LTC3411 Linear Technology, LTC3411 Datasheet - Page 7

• Manufacturer Part Number: LTC3411
• Description: 1.25A/ 4MHz/ Synchronous Step-Down DC/DC Converter
• Manufacturer: Linear Technology

OPERATIO
The LTC3411 uses a constant frequency, current mode
architecture. The operating frequency is determined by
the value of the R
external oscillator. To suit a variety of applications, the
selectable Mode pin, allows the user to trade-off noise for
efficiency.
The output voltage is set by an external divider returned to
the V
voltage with a reference voltage of 0.8V and adjusts the
peak inductor current accordingly. Overvoltage and
undervoltage comparators will pull the PGOOD output low
if the output voltage is not within 7.5%.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle
when the V
The current into the inductor and the load increases until
the current limit is reached. The switch turns off and
energy stored in the inductor flows through the bottom
switch (N-channel MOSFET) into the load until the next
clock cycle.
The peak inductor current is controlled by the voltage on
the I
amplifier compares the V
When the load current increases, the V
creases slightly below the reference. This decrease causes
the error amplifier to increase the I
average inductor current matches the new load current.
The main control loop is shut down by pulling the SHDN/R
pin to SV
which will slowly ramp the peak inductor current up over
1024 clock cycles or until the output reaches regulation,
whichever is first. Soft-start can be lengthened by ramping
the voltage on the I
section).
Low Current Operation
Three modes are available to control the operation of the
LTC3411 at low currents. All three modes automatically
switch from continuous operation to to the selected mode
when the load current is low.
TH
FB
pin, which is the output of the error amplifier.This
pin. An error amplfier compares the divided output
IN
. A digital soft-start is enabled after shutdown,
FB
voltage is below the the reference voltage.
U
T
resistor or can be synchronized to an
TH
pin (see Applications Information
FB
pin to the 0.8V reference.
TH
voltage until the
FB
voltage de-
T
To optimize efficiency, the Burst Mode operation can be
selected. When the load is relatively light, the LTC3411
automatically switches into Burst Mode operation in which
the PMOS switch operates intermittently based on load
demand. By running cycles periodically, the switching
losses which are dominated by the gate charge losses of
the power MOSFETs are minimized. The main control loop
is interrupted when the output voltage reaches the desired
regulated value. The hysteretic voltage comparator B trips
when I
reducing the power. The output capacitor and the inductor
supply the power to the load until I
turning on the switch and the main control loop which
starts another cycle.
For lower output voltage ripple at low currents, pulse
skipping mode can be used. In this mode, the LTC3411
continues to switch at a constant frequency down to very
low currents, where it will eventually begin skipping pulses.
Finally, in forced continuous mode, the inductor current is
constantly cycled which creates a fixed output voltage
ripple at all output current levels. This feature is desirable
in telecommunications since the noise is at a constant
frequency and is thus easy to filter out. Another advantage
of this mode is that the regulator is capable of both
sourcing current into a load and sinking some current
from the output.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which is
the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being
equal to the input voltage minus the voltage drops across
the internal P-channel MOSFET and the inductor.
Low Supply Operation
The LTC3411 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.5V to prevent unstable operation.
TH
is below 0.24V, shutting off the switch and
TH
/RUN exceeds 0.31V,
LTC3411
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