Datasheet
LTC3409
8
3409fc
FUNCTIONAL DIAGRAM
+
–
+
–
+
–
EA
+
–
OVDET
+
–
I
RCMP
+
–
I
COMP
1
7
RUN
PLL
SLOPE
COMP
OSC
0.613V
REFERENCE
SHUTDOWN
OV
0.675
0.4V
0.65V
SLEEP
EN
V
IN
V
FB
5
MODE
8
SYNC
BURST
V
IN
S
R
RS LATCH
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-
THRU
SOFT-
START
Q
Q
5Ω
3, 4
SW
6
GND
3409 FD
2
Main Control Loop
The LTC3409 uses a constant frequency, current mode step-
down architecture. Both the main (P-channel MOSFET) and
synchronous (N-channel MOSFET) switches are internal.
During normal operation, the internal top power MOSFET
is turned on each cycle when the oscillator sets the RS
latch, and turned off when the current comparator, I
COMP
,
resets the RS latch. The peak inductor current at which
I
COMP
resets the RS latch is controlled by the output of error
amplifi er EA. The V
FB
pin, described in the Pin Functions
section, allows EA to receive an output feedback voltage
from an external resistive divider. When the load current
increases, it causes a slight decrease in the feedback voltage
relative to the 0.613V reference, which in turn, causes the
EA amplifi er’s output voltage to increase until the average
inductor current matches the new load current. While the
top MOSFET is off, the bottom MOSFET is turned on until
either the inductor current starts to reverse, as indicated
by the current reversal comparator I
RCMP
, or the beginning
of the next clock cycle.
Comparator OVDET guards against transient overshoots
>10% by turning the main switch off and keeping it off
until the transient has ended.
Burst Mode Operation
The LTC3409 is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand. To enable Burst Mode operation, simply
connect the MODE pin to GND. To disable Burst Mode
operation and enable PWM pulse skipping mode, connect
the MODE pin to V
IN
or drive it with a logic high (V
MODE
>1.1V). In this mode, the effi ciency is lower at light loads,
but becomes comparable to Burst Mode operation when
the output load exceeds 30mA. The advantage of pulse
skipping mode is lower output ripple and less interference
OPERATION