DC857A - Demo Manual

DEMO CIRCUIT 857
LT3481
QUICK START GUIDE
LT3481EMSE/LT3481EDD
800kHz/2MHz Buck Regulator
DESCRIPTION
Demonstration circuit 857 features
LT3481EMSE and LT3481EDD in two stepdown converter circuits, one configured for high efficiency and the
other for small size. The high efficiency circuit switching at 800kHz is
designed for 3.3V output from a 4.5V
to 34V input. The small size circuit
switching at 2MHz demonstrates the
LT3481’s ability to operate with a
small inductor and output capacitor.
It is designed for 5V output from an
8.5V to 22V input. The wide input
range of the LT3481 allows a variety
of input sources. The typical sources
are automotive batteries, wall adaptors and industrial supplies. The
current mode control scheme creates
fast transient response and good loop
stability. In addition, the LT3481’s
integrated boost diode reduces the
parts count. The low ripple Burst
Mode Operation of LT3481 not only allows the circuit to have high effi-
ciency over a broad current range but
also provides the low output ripple
at light load conditions. The RUN/SS
pin can be used to set the part in
micropower shutdown mode, reducing
the supply current to less than 1uA.
The RUN/SS pin can also be used to
program soft start. In this mode, the
RUN/SS pin is driven through an external RC filter to create a voltage
ramp on this pin. The soft start
function reduces the input current
surge during start-up.
The LT3481 datasheet gives a complete
description of the part, operation
and
application
information.
The
datasheet must be read in conjunction
with this quick start guide for demo
circuit 857.
Design files for this circuit board
are available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT and PowerPath are trademarks of Linear Technology Corporation.
Performance Summary for 800kHz Buck Regulator ( TA = 25oC )
PARAMETER FOR LED DRIVER
CONDITION
Minimum input voltage
Maximum input voltage
Output voltage VOUT
Maximum output current
Typical switching frequency
VALUE
4.5V
34V
3.3V +/- 4%
2A
800kHz
Performance Summary for 2MHz Buck Regulator ( TA = 25oC )
PARAMETER FOR LED DRIVER
CONDITION
Minimum input voltage
Maximum input voltage
Output voltage VOUT
Maximum output current
Typical switching frequency
VALUE
8.5V
22V
5V +/- 4%
2A
2MHz
1
LT3481
QUICK START PROCEDURE
Demonstration circuit 857 is easy to
set up to evaluate the performance of
the LT3481. Refer to Figure 1 for
proper measurement equipment setup
and follow the procedure below:
NOTE. When measuring the input or output voltage
ripple, care must be taken to avoid a long ground
lead on the oscilloscope probe. Measure the input
or output voltage ripple by touching the probe tip
directly across the Vin or Vout and GND terminals.
See Figure 2 for proper scope probe technique.
1. Place JP1 and JP2 on the RUN po-
sition:
2. With
power off, connect the input
power supply to Vin and GND.
3. Turn on the power at the input.
NOTE. Make sure that the input voltage does not
exceed the maximum input voltage.
4. Check
for the proper output volt-
ages.
NOTE. If there is no output, temporarily
disconnect the load to make sure that the load
is not set too high.
5. Once the proper output voltages are
established,
adjust
the
loads
within the operating range and observe the output voltage regulation, ripple voltage, efficiency
and other parameters.
+
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+
+
_V
+
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_
A
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+
+
+
V
+ _
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+
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Figure 1. Proper Measurement Equipment Setup
GND
VIN
Figure 2. Measuring Input or Output Ripple
2
V
+
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A
Load
+
+
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V
Load
A
A
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LT3481
3
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