DEMO CIRCUIT 1458
QUICK START
LT3972
GUIDE
LT3972EMSE
3.5A, 33V Step-down Switching Regu-
lator with SYNC Function
DESCRIPTION
Demonstration circuit 1458 is a monolithic step-
down DC/DC switching regulator featuring LT3972.
The LT3972 can be synchronized over a 250 KHz
to 2 MHz range. The demo board is designed for
5V output from a 6.3V to 33V input with transient
up to 62V. The wide input range of the LT3972 al-
lows a variety of input sources. The typical sources
are automotive batteries, wall adaptors and indus-
trial supplies. The modes of operation (Burst Mode
or synchronization) are jumper selectable. The
Burst Mode operation increases the efficiency at
light loads.
The current mode control scheme creates fast
transient response and good loop stability. The
gate drive of the internal switch is boosted to a
voltage that is higher than the Vin to ensure satu-
ration of the switch. The LT3972’s integrated boost
diode reduces the parts count. The RUN/SS pin
can be used to set the part in micropower shut-
down 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 LT3972 datasheet gives a complete descrip-
tion of the part, operation and application informa-
tion. The datasheet must be read in conjunction
with this quick start guide for Demo circuit 1458.
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 Step-down Switching Regulator ( TA = 25oC )
PARAMETER FOR BUCK REGULATOR
Minimum Input Voltage
Maximum Input Voltage
Output Voltage V
OUT
Maximum Output Current
Typical Switching Frequency
CONDITION
VALUE
6.3V
33V
5V +/- 5%
3.5A
600kHz
QUICK START PROCEDURE
Demonstration circuit 1458 is easy to set up to
evaluate the performance of the LT3972. 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 on the RUN position.
2.
With power off, connect the input power supply
to Vin and GND.
3.
Turn on the power at the input.
4.
Check for the proper output voltage.
NOTE.
If there is no output, temporarily disconnect the load to
make sure that the load is not set too high.
1
LT3972
5.
Once the proper output voltage is established,
adjust the load within the operating range and
observe the output voltage regulation, ripple
voltage, efficiency and other parameters.
6.
An external clock can be added to the SYNC
pin when SYNC function is used. See syn-
chronization section in the datasheet for detail
+
_
A
+
+
V
+ _
_
+
_
V
_
A
+
Load
_
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
2
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