LT3581
24
3581fb
For more information www.linear.com/LT3581
APPENDIX
Inductor Current Rating
Inductors must have a rating greater than their peak
operating current, or else they could saturate and hence
contribute to losses in efficiency. The maximum inductor
current (considering start-up and steady-state conditions)
is given by:
I
L _PEAK
=I
LIM
+
IN
MIN _ PROP
where:
I
L_PEAK
= Peak Inductor Current in L
1
for a Boost
Topology, or the Peak of the sum of the
Inductor Currents in L1 and L2 for Dual
Inductor Topologies.
I
LIM
** = 3.3A with SW1 and SW2 Tied Together,
or 1.9A with just SW1 (This assumes
usage of an inductor whose core
material soft-saturates such as
powdered iron core).
T
MIN_PROP
= 100ns (Propagation Delay through the
Current Feedback Loop).
**If using an inductor whose core material saturates
hard (e.g., ferrite), then pick I
LIM
to be 5.4A with SW1
and SW2 tied together, or 3A when just SW1 is used.
Note that these equations offer conservative results for
the required inductor current ratings. The current ratings
could be lower for applications with light loads, if the SS
capacitor is sized appropriately to limit inductor currents
at start-up.
DIODE SELECTION
Schottky diodes, with their low forward voltage drops and
fast switching speeds, are recommended for use with the
LT3581. Choose a Schottky diode with low parasitic capaci
-
tance to reduce reverse current spikes through the power
switch of the L
T3581. The Central Semiconductor Corp.
CMMSH2-40 diode is a very good choice with a 40V reverse
voltage rating and an average forward current of 2A.
OUTPUT CAPACITOR SELECTION
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are an excellent choice, as
they have extremely low ESR and are available in very
small packages. X5R or X7R dielectrics are preferred, as
these materials retain their capacitance over wide voltage
and temperature ranges. A 10μF to 22μF output capacitor
is sufficient for most applications, but systems with very
low output currents may need only 2.2μF to 10μF. Always
use a capacitor with a sufficient voltage rating. Many
ceramic capacitors, particularly 0805 or 0603 case sizes,
have greatly reduced capacitance at the desired output
voltage. Tantalum Polymer or OS-CON capacitors can be
used, but it is likely that these capacitors will occupy more
board area than a ceramic, and will have higher ESR with
greater output ripple.
INPUT CAPACITOR SELECTION
Ceramic capacitors make a good choice for the input
decoupling capacitor, and should be placed such that it is
in close proximity to the V
IN
of the chip as well as to the
inductor connected to the input of the power path. If it is
not possible to optimally place a single input capacitor,
then use two separate capacitors—use one at the V
IN
of
the chip (see equation for C
VIN
in Tables 1, 2 and 3) and
one at the input to the power path (see equation for C
PWR
in Tables 1, 2 and 3) A 4.7μF to 20μF input capacitor is
sufficient for most applications.
Table 6 shows a list of several ceramic capacitor man-
ufacturers. Consult the manufacturers for detailed infor-
mation on their entire selection of ceramic parts.
Table 6: Ceramic Capacitor Manufacturers
AVX www.avxcorp.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
PMOS SELECTION
An external PMOS, controlled by the LT3581’s GATE pin,
can be used to facilitate input or output disconnect. The
GATE pin turns on the PMOS gradually during start-up
(see Soft-Start of External PMOS in the Operation section),
and turns the PMOS off when the LT3581 is in shutdown
or in fault.
