LT8697
22
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For more information www.linear.com/LT8697
APPLICATIONS INFORMATION
Enable Pin
The LT8697 is in shutdown when the EN/UV pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.0V, with 40mV of hysteresis. The EN/UV
pin can be tied to V
IN
if the shutdown feature is not used,
or tied to a logic level if shutdown control is required.
Adding a resistor divider from V
IN
to EN/UV programs the
LT8697 to regulate the output only when V
IN
is above a
desired voltage (see the Block Diagram). Typically, this
threshold, V
IN(EN)
, is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
latch low under low source voltage conditions. The V
IN(EN)
threshold prevents the regulator from operating at source
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
where the LT8697 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)
.
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 LT8697’s circuitry and must be bypassed to ground
with a minimum capacitance of 1μF. Use an X5R or an
X7R 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 SYS pin when the SYS
pin is at 3.3V or higher. SYS must be tied to the LT8697
output capacitor. If the SYS pin is below 3.3V, the internal
LDO will consume current from V
IN
. Do not load INTV
CC
with more than 100µA.
Output Voltage Tracking and Soft-Start
The LT8697 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2.2μA
source pulls up the TR/
SS pin to
INTV
CC
. Putting an external
capacitor on TR/SS enables soft starting the output to pre-
vent a 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
USB5V pin voltage to 5× that of TR/SS pin. When TR/SS
is above
0.97
V, tracking is disabled and USB5V will regulate
to 5V. 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 LT8697’s output voltage is within the ±9%
window of the regulation point, which is V
USB5V
in the
range of 4.55V to 5.45V (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 synchronize the LT8697 oscillator to an external fre
-
quency, 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).
