Datasheet

LTC3561A
9
3561af
OPERATION
The LTC3561A uses a constant frequency, current mode
architecture. The operating frequency is determined by
the value of the R
T
resistor.
The output voltage is set by an external divider returned to
the V
FB
pin. An error amplifi er compares the divided output
voltage with the reference voltage of 0.8V and adjusts the
peak inductor current accordingly.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle.
Current fl ows through this switch into the inductor and
the load, increasing until the peak inductor current reaches
the limit set by the voltage on the I
TH
pin. Then the top
switch is turned off, the bottom switch is turned on, and
the energy stored in the inductor forces the current to fl ow
through the bottom switch, and the inductor, out into the
load until the next clock cycle.
The peak inductor current is controlled by the voltage
on the I
TH
pin, which is the output of the error amplifi er.
The output is developed by the error amplifi er comparing
the feedback voltage, V
FB
, to the 0.8V reference voltage.
When the load current increases, the output voltage and
V
FB
decrease slightly. This decrease in V
FB
causes the er-
ror amplifi er to increase the I
TH
voltage until the average
inductor current matches the new load current.
The main control loop is shut down by pulling the SHDN/R
T
pin to SV
IN
, resetting the internal soft-start. Re-enabling
the main control loop by releasing the SHDN/R
T
pin
activates the internal soft-start, which slowly ramps the
output voltage over approximately 0.8ms until it reaches
regulation.
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 drop across
the internal P-channel MOSFET and the inductor.
Low Supply Operation
The LTC3561A incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.1V to prevent unstable operation.