LTC3409A_15 Datasheet, PDF (8/16 Pages) Linear Technology – 600mA Low VIN Buck Regulator in 3mm 3mm DFN

LTC3409A
OPERATION
Main Control Loop
The LTC3409A 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. 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 0.612V 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 LTC3409A 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 opera-
tion when the output load exceeds 30mA. The advantage
of pulse-skipping mode is lower output ripple and less
interference to audio circuitry. When the converter is in
Burst Mode operation, the minimum peak current of the
inductor is set to approximately 200mA regardless of the
output load. Each burst event can last from a few cycles
at light loads to almost continuously cycling with short
sleep intervals at moderate loads. In between these burst
events, the power MOSFETs and any unneeded circuitry
are turned off, reducing the quiescent current to 65μA. In
this sleep state, the load current is being supplied solely
from the output capacitor. As the output voltage droops,
the EA amplifier’s output rises above the sleep threshold
signaling the BURST comparator to trip and turn the top
MOSFET on. This process repeats at a rate that is depen-
dent on the load demand.
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