DEMO CIRCUIT 1526
LT3991
QUICK START GUIDE
LT3991
55V, 1.2A, MicroPower Buck Regulator
DESCRIPTION
Demonstration circuit 1526 is a monolithic step-
down DC/DC switching regulator featuring the
LT
®
3991. The switching frequency is adjustable
up to 2MHz. The demo circuit is designed for
3.3V, 1.2A output from a 4.5V to 55V input. The
wide input range of the LT3991 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 1.01V with 30mV of
hysteresis. When Vin is above 6V, adding a re-
sistor divider from Vin to EN can program the
LT3991 to regulate the output only when Vin is
above a desired voltage.
The circuit consumes only 2.8µA of quiescent
current. The boost diode, control and logic circuit
are integrated to reduce the components count
and solution size. The current mode control
scheme creates fast transient response and
good loop stability.
The LT3991 datasheet gives a complete de-
scription of the part, operation and application
information. The datasheet must be read in con-
junction with this quick start guide for demo cir-
cuit 1526.
Design files for this circuit board are avail-
able. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Performance Summary (TA = 25oC)
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Output Voltage V
OUT
Typical switching Frequency
Maximum Output Current
Typical efficiency
Typical output voltage Ripple
CONDITION
VALUE
4.5V
55V
3.307±2.5%
400kHz
1.2A
79.4%
20mV
V
IN
=24V, I
out
=1.2A
V
IN
=24V, I
out
=1.2A
QUICK START PROCEDURE
Demonstration circuit 1526 is easy to set up to
evaluate the performance of the LT3991. 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
LT3991
1.
2.
Place JP1 on the ON position.
With power off, connect the input
power supply to Vin and GND.
Turn on the power at the input.
Check for the proper output voltage.
5.
3.
Once the proper output voltage is
established, adjust the load within the
operating range and observe the out-
put voltage regulation, ripple voltage,
efficiency and other parameters.
NOTE.
Make sure that the input voltage does not exceed 55V.
4.
NOTE.
If there is no output, temporarily disconnect the load to
make sure that the load is not set too high.
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
VIN
GND
2
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