LTC3544B_15 Datasheet, PDF (9/18 Pages) Linear Technology – Quad Synchronous Step-Down Regulator

LTC3544B
Operation
Main Control Loop
The LTC3544B 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 rela-
tive to the 0.8V 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.
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 LTC3544B will automatically skip pulses
to maintain output regulation.
Soft-Start
Soft-start reduces surge currents on V_IN and output
overshoot during start-up. Soft-start on the LTC3544B is
implemented by internally ramping the reference signal
fed to the error amplifier over approximately a 1ms period.
Figure 1 shows the behavior of the four regulator channels
during soft-start.
V_OUT100
V_OUT200A
V_OUT200B
V_OUT300
RUNx
V_IN = 3.6V
200µs/DIV
T_A = 25°C
ALL CHANNELS UNLOADED
3544B F01
Figure 1. Regulator Soft-Start
Short-Circuit Protection
Short circuit protection is achieved by monitoring the in-
ductor current. When the current exceeds a predetermined
level, the main switch is turned off, and the synchronous
switch is turned on long enough to allow the current in the
inductor to decay below the fault threshold. This prevents
a catastrophic inductor current, run-away condition, but
will still provide current to the output. Output voltage
regulation in this condition is not achieved.
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_DS(ON) of the P-channel switch increases
(see Typical Performance Characteristics). Therefore,
the user should calculate the power dissipation when
the LTC3544B is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
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