LT3695
1
DESCRIPTION
Demonstration circuit 1578A-A and 1578A-B are micro-
power monolithic step-down DC/DC switching regulators
featuring LT3695-3.3 and LT3695-5 respectively. The
demo boards have fixed output voltages of 3.3V and 5V.
The wide input range of the LT3695 allows a variety of
input sources. The typical sources are automotive batter-
ies, wall adaptors and industrial supplies. The switching
frequency can be programmed either via oscillator resis-
tor or external clock. The modes of operation (Burst
Mode or synchronization) are jumper selectable. The
Burst Mode operation increases the efficiency at light
loads.
The LT3695 family tolerates adjacent pin shorts or an
open pin without raising the output voltage above its pro-
grammed value. NC pins on the LT3695 family must be
left floating. 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 saturation of the switch.
The LT3695 family’s integrated boost diode reduces the
parts count. The RUN/SS pin can be used to set the part
in micropower shutdown mode, reducing the supply cur-
rent 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 in-
put current surge during start-up. The LT3695 family can
run in low ripple burst mode or pulse skipping mode.
JP2 selects the mode. When in burst mode, the no load
supply current is reduced to 80uA for the demo circuit
1578A-A and 100uA for the demo circuit 1578A-B.
The LT3695 family datasheet gives a complete descrip-
tion of the part, operation and application information.
The datasheet must be read in conjunction with this
quick start guide for demo circuit 1578A-A and 1578A-B.
Design files for this circuit board are available. Call
the LTC factory.
LTM, LTC, LT and µModule are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Performance Summary for Step-down Switching Regulator ( TA = 25oC )
PARAMETER FOR BUCK REGULATOR CONDITION VALUE
Minimum Input Voltage Demo Circuit 1578A-A
Demo Circuit 1578A-B
4.5V
6.9V
Maximum Input Voltage Operating
Transient
36V
60V
Output Voltage V
OUT
Demo Circuit 1578A-A
Demo Circuit 1578A-B
3.3V +/- 1.5%
5V +/- 1.5%
Maximum Output Current 1A
Typical Efficiency Demo Circuit 1578A-A; V
IN
= 12V; I
OUT
= 1A
Demo Circuit 1578A-B; V
IN
= 12V; I
OUT
= 1A
85%
88%
Typical Switching Frequency 800kHz
QUICK START PROCEDURE
Demonstration circuit 1578A-A and 1578A-B are easy to
set up to evaluate the performance of the LT3695-3.3
and LT3695-5. 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
V
IN
or V
OUT
and GND terminals. See Figure 2 for proper scope probe
technique.
DEMO
CIRCUIT
78
A
QUICK START GUIDE
LT3695
-
3.3 and LT3695
-
5
1A Fault Tolerant Micropower Step-
down
Switching
Regulator
LT3695
-
3.3
AND
LT3695
-
5
2
1.
Place JP1 = ON; JP2 = Burst Mode.
2.
With power off, connect the input power supply to
V
IN
and GND. Connect load to V
OUT
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.
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 synchronization
section in the datasheet for details.
Figure 1.
Proper Measurement Equipment Setup
Figure 2.
Measuring Input or Output Ripple
LT3695
-
3.3
AND
LT3695
-
5
3