Datasheet

LT3800
10
3800fc
APPLICATIONS INFORMATION
Burst Mode
The LT3800 employs low current Burst Mode functional-
ity to maximize effi ciency during no load and low load
conditions. Burst Mode operation is enabled by shorting
the BURST_EN pin to SGND. Burst Mode operation can
be disabled by shorting BURST_EN to either V
FB
or V
CC
.
When the required switch current, sensed via the V
C
pin voltage, is below 15% of maximum, the Burst Mode
operation is employed and that level of sense current is
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 LT3800 shuts down most
of its internal circuitry, reducing total quiescent current
to 100µA. 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 80µA. 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 100µA. 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 LT3800 contains a reverse-current inhibit feature to
maximize effi ciency during light load conditions. This
mode of operation allows discontinuous operation, and
is sometimes referred to as “pulse-skipping” mode. Refer
to Figure 2.
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 LT3800 sense
amplifi er 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 LT3800 prevents this by incorporating a 10mV
positive offset at the sense inputs.
Figure 2. Inductor Current vs Mode
PULSE SKIP MODE
I
L
I
L
FORCED CONTINUOUS
DECREASING
LOAD
CURRENT
3800 F02