6
LT3503
3503f
PI FU CTIO S
UUU
FB (Pin 1): The LT3503 regulates its feedback pin to
780mV. Connect the feedback resistor divider tap to this
pin. Set the output voltage according to:
RR
V
V
OUT
12
078
1=
⎛
⎝
⎜
⎞
⎠
⎟
.
–
A good value for R2 is 10.0k.
GND (Pin 2): Tie the GND pin to a local ground plane below
the LT3503 and the circuit components. Return the feed-
back divider to this pin.
BOOST (Pin 3): The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal
bipolar NPN power switch.
SW (Pin 4): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
V
IN
(Pin 5): The V
IN
pin supplies current to the LT3503’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
SHDN (Pin 6): The SHDN pin is used to put the LT3503 in
shutdown mode. Tie to ground to shut down the LT3503.
Tie to 2.3V or more for normal operation. If the shutdown
feature is not used, tie this pin to the V
IN
pin. SHDN also
provides a soft-start function; see the Applications Infor-
mation section.
Exposed Pad (Pin 7): The Exposed Pad must be soldered
to the PCB and electrically connected to ground. Use a
large ground plane and thermal vias to optimize thermal
performance.
BLOCK DIAGRA
W
1
Σ
R
DRIVER
Q1
S
OSC
SLOPE
COMP
FREQUENCY
FOLDBACK
INT REG
AND
UVLO
V
C
g
m
780mV
3503 BD
2
5
6
Q
Q
3
4
BOOST
SW
FB
R2 R1
V
OUT
L1
D2
C3
C1
D1
V
IN
C2
V
IN
ON OFF
GND
C4
R3
SHDN
