LTC3779
17
Rev A
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OPERATION
Synchronous switch B is held off whenever reverse
current on the inductor is detected. At very light loads,
the current comparator, ICMP, may remain untripped for
several cycles, holding switch A off for the same number
of cycles. Synchronous switch B also remains off for the
skipped cycles. In the buck-boost region, the controller
operates alternatively in boost and buck regions in one
clock cycle, as in continuous operation. A small amount
of reverse current is allowed, to minimize ripple. For the
same reason, a narrow band of continuous buck and
boost operation is allowed on the high and low line ends
of the buck-boost region.
Forced Continuous Mode: The forced continuous
mode allows the inductor current to reverse directions
without any switches being forced “off” to prevent this
from happening. At very light load currents the inductor
current will swing positive and negative as the appropriate
average current is delivered to the output. During soft-
start, if the SS pin is lower than V
FB
, the part will be forced
into discontinuous mode to prevent pulling current from
the output to the input. After SS voltage crosses VFB or
1.32V, whichever is lower, forced continuous mode will
be enabled.
Output Overvoltage
If the output voltage is higher than the value commanded
by the VFB resistor divider, the LTC3779 will respond
according to the mode and region of operation. In
continuous conduction mode, the LTC3779 will sink
current into the input. If the input supply is capable of
sinking current, the LTC3779 will allow up to about 80mV/
RSENSE to be sunk into the input. In pulse-skipping mode
and in the buck or boost regions, switching will stop
and the output will be allowed to remain high. In pulse-
skipping mode, and in the buck-boost region as well as the
narrow band of continuous boost operation that adjoins
it, current sunk into the input through switch A is limited
to approximately 40mV/ RDS(ON) of switch A. If this level
is reached, switching will stop and the output will rise. In
pulse-skipping mode, and in the narrow continuous buck
region that adjoins the buck/ boost region, current sunk
into the input through RSENSE is limited to approximately
40mV/RSENSE.
Voltage Regulation Loop
The LTC3779 provides a constant-voltage regulation
loop, for regulating the output voltage. A resistor divider
between VOUT, VFB and GND senses the output voltage.
As with traditional voltage regulators, when V
FB
rises near
or above the reference voltage of EA (1.2V typical, see
Block Diagram), the ITH voltage is reduced to command
the amount of current that keeps VOUT regulated to the
desired voltage.
Constant-Current Regulation (IAVGSNSP and
IAVGSNSN Pins)
The LTC3779 provides a constant-current regulation
loop for either input or output current. A sensing resistor
close to the input or output capacitor will sense the
input or output current. When the current exceeds the
programmed current limit, the voltage on the ITH pin
will be pulled down to maintain the desired maximum
input or output current. The input current limit function
prevents overloading the DC input source, while the
output current limit provides a building block for battery
charger or LED driver applications. It can also serve as
an extra current limit protection for a constant-voltage
regulation application. The input or output current limit
function has an operating voltage range of GND to the
absolute maximum VIN or VOUT, respectively.
Frequency Selection and Phase-Locked Loop (FREQ
and PLLIN Pins)
The selection of switching frequency is a trade-off
between efficiency and component size. Low frequency
operation increases efficiency by reducing MOSFET
switching losses, but requires larger inductance and/or
capacitance to maintain low output ripple voltage. The
switching frequency of the LTC3779’s controllers can be
selected using the FREQ pin. If the SYNC pin is not being
driven by an external clock source, the FREQ pin can be
used to program the controller’s operating frequency
from 50kHz to 600kHz.
Switching frequency is determined by the voltage on
the FREQ pin. Since there is a precision 20µA current
flowing out of the FREQ pin, the user can program the