DEMO CIRCUIT 1647
37V, 1.2A, Buck Regulator
Demontration circuit 1647 is a current-mode PWM step- frequency at light loads, resulting in low, predictable out-
down DC/DC regulator with an internal 1.2A power put ripple above the audio and AM bands. Internal com-
switch featuring the LT3686. The demo circuit is de- pensation and an internal boost diode reduce external
signed for 3.3V output from a 5V to 37V input. The wide component count.
input range of the LT3686 allows a variety of input The LT3686 datasheet gives a complete description of
sources. The typical sources are automotive batteries the part, operation and application information. The data-
and industrial supplies. Its high maximum frequency al- sheet must be read in conjunction with this quick start
lows the use of tiny inductors and capacitors. Operating guide for Demo Circuit 1647.
frequency above the AM band avoids interfering with
radio reception, making the LT3686 particularly suitable LT3686 is available in 3mmx3mm DFN package.
for automotive applications. Design files for this circuit board are available. Call
Cycle by cycle current limit and DA current sense provide the LTC factory.
the protection against shorted outputs. Soft-start and , LTC, LTM, LT and uModule are registered trademarks of Linear Technology Corporation.
frequency foldback eliminate input current surge during All other trademarks are the property of their respective owners.
startup. An optional internal regulated active load at the
output via the BD pin keeps the LT3686 at full switching
Performance Summary ( TA = 25oC )
PARAMETER CONDITION VALUE
Minimum Input Voltage 5V
Maximum Input Voltage 37V
Output Voltage VOUT 3.3V ±3%
Switching Frequency 1MHz
Maximum Output Current VIN=5~37V 1.2A
Efficiency VIN=12V, IOUT=1.0A 80.7%
Voltage Ripple VIN=12V, IOUT=1.0A 20mV
QUICK START PROCEDURE
Demonstration circuit 1647 is easy to set up to evaluate 3. Turn on the power at the input.
the performance of the LT3686. Refer to Figure 1 for NOTE. Make sure that the input voltage does not exceed 55V.
proper measurement equipment setup and follow the 4. Check for the proper output voltage.
NOTE. When measuring the input or output voltage ripple, care must be NOTE. If there is no output, temporarily disconnect the load to make
taken to avoid a long ground lead on the oscilloscope probe. Measure the sure that the load is not set too high.
input or output voltage ripple by touching the probe tip directly across the 5. Once the proper output voltage is established, adjust
Vin or Vout and GND terminals. See Figure 2 for proper scope probe the load within the operating range and observe the
technique. output voltage regulation, ripple voltage, efficiency
1. Place JP1 on the ON position. and other parameters.
2. With power off, connect the input power supply to Vin
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
Click to View Pricing, Inventory, Delivery & Lifecycle Information:
Analog Devices Inc.: