lt3471edd-trpbf Linear Technology Corporation, lt3471edd-trpbf Datasheet - Page 6

• Manufacturer Part Number: lt3471edd-trpbf
• Description: Dual 1.3a, 1.2mhz Boost/inverter In 3mm ? 3mm Dfn
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LT3471
V
tions section for examples). In this case:
Select values of R1 and R2 according to the following
equation:
A good value for R2 is 15k, which sets the current in the
resistor divider chain to 1.00V/15k = 67µA.
Switching Frequency and Inductor Selection
The LT3471 switches at 1.2 MHz, allowing for small valued
inductors to be used. 4.7µH or 10µH will usually suffice.
Choose an inductor that can handle at least 1.4A without
saturating, and ensure that the inductor has a low DCR
(copper-wire resistance) to minimize I
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 one half
of the total switch current. For better efficiency, use similar
valued inductors with a larger volume. Many different
sizes and shapes are available from various manufactur-
ers. Choose a core material that has low losses at 1.2 MHz,
such as ferrite core.
Table 1. Inductor Manufacturers
Sumida
TDK
Murata
Soft-Start and Shutdown Features
To shut down the part, ground both SHDN/SS pins. To
shut down one switcher but not the other one, ground that
switcher’s SHDN/SS pin. The soft-start feature provides a
way to limit the inrush current drawn from the supply upon
start-up. To use the soft-start feature for either switcher,
slowly ramp up that switcher’s SHDN/SS pin. The rate of
voltage rise at the output of the switcher’s comparator (A1
or A3 for switcher 1 or switcher 2 respectively) tracks the
rate of voltage rise at the SHDN/SS pin once the SHDN/SS
6
REF
R
V
OUT
1
and R1 is between V
=
R
=
2
V
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⎜
⎝
REF
V
V
OUT
REF
⎛
⎜
⎝
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R
R
⎞
⎟
⎠
2
1
U
⎞
⎟
⎠
INFORMATION
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and V
W
OUT
www.sumida.com
www.tdk.com
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2
(see the Applica-
R power losses.
U
pin has reached about 1.1V. The soft-start function will go
away once the voltage at the SHDN/SS pin exceeds 1.8V.
See the Peak Switch Current vs SHDN/SS Voltage graph in
the Typical Performance Characteristics section. The rate
of voltage rise at the SHDN/SS pin can easily be controlled
with a simple RC network connected between the control
signal and the SHDN/SS pin. Typical values for the RC
network are 4.7kΩ and 0.33µF, giving start-up times on
the order of milliseconds. This RC time constant can be
adjusted to give different start-up times. If different values
of resistance are to be used, keep in mind the SHDN/SS
Current vs SHDN/SS voltage graph along with the Peak
Switch Current vs SHDN/SS Voltage graph, both found in
the Typical Performance Characteristics section. The im-
pedance looking into the SHDN/SS pin depends on whether
the SHDN/SS is above or below V
will not be driven above V
like 100kΩ in series with a diode. If the voltage of the
SHDN/SS pin is above V
50kΩ in series with a diode. This 100kΩ or 50kΩ imped-
ance can have a slight effect on the start-up time if you
choose the R in the RC soft-start network too large.
Another consideration is selecting the soft-start time so
that the soft-start feature is dominated by the RC network
and not the capacitor on V
section of the Applications Information for details.)
CAPACITOR SELECTION
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multi-layer ceramic capacitors are an excellent choice, as
they have extremely low ESR and are available in very
small packages. X5R dielectrics are preferred, followed by
X7R, as these materials retain the capacitance over wide
voltage and temperature ranges. A 4.7µF to 15µF output
capacitor is sufficient for most applications, but systems
with very low output currents may need only a 1µF or 2.2µF
output capacitor. Solid tantalum or OS-CON capacitors
can be used, but they will occupy more board area than a
ceramic and will have a higher ESR. Always use a capacitor
with a sufficient voltage rating.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT3471. A 4.7µF to 10µF input capacitor is
IN
IN
REF
, the impedance looks more like
, and thus the impedance looks
. (See V
IN
. Normally SHDN/SS
REF
voltage reference
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