For more information www.linear.com/LT3697
LT3697
9
3697f
pin FuncTions
V
IN
(Pins 1, 2): The V
IN
pins supply current to the LT3697’s
internal regulator and to the power switch. These pins
must be locally bypassed.
EN (Pin 3): The EN pin is used to put the LT3697 into
shutdown mode. Tie to ground to shut down the LT3697.
Tie to 2.5V or higher for normal operation. If the shutdown
feature is not used, tie this pin to the V
IN
pin.
F LT (Pin 4): The F LT pin is the open drain output of the
LT3697 fault comparator and timer. In normal operation
the F LT pin is high impedance. An overcurrent fault that
is sustained for at least 1.5ms causes the LT3697 to pull
the F LT pin low. The F LT pin then remains low until the
USB output current stays below the overcurrent threshold
for at least 1.5ms. The overcurrent fault threshold is 0.5%
below the current limit. The F LT output is valid when V
IN
is above 4V and EN is high.
SYNC (Pin 5): The SYNC pin is the external clock synchro
-
nization input. Tie to a clock source with on and off times
greater than 50ns for synchronization. Tie pin to ground if
not used. See the Synchronization section
in Applications
Information for more details.
RT (
Pin 6): The RT pin is the oscillator resistor input.
Connect a resistor from this pin to ground to set the
switching frequency.
VC (Pin 7): The VC pin is the output of the internal error
amplifier. The voltage on this pin controls the peak switch
current. Tie an R-C network from this pin to ground to
compensate the control loop.
RLIM (Pin 8): The RLIM pin provides an additional reference
to the third feedback amplifier of the LT3697 to allow the
output current limit to be programmed easily. The RLIM
pin has an accurate 11μA pull-up current. When the voltage
of the output current sense amplifier exceeds the lower
of the RLIM voltage or 1.22V, the LT3697 error amplifier
will switch to current limit mode and will regulate the USB
output current. In current limit mode, the output voltage
drops. Tie a resistor from RLIM to ground to program the
LT3697 current limit. If the USB output current exceeds
99.5% of the current limit for at least 1.5ms, the LT3697
will pull down on the F LT pin. Float the RLIM pin if not used.
RCBL (Pin 9): The RCBL pin is used to
program the USB5V
current
as a function of sense voltage (V
ISP
– V
ISN
) for
cable drop compensation. Tie a resistor from RCBL to
ground to set the USB5V input current. Float the RCBL
pin if cable drop compensation is not desired. The RCBL
pin may also be used as an USB output current monitor.
Excessive capacitive loading on the RCBL pin can cause
USB output voltage overshoot during load steps when
cable drop compensation is used. Keep the capacitive
loading on the RCBL pin below 100pF.
ISN (PIN 10): The ISN pin is the inverting input of the
LT3697’s onboard USB output current sense amplifier.
Tie a resistor R
SENSE
from ISP to ISN to sense the USB
output current. Connect ISN to ISP if the current monitor,
USB output current limit, and cable drop compensation
functions are not desired.
ISP (Pin 11): The ISP pin is the noninverting input of the
LT3697’s onboard USB output current sense amplifier. Tie
a resistor R
SENSE
from ISP to ISN to sense the USB output
current. When a USB switch is used in series between
the LT3697 and the 5V USB output, tie the ISP pin to the
USB switch output.
USB5V (Pin 12): The USB5V pin is the primary feedback
input of the internal error amplifier. In normal operation,
the LT3697 regulates the voltage on this pin to 5V. The
USB5V pin also allows the output voltage to increase as a
function of output current to compensate for voltage drop
at the point of load due to cable impedance. The USB5V
pin input current is proportional to USB output current
and is programmed by the R
CBL
resistor. Tie a resistor
from USB5V to the 5V USB output to set this cable drop
compensation.
Tie USB5V directly to the USB output if no cable drop
compensation is desired. If a USB switch is used in se
-
ries between
the LT3697 and the 5V USB output, tie the
USB5V
pin through the compensation resistor to the USB
switch output.
