LT8697
applicaTions inForMaTion
Adding a resistor divider from V to EN/UV programs the
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
isabove0.97V,trackingisdisabledandUSB5Vwillregulate
to 5V. The TR/SS pin may be left floating if the function
is not needed.
IN
LT8697 to regulate the output only when V is above a
IN
desired voltage (see the Block Diagram). Typically, this
threshold, V , is used in situations where the input
IN(EN)
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
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
latchlowunderlowsourcevoltageconditions. TheV
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:
capacitor are the EN/UV pin transitioning low, V voltage
IN
falling too low or thermal shutdown.
R3
R4
VIN(EN)
=
+ 1 •1.0V
Output Power Good
When the LT8697’s output voltage is within the 9%
window of the regulation point, which is V
wheretheLT8697willremainoffuntilV isaboveV
.
IN
IN(EN)
in the
USB5V
Duetothecomparator’shysteresis,switchingwillnotstop
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.
until the input falls slightly below V
.
IN(EN)
INꢂV Regulator
CC
Aninternallowdropout(LDO)regulatorproducesthe3.4V
supply from V that powers the drivers and the internal
IN
bias circuitry. The INTV can supply enough current for
CC
The PG pin is also actively pulled low during several fault
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
LDOcanalsodrawcurrentfromtheSYSpinwhentheSYS
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
conditions: EN/UV pin is below 1V, INTV has fallen too
CC
low, V is too low, or thermal shutdown.
IN
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).
LDO will consume current from V . Do not load INTV
with more than 100µA.
IN
CC
TheLT8697maybesynchronizedovera300kHzto2.2MHz
Output Voltage ꢂracking and Soft-Start
range. The R resistor should be chosen to set the LT8697
T
switchingfrequencyequaltoorbelowthelowestsynchro-
The LT8697 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2.2μA
nization input. For example, if the synchronization signal
will be 500kHz and higher, the R should be selected for
pulls up the TR/SS pin to INTV . Putting an external ca-
T
CC
500kHz. The slope compensation is set by the RT value,
while the minimum slope compensation required to avoid
subharmonic oscillations is established by the inductor
pacitoronTR/SSenablessoftstartingtheoutputtoprevent
current surge on the input supply. During the soft-start
ramptheoutputvoltagewillproportionallytracktheTR/SS
8697p
21
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LINEAR_DIMENSIONS [ Linear Dimensions ]
