1
dc1736af
DEMO MANUAL DC1736A
DESCRIPTION
LT3975
42V, 2.5A, 2.7µA Micropower
Buck Regulator
Demonstration circuit 1736A is a monolithic step-down
DC/DC switching regulator featuring the LT
®
3975. The
demo circuit is designed for 3.3V, 2.5A output from a 4.3V
to 42V input. The wide input range of the LT3975 allows
a variety of input sources, such as automotive batteries
and industrial supplies. The switching frequency can be
programmed either via oscillator resistor or external clock
up to 2MHz. When the circuit is synchronized to an external
clock connected to the SYNC terminal, the R
T
resistor (R5)
should be chosen to set the LT3975 internal switching
frequency at least 20% below the lowest synchroniza-
tion input frequency. Low ripple Burst Mode
®
operation
increases the efficiency at the light load while keeping the
output ripple below 15mV.
The circuit consumes only 2.7μA of quiescent current.
The LT3975 internal boost diode and loop compensation
reduce the components count and solution size. The cur-
rent mode control scheme creates fast transient response
and good loop stability. The LT3975 is in shutdown when
L
, LT, LTC, LTM, μModule, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
PERFORMANCE SUMMARY
the EN pin is low and active when the pin is high. The
threshold of the EN pin is accurate at 1.02V with 60mV
of hysteresis. Users can populate R7 and R8 to provide a
programmable undervoltage lockout. During a short-circuit
fault, the LT3975 has current limit foldback to limit the
power dissipation.
The LT3975 data sheet gives a complete description of
the part, operation and application information. The data
sheet must be read in conjunction with this demo manual
for demo circuit 1736A. The LT3975 is assembled in a
16-lead plastic MSOP package with an exposed pad for
low thermal resistance. Proper board layout is essential
for both proper operation and maximum thermal perfor-
mance. See the data sheet section PCB Layout and High
Temperature Considerations.
Design files for this circuit board are available at
http://www.linear.com/demo
T
A
= 25°C
PARAMETER
CONDITIONS
VALUE
Minimum Input Voltage
4.3V
Maximum Input Voltage
42V
Output Voltage, V
OUT
V
IN
= 4.3V ~ 42V
3.3V ± 3%
Switching Frequency
R
T
= 78.7k
600kHz ±20%
Maximum Output Current, I
OUT
V
IN
= 4.3V ~ 42V
2.5A
Typical Efficiency
V
IN
= 12V, I
OUT
= 2.5A
82.1%
Typical Output Voltage Ripple
V
IN
= 12V, I
OUT
= 2.5A, 25MHz BW
20mV
