QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
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
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
filter between the bulk capacitor and
the input capacitor can be added to
the circuit to dampen hot-plug tran-
sient ringing.
See Application Note
88 for more details.
Table 1. Typical Performance Summary
PARAMETER
Steady State Input Voltage Range
Maximum Transient Input Voltage
V
OUT
Maximum Output Current
Typical Switching Frequency
Typical Efficiency
I
OUT
≤
2A
CONDITION
VALUE
60V
5V ±3%
2A
500kHz
V
IN
= 12V, V
OUT
= 5V, I
OUT
= 2A 83%
V
OUT
= 5V, I
OUT
≤
2A, T
A
= 25°C 7.5–20V
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
Design files for this circuit board
are available. Call the LTC factory.
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
QUICK START PROCEDURE
Demonstration circuit 549 is easy to
set up to evaluate the performance of
the LT3431. Refer to Figure 1 for
proper measurement equipment setup
and follow the procedure below:
NOTE:
Make sure that the input voltage
can be left floating (disconnected)
if their functions are not being
used.
1.
Connect the power supply, load, and
meters shown in Figure 1.
2.
After
does not exceed 60V.
NOTE:
The synchronization and shutdown
all connections are made,
turn on input power and verify that
the output voltage is 5.0V.
functions are optional and their pins
+
–
BENCH
SUPPLY
+
–
–
–
+
+
–
LOAD
+
+
–
–
+
OPTIONAL
Figure 1. Proper Measurement Equipment Setup
CUSTOMIZING THE DEMONSTRATION CIRCUIT
CUSTOMIZING THE OUTPUT VOLTAGE
The components assembled on the board
are optimized for a wide input volt-
age range and a 5V output. The feed-
back resistors (R2, R3) can be
changed to adjust the output voltage
according to the following equation:
V
OUT
= 1.22 (1 + R2/R3)
The Thevenin resistance seen at the
FB pin should be less than 3.8k to
maximize frequency foldback
start-up and short-circuit.
during
For output voltages below 3V, the
boost diode should be moved from D2
to D3 to provide the minimum boost
voltage required for the internal
power switch. Make sure that the
boost capacitor (C4) has a voltage
rating greater than the output volt-
age for applications where the boost
diode is placed in D2 and greater
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
than the input voltage when the boost
diode is placed in position D3.
For output voltages greater than 5V,
the optional ‘blocking’ zener diode
D4 can be used to reduce the boost
voltage across C4 from the output
voltage to some lower voltage between
3V and 5V. This diode transfers power
dissipation from inside the IC to the
discrete component outside the IC,
allowing higher ambient temperature
operation for the part. It is recom-
mended that an SMAZ7V5 zener diode is
used in D4 when V
OUT
= 12V. To prop-
erly install D4, the small trace
shorting the anode to the cathode of
D4 on the board must be opened (an
Exacto knife works well) before D4 is
soldered to the board. In the ‘Ther-
mal Calculations’ section of Applica-
tions Information in the datasheet,
the boost power consumption in the IC
must account for the zener blocking
diode V
Z
, so the boost voltage
changes from V
OUT
to (V
OUT
– V
Z
).
COMPENSATION
Demonstration Circuit 549 has a fre-
quency compensation network that is
optimized for the ceramic output ca-
pacitor C5, the wide input voltage
range 7.5V to 20V, and 5V output. Im-
proved loop bandwidth can be achieved
for various output voltages, output
capacitors, and input voltage ranges
by adjusting R1, C1, and C2. Addi-
tional optional component locations
for feedforward capacitor (C8) and
resistor (R4 for short circuit feed-
back pin protection when feedforward
capacitor is used) are located in
parallel with R2. For more informa-
tion, see the ‘Frequency Compensa-
tion’ section in the Applications In-
formation in the datasheet, Applica-
tion Notes 19 or Application Note 76.
SOFT START
When the ratio of the input voltage
divided by the output voltage (plus
the forward voltage of the catch di-
ode) is greater than four
V
IN
/(V
OUT
+ V
F
) > 4
the soft start circuit made up of
components C9, R5, R6, and Q1 should
be used to control output voltage
rise
time
and
overshoot
during
startup. The soft start circuit is
covered in detail in ‘Buck Converter
with Adjustable Soft Start’ in the
Applications Information in the da-
tasheet.
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549
2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER
100%
90%
80%
70%
EFFICIENCY (%)
60%
50%
40%
30%
20%
10%
0%
0
500
1000
1500
2000
8.0 V in
12.0 Vin
20.0 Vin
LO AD CURRENT (mA)
Figure 2. Efficiency
2800
Tj = 125C
2600
Maximum Load Current (mA)
2A Load, Tj < 125C
2400
2200
2000
1800
1600
1400
1200
7
9
11 13 15 17 19 21 23 25
VI N ( V )
Figure 3. Maximum Load Current Capabilities
(T
A
= 25°C)
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