LT
3
686
1
DESCRIPTION
Demo circuit 1323 is a current-mode PWM step-down
DC/DC regulator with an internal 1.2A power switch fea-
turing the LT3686. The demo circuit is designed for 3.3V
output from a 5V to 37V input. The wide input range of
the LT3686 allows a variety of input sources. The typical
sources are automotive batteries and industrial supplies.
Its high maximum frequency allows the use of tiny in-
ductors and capacitors. Operating frequency above the
AM band avoids interfering with radio reception, making
the LT3686 particularly suitable for automotive applica-
tions.
Cycle by cycle current limit and DA current sense provide
the protection against shorted outputs. Soft-start and
frequency foldback eliminate input current surge during
startup. An optional internal regulated active load at the
output via the BD pin keeps the LT3686 at full switching
frequency at light loads, resulting in low, predictable out-
put ripple above the audio and AM bands. Internal com-
pensation and an internal boost diode reduce external
component count.
The LT3686 datasheet gives a complete description of
the part, operation and application information. The data-
sheet must be read in conjunction with this quick start
guide for Demo Circuit 1323.
LT3686 is available in 3mmx3mm DFN package.
Design files for this circuit board are available. Call
the LTC factory.
, LTC, LTM, LT and uModule are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Performance Summary ( TA = 25oC )
PARAMETER CONDITION VALUE
Minimum Input Voltage 5V
Maximum Input Voltage 37V
Output Voltage V
OUT
3.3V ±3%
Switching Frequency 1MHz
Maximum Output Current V
IN
=5~37V 1.2A
Efficiency V
IN
=12V, I
OUT
=1.0A 80.7%
Voltage Ripple V
IN
=12V, I
OUT
=1.0A 20mV
DEMO CIRCUIT 1323
QUICK START GUIDE
LT3686
37V, 1.2A,
Buck Regulator
LT
3
686
2
QUICK START PROCEDURE
Demonstration circuit 1323 is easy to set up to evaluate
the performance of the LT3686. 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 ON position
.
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 55V.
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.
Figure 1.
Proper Measurement Equipment Setup
VIN
GND
Figure 2.
Measuring Input or Output Ripple
LT
3
686
3