Datasheet

LT3845
12
3845fd
APPLICATIONS INFORMATION
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 LT3845 contains a reverse-current inhibit feature to
maximize effi ciency 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 LT3845 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 LT3845 prevents this by incorporating a 10mV
positive offset at the sense inputs.
With the reverse-current inhibit feature enabled, an LT3845
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 effi ciencies during
light-load conditions.
An LT3845 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 LT3845, this require-
ment 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).
Soft-Start
The soft-start function controls the slew rate of the power
supply output voltage during start-up. A controlled output
voltage ramp minimizes output voltage overshoot, reduces
inrush current from the V
IN
supply, and facilitates supply
sequencing. A capacitor, C
SS
, connected from the C
SS
pin
to SGND, programs the slew rate. The capacitor is charged
from an internal 2μA current source producing a ramped
voltage. The capacitor voltage overrides the internal refer-
ence to the error amplifi er. If the V
FB
pin voltage exceeds
the C
SS
pin voltage then the current threshold set by the
DC control voltage, V
C
, is decreased and the inductor cur-
rent is lowered. This in turn decreases the output voltage
slew rate allowing the C
SS
pin voltage ramp to catch up to
the V
FB
pin voltage. An internal 100mV offset is added to
the V
FB
pin voltage relative to the C
SS
pin voltage so that
Figure 1. Inductor Current vs Mode
PULSE SKIP MODE
I
L
I
L
FORCED CONTINUOUS
DECREASING
LOAD
CURRENT
3845 F01