Datasheet

LT8614
18
For more information www.analog.com
APPLICATIONS INFORMATION
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
they satisfy the following equation:
V
IN(EN)
=
R3
R4
+ 1
⎛
⎝
⎜
⎞
⎠
⎟
•1.0V
(10)
where the LT8614 will remain off until V
IN
is above V
IN(EN)
.
Due to the comparator’s hysteresis, switching will not
stop until the input falls slightly below V
IN(EN)
.
When operating in Burst Mode operation for light load
currents, the current through the V
IN(EN)
resistor network
can easily be greater than the supply current consumed
by the LT8614. Therefore, the V
IN(EN)
resistors should be
large to minimize their effect on efficiency at low loads.
INTV
CC
Regulator
An internal low dropout (LDO) regulator produces the
3.4V supply from V
IN
that powers the drivers and the
internal bias circuitry. The INTV
CC
can supply enough cur-
rent for the LT8614’s circuitry and must be bypassed to
ground with a minimum of 1μF ceramic capacitor. Good
bypassing is necessary to supply the high transient cur
-
rents required by the power MOSFET gate drivers. To
improve efficiency the internal LDO can also draw cur-
rent 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 LT8614, or can be tied to an external supply of 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 Voltage Tracking and Soft-Start
T he LT8614 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2.2μA
pulls up the TR/SS pin to INTV
CC
. Putting an external
capacitor on TR/SS enables soft starting the output to
prevent current surge on the input supply. During the soft-
start ramp the output voltage will proportionally track the
TR/SS pin voltage. For output tracking applications, TR/
SS can be externally driven by another voltage source.
From 0V to 0.97V, the TR/SS voltage will override the
internal 0.97V reference input to the error amplifier, thus
regulating the FB pin voltage to that of TR/SS pin. When
TR/SS is above 0.97V, tracking is disabled and the feed-
back voltage will regulate to the internal reference voltage.
The TR/SS pin may be left floating if the function is not
needed.
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, V
IN
voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8614’s output voltage is within the ±9% win -
dow of the regulation point, which is a V
FB
voltage in the
range of 0.883V to 1.057V (typical), the output voltage
is considered good and the open-drain PG pin goes high
impedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and
lower thresholds include 1.2% of hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTV
CC
has fallen too
low, V
IN
is too low, or thermal shutdown.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.4V (this can be ground or a logic low out
-
put). To synchronize the LT8614 oscillator to an external
frequency connect 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 2.4V
(up to 6V).
The LT8614 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation. The LT8614
may be synchronized over a 200kHz to 3MHz range. The
R
T
resistor should be chosen to set the LT8614 switching
frequency equal to or below the lowest synchronization
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.