LT1373
U
U
U
PI FU CTIO S
VC (Pin 1): Compensation Pin. The VC pin is used for
frequency compensation, current limiting and soft start. It
is the output of the error amplifier and the input of the
currentcomparator. Loopfrequencycompensationcanbe
performed with an RC network connected from the VC pin
to ground.
floating. To synchronize switching, drive the S/S pin be-
tween 300kHz and 340kHz.
VIN (Pin 5): Input Supply Pin. Bypass VIN with 10µF or
more. The part goes into undervoltage lockout when VIN
drops below 2.5V. Undervoltage lockout stops switching
and pulls the VC pin low.
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.
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 VC compensation network
free of large ground currents.
NFB(Pin3):Thenegativefeedbackpinisusedfornegative
output voltage sensing. It is connected to the inverting
input of the negative feedback amplifier through a 400k
source resistor.
GND (Pin 7): The ground pin is the emitter connection of
thepowerswitchandhaslargecurrentsflowingthroughit.
It should be connected directly to a good quality ground
plane.
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 VIN or leave it
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.
W
BLOCK DIAGRA
V
SW
IN
SHUTDOWN
DELAY AND RESET
LOW DROPOUT
2.3V REG
S/S
ANTI-SAT
250kHz
LOGIC
DRIVER
SWITCH
SYNC
OSC
5:1 FREQUENCY
SHIFT
+
NEGATIVE
FEEDBACK
400k
200k
AMP
NFB
FB
–
COMP
–
+
–
ERROR
CURRENT
AMP
0.08Ω
AMP
+
A
≈ 6
V
V
C
1.245V
REF
GND LT1373 • BD
GND SENSE
5
Linear [ Linear ]
