LT3474/LT3474-1
6
3474fd
PIN FUNCTIONS
DNC (Pins 1, 16): Do not connect external circuitry to these
pins, or tie them to GND. Leave the DNC pins fl oating.
OUT (Pin 2): The OUT pin is the input to the current sense
resistor. Connect this pin to the inductor and the output
capacitor.
LED (Pin 3): The LED pin is the output of the current sense
resistor. Connect the anode of the LED here.
V
IN
(Pin 4): The V
IN
pin supplies current to the internal
circuitry and to the internal power switch and must be
locally bypassed.
SW (Pin 5): The SW pin is the output of the internal power
switch. Connect this pin to the inductor and switching
diode.
BOOST (Pin 6): The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
BIAS (Pin 7): The BIAS pin connects through a Schottky
diode to BOOST. Tie to OUT.
GND (Pins 8, 15, Exposed Pad Pin 17): Ground. Tie both
GND pins and the Exposed Pad directly to the ground
plane. The Exposed Pad metal of the package provides both
electrical contact to ground and good thermal contact to
the printed circuit board. It must be soldered to the circuit
board for proper operation.
R
T
(Pin 9): The R
T
pin is used to set the internal oscilla-
tor frequency. Tie an 80.6k resistor from R
T
to GND for a
500kHz switching frequency.
SHDN (Pin 10): The SHDN pin is used to shut down the
switching regulator and the internal bias circuits. The
2.6V switching threshold can function as an accurate
under-voltage lockout. Pull below 0.3V to shut down the
LT3474/LT3474-1. Pull above 2.65V to enable the LT3474/
LT3474-1. Tie to V
IN
if the SHDN function is unused.
REF (Pin 11): The REF pin is the buffered output of the
internal reference. Either tie the REF pin to the V
ADJ
pin for
a 1A output current, or use a resistor divider to generate a
lower voltage at the V
ADJ
pin. Leave this pin unconnected
if unused.
V
C
(Pin 12): The V
c
pin is the output of the internal error
amp. The voltage on this pin controls the peak switch
current. Use this pin to compensate the control loop.
V
ADJ
(Pin 13): The V
ADJ
pin is the input to the internal
voltage to current amplifi er. Connect the V
ADJ
pin to the
REF pin for a 1A output current. For lower output cur-
rents, program the V
ADJ
pin using the following formula:
I
LED
= 1A • V
ADJ
/1.25V.
PWM (Pin 14): The PWM pin controls the connection of
the V
C
pin to the internal circuitry. When the PWM pin is
low, the V
C
pin is disconnected from the internal circuitry
and draws minimal current. If the PWM feature is unused,
leave this pin unconnected.
