QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
LT3431
DESCRIPTION
Demonstration circuit 549 is a mono-
lithic step-down DC/DC switching con-
verter using the LT3431. The board is
optimized for 5V output at up to 2A
load current for a steady state input
voltage range of 7.5V to 20V. The
LT3431 is capable of withstanding in-
put transients to 60V and runs at
500kHz with steady state input volt-
ages that require duty cycles between
25% and 75%. For duty cycles below
25% the part may pulse skip to effec-
tively average the lower duty cycle
and still provide minimum on-time
switch cycles. With its wide input
voltage range, 3A internal power
switch, high 500kHz switching fre-
quency and thermally enhanced pack-
age, the LT3431 is a very versatile
and powerful IC for DC/DC converters
that require compact size, high effi-
ciency and tolerance to high input
voltage transients.
The LT3431 500kHz switching frequency
allows all of the components to be
small, surface mount devices. Syn-
chronization with an external clock
of up to 700kHz is possible. The cur-
rent-mode control topology creates
fast transient response and good loop
stability with a minimum number of
external compensation components. In
particular, the current-mode archi-
tecture allows the use of ceramic in-
put and output capacitors for in-
creased reliability, extremely low
output ripple voltage and small com-
ponent size. The low resistance in-
ternal power switch (0.1Ω) maintains
high efficiencies of up to 87%. The
SHDN pin can be used to program un-
dervoltage lockout or place the part
in micropower shutdown, reducing sup-
ply current to 30µA by driving the
pin low.
The LT3431 datasheet gives a complete
description of the part, operation
and applications information. The da-
tasheet must be read in conjunction
with this Quick Start Guide for dem-
onstration circuit 549. In particu-
lar, the datasheet section on “Ther-
mal Calculations” is important for
estimating whether a given applica-
tion’s combination of input voltage,
load current and frequency will cause
the LT3431 to exceed it’s absolute
maximum rated junction temperature.
The part is assembled in a small 16-
pin thermally enhanced package with
exposed pad where proper board layout
is essential for maximum thermal per-
formance. See the datasheet section
“Layout Considerations”.
NOTE: Do not hot-plug the input volt-
age terminal V
IN
. The absolute maxi-
mum voltage on V
IN
is 60V and hot-
plugging a power supply through wire
leads to the demonstration card can
cause the voltage on the extremely
low-ESR ceramic input capacitor to
ring to twice its DC value. This is
due to high currents instantaneously
generated in the inductive supply
leads from an input voltage step on
the low-ESR ceramic input capacitor.
An over-voltage can result if care is
not taken to ensure that this does
not happen. A bulky higher-ESR ca-
pacitor, and an additional inductive
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