Datasheet

LT8619/LT8619-5
19
Rev A
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APPLICATIONS INFORMATION
threshold can be adjusted by setting the values R3 and
R4 such that they satisfy the following equation:
V
IN(EN/UV)
= 1+
R3
R4
⎛
⎝
⎜
⎞
⎠
⎟
• 1V
where the LT8619 will remain off until V
IN
is above
V
IN(EN/UV)
. Due to the comparator’s hysteresis, switching
will not stop until the input falls slightly below V
IN(EN/UV)
.
When in Burst Mode operation for light load currents,
the current through the V
IN(EN/UV)
resistor network can
easily be greater than the supply current consumed by
the LT8619. Therefore, the V
IN(EN/UV)
resistors should
be large enough to minimize their impact on efficiency
at low loads.
INTV
CC
Regulator
An internal low dropout (LDO) regulator produces the 3.3V
supply from V
IN
that powers the drivers and the internal
bias circuitry. The INTV
CC
can supply enough current for
the LT8619’s circuitry and must be bypassed to ground
with at least a 1μF ceramic capacitor. Good bypassing is
necessary to supply the high transient currents required
by the power MOSFET gate drivers. To improve efficiency
the internal LDO can draw current from the BIAS pin when
the BIAS pin is at 3.1V or higher. Typically the BIAS pin can
be tied to the output of the switching regulator, or can be
tied to an external supply which must also be at 3.3V or
above. If BIAS is connected to a supply other than V
OUT
,
be sure to bypass with a local ceramic capacitor. If the
BIAS pin is below 3.0V, the internal LDO will consume
current from V
IN
. Applications with high input voltage and
high switching frequency where the internal LDO pulls
current from V
IN
will increase die temperature because
of the higher power dissipation across the LDO. Do not
connect an external load to the INTV
CC
pin.
Output Power Good
When the LT8619’s output voltage is within the ±7.5%
window of the regulation point, the open-drain PG pin
goes high impedance and is typically pulled high with an
external resistor. Otherwise, the internal open-drain tran-
sistor will pull the PG pin low. The PG pin is also actively
pulled low during several fault conditions:
EN/UV pin is
below 1V, INTV
CC
drops below its UVLO threshold, V
IN
is
too low, or thermal shutdown.
Synchronization
Synchronizing the LT8619 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square wave
amplitude should have valleys that are below 0.4V and
peaks above 2V (up to 6V). During frequency synchroni-
zation, the part operates in forced continuous mode with
the SW rising edge synchronized to the SYNC positive
edge. The LT8619 may be synchronized over a 300kHz
to 2.2MHz range. The R
T
resistor must be chosen to set
the LT8619 switching frequency equal or below the lowest
synchronization input. For example, if the synchroniza-
tion signal will be 500kHz and higher, the R
T
should be
selected for 500kHz.
Start-Up Inrush Current, Short-Circuit Protection
Upon start-up, the internal soft-start action regulates
the V
OUT
slew rate; the LT8619 provides the maximum
rated output current to charge up the output capacitor
as quickly as possible. During start-up, if the output is
overloaded, the regulator continues to provide the maxi-
mum sourcing current to overcome the output load, but
at the same time, the bottom switch current is monitored
such that if the inductor current is beyond the safe levels,
switching of the top switch will be delay until such time
as the inductor current falls to safe levels.
Once the
soft-start period has expired and the FB voltage
is higher than 0.74V, the LT8619 switching frequency will
be folded back if the external load pulls down the output.
At the same time, the bottom switch current will continue
to be monitored to limit the short-circuit current. Figure 8
shows the frequency foldback transfer curve and Figure 9
shows the short circuit waveform. During this overcurrent
condition, if the SYNC pin is connected to a clock source,
the LT8619 will get out from the synchronization mode.
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