Datasheet

LT8611
17
8611fa
For more information www.linear.com/LT8611
APPLICATIONS INFORMATION
capacitor on TR/SS enables soft starting the output to pre-
vent current surge on the input supply. During the soft-start
ramp the output voltage will proportionally track the TR/SS
p
in 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 feedback 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 LT8611’s output voltage is within the ±9%
window 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 1.3% 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 output). To
synchronize the LT8611 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 val
-
leys that are below 0.4V and peaks above 2.4V (up to 6V).
Figure 4. LT8611 Sense Voltage vs IMON Voltage
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 current for
the LT8611’s circuitry and must be bypassed to ground
with a minimum of 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 also 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 LT8611, 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 LT8611 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
V
ISP
-V
ISN
(mV)
0
0
V
IMON
(mV)
200
400
600
800
1000
1200
10 20 30 40
8611 G45
50
V
SYNC
= 3.3V
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