LT3973/LT3973-3.3/LT3973-5
9
3973fb
For more information www.linear.com/LT3973
PIN FUNCTIONS
FB (Pin 1, LT3973 Only): The LT3973 regulates the FB
pin to 1.21V. Connect the feedback resistor divider tap
to this pin.
V
OUT
(Pin 1, LT3973-3.3 and LT3973-5 Only): The
LT3973-3.3 and
LT3973-5 regulate the V
OUT
pin to 3.3V
and 5V, respectively. This pin connects to the internal
feedback divider that programs the fixed output voltage.
OUT (Pin 2): The LT3973 regulates the V
IN
to V
OUT
voltage
for dropout conditions. It will also pull current from this
pin to charge the boost capacitor when needed. Connect
this pin to the output. If programmed output is greater
than 14V, tie this pin to GND.
EN/UVLO (Pin 3): The part is in shutdown when this pin is
low and active when this pin is high. The threshold voltage
is 1.19V going up with 30mV of hysteresis. Tie to V
IN
if
shutdown feature is not used. The EN/UVLO threshold is
accurate only when V
IN
is above 4.2V. If V
IN
is lower than
4.2V, ground EN/UVLO to place the part in shutdown.
V
IN
(Pin 4): The V
IN
pin supplies current to the LT3973’s
internal circuitry and to the internal power switch. This
pin must be locally bypassed.
GND
(Pin 5, Exposed Pad Pin 11): Ground. The exposed
pad must be soldered to the PCB.
SW (Pin 6): The SW pin is the output of an internal power
switch. Connect this pin to the inductor.
BOOST (Pin 7): This pin is used to provide a drive volt
-
age, higher than the input voltage, to the internal bipolar
NPN power switch.
BD
(Pin 8): This pin connects to the anode of the boost
diode. This pin also supplies current to the LT3973’s
internal regulator when BD is above 3.2V.
PG (Pin 9): The PG pin is the open-drain output of an
internal comparator. PG remains low until the FB pin is
within 10% of the final regulation voltage. PG is valid when
V
IN
is above 4.2V and EN/UVLO is high.
RT (Pin 10): A resistor is tied between RT and ground to
set the switching frequency.
