LTC3415 Datasheet, PDF (8/28 Pages) Linear Technology – 7A, PolyPhase Synchronous Step-Down Regulator

LTC3415
OPERATION
Main Control Loop
The LTC3415 is a constant frequency, current mode,
monolithic step down regulator. In normal operation,
the internal top P-channel power MOSFET turns on each
cycle when the oscillator sets the RS latch, and turns
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 voltage on the I_TH pin,
which is the output of error amplifier EA. The FB pin
allows EA to receive an output feedback voltage from
an external resistive divider. When the load current in-
creases, it causes a slight decrease in the feedback volt-
age relative to the 0.596V reference, which in turn causes
I_TH voltage to increase until the average inductor current
matches the new load current. While the top P-channel
power MOSFET is off, the bottom N-Channel power MOSFET
is turned on until either the inductor current starts to re-
verse, as indicated by the current reversal comparator
I_RCMP, or the beginning of the next cycle.
The main control loop is shut down by pulling the RUN pin
below 1.5V (V_TRACK = SV_IN or V_TRACK < 0.18V). Tying RUN
higher than 1.5V allows operation to begin. To control the
rise time of the output, a voltage ramp can be applied to
the TRACK pin. The FB voltage will servo to the TRACK
voltage until TRACK goes above 0.57V, which is when
PGOOD is high and the output is in normal regulation.
If TRACK is not used (tied high), then an internal 100μs
soft-start will ramp up the output.
Burst Mode Operation
The LTC3415 is capable of Burst Mode operation in which
the power MOSFETs operate intermittently based on load
demand, thus saving quiescent current. For applications
where maximizing the efficiency at very light loads is a
high priority, Burst Mode operation should be applied. To
enable Burst Mode operation, simply tie the MODE pin to
V_IN. During this operation, the peak current of the inductor
is set to approximately 20% of the maximum peak current
value in normal operation even though the voltage at the
I_TH pin indicates a lower value. The voltage at the I_TH pin
drops when the inductor’s average current is greater than
the load requirement. As the I_TH voltage drops below 0.2V,
the BURST comparator trips, causing the internal sleep
line to go high and turn off both power MOSFETs.
In sleep mode, the internal circuitry is partially turned off,
reducing the quiescent current to about 450μA. The load
current is now being supplied from the output capacitor.
When the output voltage drops, causing I_TH to rise above
0.25V, the internal sleep line goes low, and the LTC3415
resumes normal operation. The next oscillator cycle will
turn on the top power MOSFET and the switching cycle
repeats.
Pulse-Skipping Mode Operation
In applications where fixed frequency operation, low
output ripple and high efficiency at intermediate current
is desired, pulse-skipping mode should be used. Pulse-
skipping operation allows the LTC3415 to skip cycles at
low output loads, thus increasing efficiency by reducing
switching current. Tying the MODE pin to V_IN/2 enables
pulse-skipping operation. This allows discontinuous
conduction mode (DCM) operation down to near the limit
defined by the chip’s minimum on-time (about 100ns).
Below this output current level, the converter will begin
to skip cycles in order to maintain output regulation.
Increasing the output load current slightly, above the
minimum required for discontinuous conduction mode,
allows constant frequency PWM.
Forced Continuous Operation
In applications where fixed frequency operation is more
critical than low current efficiency, and where the lowest
output ripple is desired, forced continuous operation should
be used. Forced continuous operation can be enabled by
tying the MODE pin to GND. In this mode, inductor cur-
rent is allowed to reverse during low output loads, the I_TH
voltage is in control of the current comparator threshold
throughout, and the top MOSFET always turns on with each
oscillator pulse. During start-up, forced continuous mode is
disabled and inductor current is prevented from reversing
until the LTC3415’s output voltage is in regulation.
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