LTC3409 Datasheet, PDF (8/16 Pages) Linear Technology – 600mA Low Vin Buck Regulator in 3mm x 3mm DFN

LTC3409
W
FU CTIO AL DIAGRA
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
U
OPERATIO
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 com-
parator, 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 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
3409f
8