LTC3407-3 Linear Technology, LTC3407-3 Datasheet - Page 7

• Manufacturer Part Number: LTC3407-3
• Description: Dual Synchronous 1.8V/0.8A and 3.3V/0.8A 2.25MHz Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

OPERATIO
The LTC3407-3 uses a constant frequency, current mode
architecture. The operating frequency is set at 2.25MHz
and can be synchronized to an external oscillator. Both
channels share the same clock and run in-phase. To suit
a variety of applications, the selectable Mode pin allows
the user to choose between low noise and light load
efficiency.
The output voltage is set by an internal divider. An error
amplfier compares the divided output voltage with a
reference voltage of 0.6V and adjusts the peak inductor
current accordingly. An undervoltage comparator pulls
the POR output low if the output voltage is not above
–8.5% of the reference voltage. The POR output will go
high after 262,144 clock cycles of achieving regulation.
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 flows 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 internally
compensated I
amplifier.This amplifier compares the V
gram) to the 0.6V reference. When the load current in-
creases, the V
reference. This decrease causes the error amplifier to
increase the I
matches the new load current.
The main control loop is shut down by pulling the RUN pin
to ground.
Low Current Operation
Two modes are available to control the operation of the
LTC3407-3 at low currents. Both modes automatically
switch from continuous operation to the selected mode
when the load current is low.
OUT
TH
TH
voltage is below the the regulated voltage.
FB
voltage until the average inductor current
U
voltage, which is the output of the error
voltage decreases slightly below the
FB
(see Block Dia-
To optimize efficiency, the Burst Mode operation can be
selected. When the load is relatively light, the LTC3407-3
automatically switches into Burst Mode operation, in
which the PMOS switch operates intermittently based on
load demand with a fixed peak inductor current. By run-
ning 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. A
voltage comparator trips when I
ting off the switch and reducing the power. The output
capacitor and the inductor supply the power to the load
until I
control loop which starts another cycle.
For lower ripple noise at low currents, the pulse skipping
mode can be used. In this mode, the LTC3407-3 continues
to switch at a constant frequency down to very low
currents, where it will begin skipping pulses. The effi-
ciency in pulse skip mode can be improved slightly by
connecting the SW node to the MODE/SYNC input which
reduces the clock frequency by approximately 30%.
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.
An important design consideration is that the R
the P-channel switch increases with decreasing input
supply voltage (See Typical Performance Characteristics).
Therefore, the user should calculate the power dissipation
when the LTC3407-3 is used at 100% duty cycle with low
input voltage (See Thermal Considerations in the Applica-
tions Information Section).
Low Supply Operation
To prevent unstable operation, the LTC3407-3 incorpo-
rates an Undervoltage Lockout circuit which shuts down
the part when the input voltage drops below about 1.65V.
TH
exceeds 0.65V, turning on the switch and the main
TH
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is below 0.65V, shut-
LTC3407-3
DS(ON)
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