Datasheet

LTC3406AB
6
3406abfa
FUNCTIONAL DIAGRAM
+
–
+
–
EA
+
–
I
RCMP
+
–
I
COMP
5
1
RUN
OSC
SLOPE
COMP
OSC
FREQ
SHIFT
0.6V
0.6V REF
SHUTDOWN
V
IN
V
FB
V
IN
S
R
RS LATCH
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-
THRU
Q
Q
5Ω
4
SW
3
GND
3406AB BD
2
OPERATION
(Refer to Functional Diagram)
Main Control Loop
The LTC3406AB 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. When the load current increases,
it causes a slight decrease in the feedback voltage, FB,
relative to the 0.6V 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.
The main control loop is shut down by grounding RUN,
resetting the internal soft-start. Re-enabling the main
control loop by pulling RUN high activates the internal
soft-start, which slowly ramps the output voltage over
approximately 0.9ms until it reaches regulation.
Pulse Skipping Mode Operation
At light loads, the inductor current may reach zero or
reverse on each pulse. The bottom MOSFET is turned off
by the current reversal comparator, I
RCMP
, and the switch
voltage will ring. This is discontinuous mode operation,
and is normal behavior for the switching regulator. At
very light loads, the LTC3406AB will automatically skip
pulses in pulse skipping mode operation to maintain
output regulation. Refer to the LTC3406A data sheet if
Burst Mode operation is preferred.