LTC3405 Linear Technology, LTC3405 Datasheet - Page 6

• Manufacturer Part Number: LTC3405
• Description: Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger
• Manufacturer: Linear Technology

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FU CTIO AL DIAGRA
LTC3405
OPERATIO
Main Control Loop
The LTC3405 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
6
MODE
RUN
V
1
6
5
FB
U
COMP
0.8V REF
V
IN
U
SHUTDOWN
, resets the RS latch. The peak inductor
COMP
U
0.85V
SHIFT
FREQ
(Refer to Functional Diagram)
OSC
resets the RS latch, is controlled by
0.8V
+
–
OVDET
+
–
EA
SLOPE
COMP
W
FB
–
+
OV
pin, described in
OSC
0.4V
feedback voltage from an external resistive divider. When
the load current increases, it causes a slight decrease in
the feedback voltage relative to the 0.8V reference, which
in turn, causes the EA amplifier’s output voltage to in-
crease 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
cycle.
RS LATCH
–
+
S
R
BURST
EN
Q
Q
SLEEP
SWITCHING
BLANKING
CIRCUIT
LOGIC
AND
RCMP
0.65V
, or the beginning of the next clock
–
I
SHOOT-
COMP
ANTI-
THRU
I
RCMP
+
+
–
5
sn3405 3405fs
3405 BD
4
3
2
V
SW
GND
IN