Datasheet

LT3957A
7
3957afa
PIN FUNCTIONS
NC (Pins 1, 2, 10, 35, 36): No Internal Connection. Leave
these pins open or connect them to the adjacent pins.
SENSE2 (Pin 3): The Current Sense Input for the Control
Loop. Connect this pin to SENSE1 pin directly or through
a low pass filter (connect this pin to SENSE1 pin through
a resistor, and to SGND through a capacitor).
SGND (Pins 4, 23, 24, Exposed Pad Pin 37): Signal
Ground. All small-signal components should connect to
this ground. SGND is connected to GND inside the IC to
ensure Kelvin connection for the internal switch current
sensing. Do not connect SGND and GND externally.
SENSE1 (Pin 6): The Current Sense Output of the Inter-
nal N-channel MOSFET. Connect this pin to SENSE2 pin
directly or through a lowpass filter (connect this pin to
SENSE1 pin through a resistor, then connect SENSE2 to
SGND through a capacitor).
SW (Pins 8, 9, 20, 21, Exposed Pad Pin 38): Drain of
Internal Power N-channel MOSFET.
GND (Pins 12, 13, 14, 15, 16, 17): Ground. These pins
connect to the source terminal of internal power N-channel
MOSFET through an internal sense resistor. GND is con-
nected to SGND inside the IC to ensure Kelvin connection
for the internal switch current sensing. Do not connect
GND and SGND externally.
EN/UVLO (Pin 25): 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
soft-start. Tie to 0.4V, or less, to disable the device and
reduce V
IN
quiescent current below 1µA.
V
IN
(Pin 27): Input Supply Pin. The V
IN
pin can be locally
bypassed with a capacitor to GND (not SGND).
INTV
CC
(Pin 28): Regulated Supply for Internal Loads
and Gate Driver. Supplied from V
IN
and regulated to
5.2V (typical). INTV
CC
must be bypassed to SGND 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 8V.
INTV
CC
can also be connected to a power supply whose
voltage is higher than 5.5V, and lower than V
IN
, provided
that supply does not exceed 8V.
V
C
(Pin 30): Error Amplifier Compensation Pin. Used to
stabilize the voltage loop with an external RC network. Place
compensation components between the V
C
pin and SGND.
FBX (Pin 31): Positive and Negative Feedback Pin. Re-
ceives the feedback voltage from the external resistor
divider between the output and SGND. Also modulates the
switching frequency during start-up and fault conditions
when FBX is close to SGND.
SS (Pin 32): Soft-Start Pin. This pin modulates compen-
sation pin voltage (V
C
) clamp. The soft-start interval is
set with an external capacitor between SS pin and SGND.
The pin has a 10µA (typical) pull-up current source to an
internal 2.5V rail. The soft-start pin is reset to SGND by an
undervoltage condition at EN/UVLO, an INTV
CC
undervolt-
age or overvoltage condition or an internal thermal lockout.
RT (Pin 33): Switching Frequency Adjustment Pin. Set
the frequency using a resistor to SGND. Do not leave this
pin open.
SYNC (Pin 34): 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 SGND if this
feature is not used. SYNC is bypassed when FBX is close
to SGND.