lt3580ems8e-trpbf Linear Technology Corporation, lt3580ems8e-trpbf Datasheet - Page 9

• Manufacturer Part Number: lt3580ems8e-trpbf
• Description: Boost/inverting Dc/dc Converter With 2a Switch, Soft-start, And Synchronization
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Setting Output Voltage
The output voltage is set by connecting a resistor (R
from V
lowing equation:
where V
(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:
where T
the Electrical Characteristics) is typically 60ns.
The application should be designed so that the operating
duty cycle does not exceed DC
Duty cycle equations for several common topologies are
given below, where V
and V
For the boost topology:
For the SEPIC or dual inductor inverting topology (see
Figures 1 and 2):
The LT3580 can be used in confi gurations where the duty
cycle is higher than DC
discontinuous conduction mode so that the effective duty
cycle is reduced.
R
DC
DC
DC
FB
CESAT
MAX
OUT
=
FB
P
V
V
| V
OUT
IN
is the clock period and Min Off Time (found in
is 1.215V (typical) for non-inverting topologies
V
=
to the FB pin. R
OUT
is typically 300mV at 1.5A.
83.3µA
OUT
+ | V
(T
+ V
P
OUT
V
D
V
D
V
Min Off Time)
FB
IN
+| V
| + V
T
D
V
|
+ V
P
CESAT
MAX
is the diode forward voltage drop
OUT
D
D
, but it must be operated in the
FB
|
V
is determined from the fol-
CESAT
MAX
• 100%
.
FB
)
Inductor Selection
General Guidelines : The high frequency operation of the
LT3580 allows for the use of small surface mount inductors.
For high effi ciency, choose inductors with high frequency
core material, such as ferrite, to reduce core losses. To
improve effi ciency, choose inductors with more volume
for a given inductance. The inductor should have low DCR
(copper wire resistance) to reduce I
able to handle the peak inductor current without saturat-
ing. 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 frac-
tion 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 mini-
mize 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.
Table 1.Inductor Manufacturers
Coilcraft
Coiltronics DR, LD and CD Series
Murata
Sumida
TDK
Würth
Minimum Inductance : Although there can be a tradeoff
with effi ciency, 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.
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
provided to a load (I
load current, L should be at least:
L >
2(f) I
DO3316P , MSS7341 and LPS4018
Series
LQH55D and LQH66S Series
CDRH5D18B/HP , CDR6D23MN,
CDRH6D26/HP , CDRH6D28,
CDR7D28MN and CDRH105R Series
RLF7030 and VLCF4020 Series
WE-PD and WE-PD2 Series
LIM
DC • V
| V
OUT
OUT
IN
V
IN
). In order to provide adequate
| • I
•
OUT
2
R losses, and must be
www.coilcraft.com
www.coiltronics.com
www.murata.com
www.sumida.com
www.tdk.com
www.we-online.com
LT3580
3580fa
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