Datasheet

LT3757/LT3757A
7
3757afe
For more information www.linear.com/LT3757
pin FuncTions
V
C
(Pin 1): Error Amplifier Compensation Pin. Used to
stabilize the voltage loop with an external RC network.
FBX (Pin 2): Positive and Negative Feedback Pin. Receives
the feedback voltage from the external resistor divider
across the output. Also modulates the frequency during
start-up and fault conditions when FBX is close to GND.
SS (Pin 3): Soft-Start Pin. This pin modulates compensa-
tion pin voltage (V
C
) clamp. The soft-start interval is set
with an external capacitor. The pin has a 10µA (typical)
pull-up current source to an internal 2.5V rail. The soft-
start pin is reset to GND by an undervoltage condition
at SHDN/UVLO, an INTV
CC
undervoltage or overvoltage
condition or an internal thermal lockout.
RT (Pin 4): Switching Frequency Adjustment Pin. Set the
frequency using a resistor to GND. Do not leave this pin
open.
SYNC (Pin 5): Frequency Synchronization Pin. Used to
synchronize the switching frequency to an outside clock.
If this feature is used, an R
T
resistor should be chosen
to program a switching frequency 20% slower than the
SYNC pulse frequency. Tie the SYNC pin to GND if this
feature is not used. SYNC is ignored when FBX is close
to GND.
SENSE (Pin 6): The Current Sense Input for the Control
Loop. Kelvin connect this pin to the positive terminal of
the switch current sense resistor in the source of the
N-channel MOSFET. The negative terminal of the current
sense resistor should be connected to GND plane close
to the IC.
GATE (Pin 7): N-Channel MOSFET Gate Driver Output.
Switches between INTV
CC
and GND. Driven to GND when
IC is shut down, during thermal lockout or when INTV
CC
is above or below the OV or UV thresholds, respectively.
INTV
CC
(Pin 8): Regulated Supply for Internal Loads and
Gate Driver. Supplied from V
IN
and regulated to 7.2V (typi -
cal). INTV
CC
must be bypassed with a minimum of 4.7µF
capacitor placed close to pin. INTV
CC
can be connected
directly to V
IN
, if V
IN
is less than 17.5V. INTV
CC
can also
be connected to a power supply whose voltage is higher
than 7.5V, and lower than V
IN
, provided that supply does
not exceed 17.5V.
SHDN/UVLO (Pin 9): Shutdown and Undervoltage Detect
Pin. An accurate 1.22V (nominal) falling threshold with
externally programmable hysteresis detects when power
is okay to enable switching. Rising hysteresis is generated
by the external resistor divider and an accurate internal
2µA pull-down current. An undervoltage condition resets
sort-start. Tie to 0.4V, or less, to disable the device and
reduce V
IN
quiescent current below 1µA.
V
IN
(Pin 10): Input Supply Pin. Must be locally bypassed
with a 0.22µF, or larger, capacitor placed close to the pin.
Exposed Pad (Pin 11): Ground. This pin also serves as
the negative terminal of the current sense resistor. The
Exposed Pad must be soldered directly to the local ground
plane.
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