LT3580
8
3580fg
Setting Output Voltage
The output voltage is set by connecting a resistor (R
FB
)
from V
OUT
to the FB pin. R
FB
is determined from the
following equation:
R
FB
=
|V
OUT
− V
FB
|
83.3µA
where V
FB
is 1.215V (typical) for non-inverting topologies
(i.e., boost and SEPIC regulators) and 5mV (typical) for
inverting topologies (see the Electrical Characteristics).
Power Switch Duty Cycle
In order to maintain loop stability and deliver adequate
current to the load, the power NPN (Q1 in the Block Dia-
gram) cannot remain “on” for 100% of each clock cycle.
The maximum allowable duty cycle is given by:
DC
MAX
=
(T
P
−Min Off Time)
T
P
•100%
where T
P
is the clock period and Min Off Time (found in
the Electrical Characteristics) is typically 60ns.
The application should be designed so that the operating
duty cycle does not exceed DC
MAX
.
Duty cycle equations for several common topologies are
given below, where V
D
is the diode forward voltage drop
and V
CESAT
is typically 300mV at 1.5A.
For the boost topology:
DC ≅
V
OUT
− V
IN
+ V
D
V
OUT
+ V
D
− V
CESAT
For the SEPIC or dual inductor inverting topology (see
Figures 1 and 2):
DC ≅
V
D
+|V
OUT
|
V
IN
+ |V
OUT
| + V
D
− V
CESAT
The LT3580 can be used in configurations where the duty
cycle is higher than DC
MAX
, but it must be operated in the
discontinuous conduction mode so that the effective duty
cycle is reduced.
APPLICATIONS INFORMATION
Inductor Selection
General Guidelines
: The high frequency operation of the
LT3580 allows for the use of small surface mount inductors.
For high efficiency, choose inductors with high frequency
core material, such as ferrite, to reduce core losses. To
improve efficiency, choose inductors with more volume
for a given inductance. The inductor should have low
DCR (copper wire resistance) to reduce I
2
R losses, and
must be able to handle the peak inductor current without
saturating. Note that in some applications, the current
handling requirements of the inductor can be lower, such
as in the SEPIC topology, where each inductor only carries
a fraction of the total switch current. Molded chokes or chip
inductors usually do not have enough core area to sup-
port peak inductor currents in the 2A to 3A range. To
minimize radiated noise, use a toroidal or shielded inductor.
Note that the inductance of shielded types will drop more
as current increases, and will saturate more easily. See
Table 1 for a list of inductor manufacturers. Thorough lab
evaluation is recommended to verify that the following
guidelines properly suit the final application.
Table 1.Inductor Manufacturers
Coilcraft DO3316P, MSS7341 and LPS4018
Series
www.coilcraft.com
Coiltronics DR, LD and CD Series www.coiltronics.com
Murata LQH55D and LQH66S Series www.murata.com
Sumida CDRH5D18B/HP, CDR6D23MN,
CDRH6D26/HP, CDRH6D28,
CDR7D28MN and CDRH105R Series
www.sumida.com
TDK RLF7030 and VLCF4020 Series www.tdk.com
Würth WE-PD and WE-PD2 Series www.we-online.com
Minimum Inductance
: Although there can be a tradeoff with
efficiency, it is often desirable to minimize board space by
choosing smaller inductors. When choosing an inductor,
there are two conditions that limit the minimum inductance;
(1) providing adequate load current, and (2) avoidance of
subharmonic oscillation. Choose an inductance that is high
enough to meet both of these requirements.
Adequate Load Current
: Small value inductors result in
increased ripple currents and thus, due to the limited peak
switch current, decrease the average current that can be
