6
LTC3402
3402fb
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 and high
ringing down. For discontinuous inductor current, a
controlled impedance is placed from SW to V
IN
from the
UU
U
PI FU CTIO S
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 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, refer to the section
“Output Disconnect Circuits.”
