Datasheet
LT3845A
12
3845afa
APPLICATIONS INFORMATION
latched onto the IC control path. If the output load requires
less than this latched current level, the converter will
overdrive the output slightly during each switch cycle.
This overdrive condition is sensed internally and forces
the voltage on the V
C
pin to continue to drop. When the
voltage on V
C
drops 150mV below the 15% load level,
switching is disabled and the LT3845A shuts down most
of its internal circuitry, reducing total quiescent current
to 120µA. When the converter output begins to fall, the
V
C
pin voltage begins to climb. When the voltage on the
V
C
pin climbs back to the 15% load level, the IC returns
to normal operation and switching resumes. An internal
clamp on the V
C
pin is set at 100mV below the switch
disable threshold, which limits the negative excursion of
the pin voltage, minimizing the converter output ripple
during Burst Mode operation.
During Burst Mode operation, V
IN
pin current is 20µA and
V
CC
current is reduced to 100µA. If no external drive is
provided for V
CC
, all V
CC
bias currents originate from the
V
IN
pin, giving a total V
IN
current of 120µA. Burst current
can be reduced further when V
CC
is driven using an output
derived source, as the V
CC
component of V
IN
current is
then reduced by the converter buck ratio.
Reverse-Current Inhibit
The LT3845A contains a reverse-current inhibit feature
to maximize efficiency during light load conditions. This
mode of operation allows discontinuous operation and
pulse-skipping mode at light loads. Refer to Figure 1.
This feature is enabled with Burst Mode operation, and can
also be enabled while Burst Mode operation is disabled
by shorting the BURST_EN pin to V
FB
.
When reverse-current inhibit is enabled, the LT3845A sense
amplifier detects inductor currents approaching zero and
disables the synchronous switch for the remainder of
the switch cycle. If the inductor current is allowed to go
negative before the synchronous switch is disabled, the
switch node could inductively kick positive with a high
dv/dt. The LT3845A prevents this by incorporating a 10mV
positive offset at the sense inputs.
With the reverse-current inhibit feature enabled, an LT3845A
converter will operate much like a nonsynchronous
converter during light loads. Reverse-current inhibit
reduces resistive losses associated with inductor ripple
currents, which improves operating efficiencies during
light-load conditions.
An LT3845A DC/DC converter that is operating in reverse-
current inhibit mode has a minimum load requirement
of 1mA (BURST_EN = V
FB
). Since most applications use
output-generated power for the LT3845A, this requirement
is met by the bias currents of the IC, however, for
applications that do not derive power from the output,
this requirement is easily accomplished by using a 1.2k
resistor connected from V
FB
to ground as one of the
converter output voltage programming resistors (R1).
There are no minimum load restrictions when in Burst
Mode operation (BURST_EN < 0.5V) or continuous
conduction mode (BURST_EN > 2.5V).
Figure 1. Inductor Current vs Mode
PULSE SKIP MODE
I
L
I
L
FORCED CONTINUOUS
DECREASING
LOAD
CURRENT
3845A F01