Datasheet
LT8619/LT8619-5
12
Rev A
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OPERATION
The LT8619 is a monolithic, constant frequency current
mode step-down DC/DC converter. An oscillator, with fre-
quency 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 cur-
rent 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 volt-
age on the FB pin with an internal 0.8V 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 induc-
tor current matches the new load current. When the top
power switch turns off, the bottom power switch turns on
until the next clock cycle begins or inductor current falls
to zero (Burst Mode operation or pulse-skipping mode).
If overload conditions result in more than 1.8A flowing
through the bottom switch, the next clock cycle will be
delayed until the switch current returns to a safe level.
If the EN/UV pin is low, the LT8619 is shut down and
draws less than 0.6µA from the input. When the EN/UV
pin is above 1V, the switching regulator starts operation.
First, the internal LDO powers up, followed by the switch-
ing regulator 200μs soft-start ramp. During the soft-start
phase, the switcher operates in pulse-skipping mode and
gradually switches to forced continuous mode when V
OUT
approaches the set point (if SYNC pin is forced high or
connected to an external clock). Typically, upon EN/UV
rising edge, it takes about 660μs for the switcher output
voltage to reach regulation and PG to be asserted.
To optimize efficiency at light loads, configure the LT8619
to operate in Burst Mode by grounding the SYNC pin.
At light load, in between bursts, all circuitry associated
with controlling the output switch is shut down, reducing
the input supply current. In a typical application, 6μA will
be consumed from the supply when regulating with no
load. Float the SYNC pin to enable pulse-skipping mode
operation. While in pulse-skipping mode, the oscillator
operates continuously and the bottom power switch turns
off when the inductor current falls to zero. During light
loads, switch pulses are skipped to regulate the output
and the quiescent current will be higher than Burst Mode
operation. Connecting the SYNC pin to INTV
CC
enables
forced continuous mode operation. In forced continuous
mode, the inductor current is allowed to reverse and the
switcher operates at a fixed frequency. If a clock is applied
to the SYNC pin, the part operates in forced continuous
mode and synchronizes to the external clock frequency;
with the rising SW signal synchronized to the external
clock positive edge.
To improve efficiency across all loads, supply current
to internal circuitry can be sourced from the BIAS pin
when biased above 3.1V. Else, the internal circuitry will
draw current from V
IN
. The BIAS pin should be connected
to V
OUT
if the LT8619 output is programmed to 3.3V or
above.
An overvoltage comparator, OV, guards against transient
overshoots. If V
FB
is higher than 0.83V, the OV compara-
tor trips, disables the top MOSFET and turns on the bot-
tom power switch until the next clock cycle begins or the
inductor reverse current reaches 0.55A. With high reverse
current, both top and bottom MOSFETs shut off till the
next cycle. Positive and negative power good compara-
tors pull the PG pin low if the FB voltage varies more than
±
7.5% (typical) from the set point.
The oscillator reduces the
LT8619’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 overcurrent conditions.
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