LT3471
7
3471fb
APPLICATIONS INFORMATION
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
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 differ-
ent 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 impedance looking into the SHDN/SS pin depends
on whether the SHDN/SS is above or below V
IN
. Normally
SHDN/SS will not be driven above V
IN
, and thus the imped-
ance looks like 100kΩ in series with a diode. If the voltage
of the SHDN/SS pin is above V
IN
, the impedance looks
more like 50k Ω in series with a diode. This 100k Ω or 50k Ω
impedance 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
REF
. (See V
REF
voltage reference
section of the Applications Information for details.)
The soft-start feature is of particular importance in ap-
plications where the switch will see voltage levels of 30V
or higher. In these applications, the simultaneous presence
of high current and voltage during startup may cause an
overstress condition to the switch. Therefore, depending
on input and output voltage conditions, higher RC time
constant values may be necessary to improve the rug-
gedness of the design.
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 suffi cient 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 suffi cient 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 suffi cient for most applications. Table 2 shows a list
of several ceramic capacitor manufacturers. Consult the
manufacturers for detailed information on their entire
selection of ceramic parts.
Table 2. Ceramic Capacitor Manufacturers
Taiyo Yuden (408) 573-4150 www.t-yuden.com
AVX (803) 448-9411 www.avxcorp.com
Murata (714) 852-2001 www.murata.com
The decision to use either low ESR (ceramic) capacitors
or the higher ESR (tantalum or OS-CON) capacitors can
affect the stability of the overall system. The ESR of any
capacitor, along with the capacitance itself, contributes
a zero to the system. For the tantalum and OS-CON ca-
pacitors, this zero is located at a lower frequency due to
the higher value of the ESR, while the zero of a ceramic
capacitor is at a much higher frequency and can generally
be ignored.
A phase lead zero can be intentionally introduced by placing
a capacitor (C
PL
) in parallel with the resistor (R3) between
V
OUT
and V
FB
as shown in Figure 2. The frequency of the
zero is determined by the following equation.
