clamped from V
C
.
A detailed diagram of this function is shown in Figure 3.
The components R
SS
and C
SS
provide a slow ramping
voltage on SHDN/SS to provide a soft-start function. To
ensure that V
C
is not being clamped, SHDN/SS must be
raised above 1.6V.
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3530 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•I
L
•V
OUT
H
L
BUCK
>
V
OUT
•(V
IN(MAX)
–V
OUT
)
f•I
L
•V
IN(MAX)
H
where f = operating frequency, Hz
ΔI
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 effi ciency, choose a ferrite inductor with a high
frequency core material to reduce core loses. 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 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 fi lter 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 =
1
8LCf
2
(V
IN(MAX)
–V
OUT
) • 100
V
IN(MAX)
%
where C
OUT
= output fi lter capacitor in Farads and
f = switching frequency in Hz.
V
IN
V
C
V
CI
SHDN/SS
3530 F05
Figure 3.
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
OPERATION