LTC3409 LINER [Linear Technology], LTC3409 Datasheet - Page 8

• Manufacturer Part Number: LTC3409
• Description: 600mA Low Vin Buck Regulator in 3mm x 3mm DFN
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC3409
FU CTIO AL DIAGRA
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
current at which I
the output of error amplifier EA. The V
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
8
MODE
SYNC
RUN
V
7
5
8
1
FB
U
COMP
REFERENCE
V
IN
U
, resets the RS latch. The peak inductor
SHUTDOWN
COMP
U
PLL
resets the RS latch is controlled by
0.613V
0.675
START
SOFT-
+
–
+
OVDET
–
EA
SLOPE
COMP
W
FB
OV
–
+
pin, described in
OSC
0.4V
starts to reverse, as indicated by the current reversal
comparator I
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
>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
RS LATCH
–
+
R
S
BURST
EN
Q
Q
SLEEP
SWITCHING
BLANKING
CIRCUIT
LOGIC
AND
RCMP
, or the beginning of the next clock
IN
0.65V
–
or drive it with a logic high (V
I
SHOOT-
COMP
ANTI-
THRU
I
RCMP
+
+
–
5Ω
3409 FD
3, 4
6
2
MODE
V
SW
GND
3409f
IN