Datasheet

LTC3789
14
3789fa
operaTion
The duty cycle of switch C decreases until the minimum
duty cycle of the converter reaches D
MIN_BOOST
, given by:
D
MIN_BOOST
=
1
12






• 100% = 8.33%
Figure 5 shows typical boost region waveforms. If V
IN
approaches V
OUT
, the buck-boost region is reached.
(4a) Buck-Boost Region (V
IN
≥ V
OUT
)
(4b) Buck-Boost Region (V
IN
≤ V
OUT
)
Figure 4. Buck-Boost Region
Light Load Current Operation
The LTC3789 can be enabled to enter pulse-skipping mode
or forced continuous conduction mode. To select forced
continuous operation, tie the MODE/PLLIN pin to a DC
voltage below 0.8V (e.g., SGND). To select pulse-skipping
mode of operation, tie the MODE/PLLIN pin to INTV
CC
.
When the LTC3789 enters pulse-skipping mode, in the
boost region, synchronous switch D is held off whenever
reverse current through switch A is detected. At very
light loads, the current comparator, I
CMP
, may remain
tripped for several cycles and force switch C to stay off
for the same number of cycles (i.e., skipping pulses). In
the buck region, the inductor current is not allowed to re-
verse. Synchronous switch B is held off whenever reverse
current on the inductor is detected. At very light loads,
the current comparator, I
CMP
, 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 region 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.
Output Overvoltage
If the output voltage is higher than the value commanded
by the V
FB
resistor divider, the LTC3789 will respond ac-
cording to the mode and region of operation. In continuous
conduction mode, the LTC3789 will sink current into the
input. If the input supply is capable of sinking current, the
LTC3789 will allow up to about 160mV/R
SENSE
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 continu-
ous boost operation that adjoins it, current sunk into the
input through switchA is
limited to approximately 40mV/
R
DS(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 R
SENSE
is limited to approximately 40mV/R
SENSE
.
Figure 5. Boost Region (V
IN
<< V
OUT
)
SWITCH A
CLOCK
SWITCH B
SWITCH C
SWITCH D
I
L
3789 F04a
SWITCH A
CLOCK
SWITCH B
SWITCH C
SWITCH D
I
L
3789 F04b
SWITCH A
CLOCK
SWITCH B
SWITCH C
SWITCH D
I
L
0V
HIGH
3789 F05