LTC3560_15 Datasheet, PDF (7/16 Pages) Linear Technology – 2.25MHz, 800mA Synchronous Step-Down Regulator in ThinSOT

LTC3560
OPERATION (Refer to Functional Diagram)
Main Control Loop
The LTC3560 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 0.6V 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.
Burst Mode Operation
The LTC3560 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 SYNC/MODE pin to GND. To disable Burst
Mode operation and enable PWM pulse-skipping mode,
connect the SYNC/MODE pin to V_IN or drive it with a logic
high (V_MODE > 1.5V). In this mode, the efficiency is lower
at light loads, but becomes comparable to Burst Mode
operation when the output load exceeds 100mA. 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 peak
current of the inductor is set to approximately 150mA
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 16μA. 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 dependent on the load demand.
Frequency Synchronization
When the LTC3560 is clocked by an external source, Burst
Mode operation is disabled; the LTC3560 then operates in
PWM pulse-skipping mode. In this mode, when the output
load is very low, current comparator I_COMP may remain
tripped for several cycles and force the main switch to stay
off for the same number of cycles. Increasing the output
load slightly allows constant frequency PWM operation
to resume. This mode exhibits low output ripple as well
as low audio noise and reduced RF interference while
providing reasonable low current efficiency.
Dropout Operation
As the input supply voltage decreases to a value approaching
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.
Another important detail to remember is that at low input
supply voltages, the R_DS(ON) of the P-channel switch
increases (see Typical Performance Characteristics).
Therefore, the user should calculate the power dissipation
when the LTC3560 is used at 100% duty cycle with low
input voltage (See Thermal Considerations in the Applica-
tions Information section).
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