LT1373
PI FU CTIO S
V
C
(Pin 1):
Compensation Pin. The V
C
pin is used for
frequency compensation, current limiting and soft start. It
is the output of the error amplifier and the input of the
current comparator. Loop frequency compensation can be
performed with an RC network connected from the V
C
pin
to ground.
FB (Pin 2):
The feedback pin is used for positive output
voltage sensing and oscillator frequency shifting. It is the
inverting input to the error amplifier. The noninverting
input of this amplifier is internally tied to a 1.245V
reference. Load on the FB pin should not exceed 100µA
when the NFB pin is used. See Applications Information.
NFB (Pin 3):
The negative feedback pin is used for negative
output voltage sensing. It is connected to the inverting
input of the negative feedback amplifier through a 400k
source resistor.
S/S (Pin 4):
Shutdown and Synchronization Pin. The S/S
pin is logic level compatible. Shutdown is active low and
the shutdown threshold is typically 1.3V. For normal
operation, pull the S/S pin high, tie it to V
IN
or leave it
floating. To synchronize switching, drive the S/S pin be-
tween 300kHz and 340kHz.
V
IN
(Pin 5):
Input Supply Pin. Bypass V
IN
with 10µF or
more. The part goes into undervoltage lockout when V
IN
drops below 2.5V. Undervoltage lockout stops switching
and pulls the V
C
pin low.
GND S (Pin 6):
The ground sense pin is a “clean” ground.
The internal reference, error amplifier and negative feed-
back amplifier are referred to the ground sense pin. Con-
nect it to ground. Keep the ground path connection to the
output resistor divider and the V
C
compensation network
free of large ground currents.
GND (Pin 7):
The ground pin is the emitter connection of
the power switch and has large currents flowing through it.
It should be connected directly to a good quality ground
plane.
V
SW
(Pin 8):
The switch pin is the collector of the power
switch and has large currents flowing through it. Keep the
traces to the switching components as short as possible to
minimize radiation and voltage spikes.
BLOCK DIAGRA
S/S
400k
NFB
200k
FB
1.245V
REF
GND SENSE
+
W
–
U
U
U
V
IN
SHUTDOWN
DELAY AND RESET
250kHz
OSC
5:1 FREQUENCY
SHIFT
LOW DROPOUT
2.3V REG
SW
ANTI-SAT
SYNC
LOGIC
DRIVER
SWITCH
+
NEGATIVE
FEEDBACK
AMP
–
COMP
+
CURRENT
AMP
V
C
A
V
≈
6
0.08Ω
ERROR
AMP
–
GND
LT1373 • BD
5
LINER [ LINEAR TECHNOLOGY ]
