LTC3406A LINER [Linear Technology], LTC3406A Datasheet - Page 7

• Manufacturer Part Number: LTC3406A
• Description: 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT
• Manufacturer: LINER [Linear Technology]

OPERATION
Main Control Loop
The LTC3406A 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
which I
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
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.
Burst Mode Operation
The LTC3406A is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand.
In Burst Mode operation, the peak current of the inductor
is set to approximately 100mA 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
COMP
, resets the RS latch. The peak inductor current at
COMP
resets the RS latch, is controlled by the output
(Refer to Functional Diagram)
RCMP
, or the beginning
off, reducing the quiescent current to 20μA. In this sleep
state, the load current is being supplied solely from the
output capacitor. As the output voltage droops, the EA
amplifi er’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 dependent on
the load demand.
Dropout Operation
As the input supply voltage decreases to a value approach-
ing the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one cycle
until it reaches 100% duty cycle. The output voltage will
then be determined by the input voltage minus the voltage
drop across the P-channel MOSFET and the inductor.
An important detail to remember is that at low input supply
voltages, the R
(see Typical Performance Characteristics). Therefore,
the user should calculate the power dissipation when
the LTC3406A is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current signal
at duty cycles in excess of 40%. Normally, this results in
a reduction of maximum inductor peak current for duty
cycles > 40%. However, the LTC3406A uses a patented
scheme that counteracts this compensating ramp, which
allows the maximum inductor peak current to remain
unaffected throughout all duty cycles.
DS(ON)
of the P-channel switch increases
LTC3406A
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