LTC3549 Datasheet, PDF (8/16 Pages) Linear Technology – 250mA Low VIN Buck Regulator in 2mm × 3mm DFN

LTC3549
U
OPERATIO
Main Control Loop
The LTC3549 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 oscil-
lator 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 0.611V 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 LTC3549 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 operation when
the output load exceeds 50mA. 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 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
off, reducing the quiescent current to 50µA. In this sleep
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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.
Short-Circuit Protection
When the output is shorted to ground the LTC3549 limits the
synchronous switch current to 0.45A. If this limit is exceeded,
the top power MOSFET is inhibited from turning on until
the current in the synchronous switch falls below 0.45A.
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.
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 LTC3549 is used at 100% duty cycle with low
input voltage (see Thermal Considerations in the Applica-
tions Information section).
Slope Compensation
Slope compensation provides stability in constant-fre-
quency architectures by preventing subharmonic oscil-
lations 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%.
Internal Soft-Start
At start-up when the RUN pin is brought high, the internal
reference is linearly ramped from 0V to 0.611V in 1ms. The
regulated feedback voltage will follow this ramp, resulting
in the output voltage ramping from 0% to 100% in 1ms.
The average current in the inductor during soft-start will
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