QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
LT1765EFE
DESCRIPTION
Demonstration circuit 476 is a 1.25MHz 3A monolithic
step-down DC/DC switching converter using the
LT1765EFE. The LT1765 features fast switching speed,
a 3A internal power switch, and a wide input voltage
range, making it a versatile and powerful IC that fits
easily into space-constrained applications. The con-
stant 1.25MHz switching frequency allows for the use
of tiny, surface mount external components. The cur-
rent-mode control topology yields fast transient re-
sponse and good loop stability, requiring a minimum
number of external compensation components and
allowing the use of ceramic input and output capaci-
tors. The low R
DS(ON)
internal power switch (0.09
Ω
)
maintains high efficiencies (as high as 90%) over a
wide range of input voltages and loads. Its 15µA shut-
down current (activated via the SHDN pin) extends
battery life. The wide V
IN
range of the LT1765 allows
step-down configurations from 3V to 25V
input. Syn-
chronization of switching frequencies up to 2MHz is
possible.
The demonstration circuit is designed to provide either
3.3V at 2A or 5V at 2A output, from a 7V–25V input or
4.7V–25V
input, respectively, covering the common
values used in cable modems, handhelds, automotive,
and desktop computer applications. The 5V or 3.3V
output voltage is jumper selectable.
This board is designed for applications that require 2A
of load current from a wide input voltage range plus
simplicity, small circuit size, and low component
count. The use of ceramic capacitors in this circuit not
only demonstrates small size and low cost, but the
advantage of current-mode control in step-down ap-
plications with a simple compensation network and a
feedforward capacitor for more rugged stability and
excellent transient response.
Design files for this circuit board are available. Call
the LTC factory.
†
Higher input voltages may pulse-skip due to minimum on-time restrictions.
Compensation component changes may be necessary to optimize pulse-
skipping during high-temperature, high-voltage conditions and maintain
control of switch current.
Figure 1.
Demonstration Circuit 476
1
