DC419A – LTC1779 Constant Frequency Step-Down Converter
Description
Demonstration Circuit DC419A is a 550kHz 250mA step-down DC/DC switching
converter using the LTC1779. With its internal power switch and ThinSOT package, the
LTC1779 makes a very compact and low-parts-count DC/DC converter. The 550kHz
switching frequency allows all of the components to be small, surface mount devices.
The current-mode control topology creates fast transient response and good loop stability
with a minimum number of external compensation components. The 100% duty cycle
feature provides very low dropout voltages and high efficiency. The 8uA shutdown
current extends battery life. The wide Vin range of the LTC1779 allows step-down
configurations from 2.5Vin up to 9.8Vin.
Demonstration Circuit DC419A is a step-down converter using the LTC1779. The
circuit is designed for a single or dual Lithium-Ion battery input to 1.8V output at 250mA
for portable electronics. This board was designed for low power, hand-held applications
where small circuit size and low component count are important. The external sense
resistor is set to reduce the short circuit current limit between 260mA and 300mA over
the input voltage ranges of 2.7V to 4.2V for single Li-Ion batteries and 5.4V to 8.4V for
dual. The components built on the board are optimized for single Li-Ion battery
applications, but can be simply adjusted for dual or higher input voltages up to 9.8Vin
and down to 2.5Vin.
Typical Performance Summary (TA = 25C)
Step-Down Converter
Vin 2.7 to 4.2V
Vout 1.8V
Iout 250mA(max)
Eff. up to 85% at 250mA and up to 91% at 100mA output
Short Circuit up to 270mA output current
Quick Start Guide
Refer to Figure 1 for proper measurement equipment setup and follow the procedure
outlined below:
1. Connect the 2.7 to 4.2V input power supply to the Vin and Gnd terminals on the
board.
2. Connect an ammeter in series with the input supply to measure input current.
3. Connect either power resistors or an electronic load to the Vout and Gnd
terminals on the board.
4. Connect an ammeter in series with output load to measure output current.
5. Connect a voltmeter across the Vin and Gnd terminals to measure input voltage.
6. Connect a voltmeter across the Vout and Gnd terminal to measure output voltage.
7. After all connections are made, turn on input power and verify that the output
voltage is 1.8V.
Figure 1… Quick Start connections for DC419A.
