Linear Technology LT3971 User manual

1
LT
3
97
1
DESCRIPTION
Demonstration circuit 1527 is a monolithic step-down
DC/DC switching regulator featuring the LT3971 The
switching frequency is adjustable up to 2MHz The
demo circuit is designed for 3 3V, 1 2A output from a
4 5V to 38V input The wide input range of the LT3971
allows a variety of input sources including automotive
batteries and 24V industrial supplies Low ripple Burst
Mode increases the efficiency at the light load while
keeping the output ripple below 15mV The part is in
shutdown when the EN pin is low and active when the
pin is high The threshold of the EN pin is accurate at
1V with 30mV of hysteresis When Vin is above 6V,
adding a resistor divider from Vin to EN can program
the LT3971 to regulate the output only when Vin is
above a desired voltage
The circuit consumes only 2 8 µA of quiescent current
The boost diode, control and logic circuit are inte-
grated to reduce the components count and solution
size The current mode control scheme creates fast
transient response and good loop stability
The LT3971 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 1527
Design files for this circuit board are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation
Performance Summary (TA = 2 oC)
PARAMETER CONDITION VALUE
Minimum Input Voltage 4 5V
Maximum Input Voltage 38V
Output Voltage V
OUT
3 307±2 5%
Typical switching Frequency 800kHz
Maximum Output Current 1 2A
Typical efficiency V
IN
=12V, I
out
=1 2A 82 43%
Typical output voltage Ripple V
IN
=12V, I
out
=1 2A 10mV
QUICK START PROCEDURE
Demonstration circuit 1527 is easy to set up to
evaluate the performance of the LT3971 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. Place JP2 on the RT FREQ position
3. With power off, connect the input
power supply to Vin and GND
4. Turn on the power at the input
NOTE. Make sure that the input voltage does not exceed 38V
5. Check for the proper output voltage
D
EMO
C
IRCUIT 1
5
27
QUICK START GUIDE
LT397
1
38V, 1.2A
Micropo er Buck Regulator

2
LT
3
97
1
NOTE. If there is no output, temporarily disconnect the load to
make sure that the load is not set too high
6. Once the proper output voltage is es-
tablished, adjust the load within the op-
erating range and observe the output
voltage regulation, ripple voltage, effi-
ciency and other parameters
Figure 1. Proper Measurement Equipment Setup
VIN GND
Figure 2. Measuring Input or Output Ripple

3
LT
3
97
1