6
LTC3401
3401fb
R
t
(Pin 1): Timing Resistor to Program the Oscillator
Frequency.
f
R
Hz
OSC
t
=
310
10
•
MODE/SYNC (Pin 2): Burst Mode Select and Oscillator
Synchronization.
MODE/SYNC = High. Enable Burst Mode operation. The
inductor peak inductor current will be 1/3 the current
limit value and return to zero current on each cycle.
During Burst Mode operation the operation is variable
frequency, providing a significant efficiency improve-
ment at light loads. It is recommended the Burst Mode
operation only be entered once the part has started up.
MODE/SYNC = Low. Disable Burst Mode operation and
maintain low noise, constant frequency operation.
MODE/SYNC = External CLK. Synchronization of the
internal oscillator and Burst Mode operation disable. A
clock pulse width of 100ns to 2μs is required to
synchronize.
V
IN
(Pin 3): Input Supply Pin.
SW (Pin 4): Switch Pin. Connect inductor and Schottky
diode here. For applications with output voltages over
4.3V, a Schottky diode is required to ensure that the SW
pin voltage does not exceed its absolute maximum rating.
Minimize trace length to keep EMI down. For discontinu-
ous inductor current, a controlled impedance is placed
UU
U
PI FU CTIO S
from SW to V
IN
from the IC to eliminate high frequency
ringing due to the resonant tank of the inductor and SW
node capacitance, therefore reducing EMI radiation.
GND (Pin 5): Signal and Power Ground for the IC.
PGOOD (Pin 6): Power Good Comparator Output. This
open-drain output is low when V
FB
< –9% from its
regulation voltage.
V
OUT
(Pin 7): Output of the Synchronous Rectifier and
Bootstrapped Power Source for the IC. A ceramic capaci-
tor of at least 1μF is required and should be located as
close to the V
OUT
and GND pins as possible (Pins 7 and 5).
FB (Pin 8): Feedback Pin. Connect resistor divider tap
here. The output voltage can be adjusted from 2.6V to
5.5V. The feedback reference voltage is typically 1.25V.
V
C
(Pin 9): Error Amp Output. A frequency compensation
network is connected to this pin to compensate the loop.
See the section “Compensating the Feedback Loop” for
guidelines.
SHDN (Pin 10): Shutdown. Grounding this pin shuts down
the IC. Tie to >1V to enable (V
IN
or digital gate output). To
operate with input voltages below 1V once the converter
has started, a 1M resistor from SHDN to V
IN
, and a 5M
resistor from SHDN to V
OUT
will provide sufficient hyster-
esis During shutdown the output voltage will hold up to V
IN
minus a diode drop due to the body diode of the PMOS
synchronous switch. If the application requires a com-
plete disconnect during shutdown then refer to section
“Output Disconnect Circuits”.
