LTC3406AB Datasheet, PDF (6/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406AB
FUNCTIONAL DIAGRAM
SLOPE
COMP
OSC
OSC
FREQ
SHIFT
V_FB
5
0.6V ⁺
– ^EA
V_IN
RUN
1
0.6V REF
SHUTDOWN
I_COMP
SQ
RQ
RS LATCH
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-
THRU
I_RCMP
4 V_IN
5Ω
3 SW
2 GND
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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 amplifier 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 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.
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.
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