Datasheet

LT8614
11
Rev. E
For more information www.analog.com
OPERATION
The LT8614 is a monolithic, constant frequency, current
mode step-down DC/DC converter. An oscillator, with
frequency set using a resistor on the RT pin, turns on
the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which the
top switch turns off is controlled by the voltage on the
internal VC node. The error amplifier servos the VC node
by comparing the voltage on the V
FB
pin with an inter-
nal 0.97V reference. When the load current increases it
causes a reduction in the feedback voltage relative to the
reference leading the error amplifier to raise the VC volt
-
age until the average inductor current matches the new
load current. When the top power switch turns off, the
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in more than 6.9A flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
If the EN/UV pin is low, the LT8614 is shut down and
draws 1µA from the input. When the EN/UV pin is above
1V, the switching regulator will become active.
To optimize efficiency at light loads, the LT8614 operates
in Burst Mode operation in light load situations. Between
bursts, all circuitry associated with controlling the output
switch is shut down, reducing the input supply current to
1.7μA. In a typical application, 2.5μA will be consumed
from the input supply when regulating with no load. The
SYNC pin is tied low to use Burst Mode operation and can
be tied to a logic high to use pulse-skipping mode. If a
clock is applied to the SYNC pin the part will synchronize
to an external clock frequency and operate in pulse-skip
-
ping mode. While in pulse-skipping mode the oscillator
operates continuously and positive SW transitions are
aligned to the clock. During light loads, switch pulses are
skipped to regulate the output and the quiescent current
will be several hundred µA.
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will
draw current from V
IN
. The BIAS pin should be connected
to V
OUT
if the LT8614 output is programmed at 3.3V or
above.
Comparators monitoring the FB pin voltage will pull the PG
pin low if the output voltage varies more than ±9% (typi-
cal) from the set point, or if a fault condition is present.
The oscillator reduces the
LT8614’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the inductor current when the
output voltage is lower than the programmed value which
occurs during start-up or overcurrent conditions. When
a clock is applied to the SYNC pin or the SYNC pin is
held DC high, the frequency foldback is disabled and the
switching frequency will slow down only during overcur-
rent conditions.
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