LTC3409_15 Datasheet, PDF (8/16 Pages) Linear Technology – 600mA Low VIN Buck Regulator in 3mm 3mm DFN

LTC3409
FUNCTIONAL DIAGRAM
MODE
5
SYNC
8
SLOPE
COMP
PLL
OSC
V_FB
1
0.613V
+
–
EA
SOFT-
START
V_IN
RUN
7
REFERENCE
+
OV
OVDET
0.675 –
SHUTDOWN
0.65V
EN
–
SLEEP
0.4V
+
BURST
I_COMP
SQ
RQ
RS LATCH
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-
THRU
I_RCMP
V_IN
3, 4
5Ω
6 SW
2 GND
3409 FD
OPERATION
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
amplifier 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 amplifier’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
8
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 efficiency 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
3409fc