LT8611
11
8611fa
For more information
OPERATION
The LT8611 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
internal 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
voltage 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 3.3A flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
The LT8611 includes a current control and monitoring
loop using the ISN, ISP, IMON and ICTRL pins. The ISP/
ISN pins monitor the voltage across an external sense
resistor such that the V
ISP
-V
ISN
does not exceed 50mV
by limiting the peak inductor current controlled by the VC
node. The current sense amplifier inputs (ISP/ISN) are rail-
to-rail such that input, output, or other system currents
may be monitored and regulated. The IMON pin outputs
a ground-referenced voltage equal to 20 times the voltage
between the ISP-ISN pins for monitoring system currents.
The ICTRL pin can be used to override the internal 50mV
limit between the ISP, ISN pin to a lower set point for the
current control loop.
If the EN/UV pin is low, the LT8611 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 LT8611 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-skipping
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 LT8611 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%
(typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8611’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.
