LT1372/LT1377
U
U
U
PIN FUNCTIONS
VC (Pin 1): The Compensation 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 per-
formed with an RC network connected from the VC pin to
ground.
VIN (Pin5):Bypassinputsupplypinwith10µFormore.The
part goes into undervoltage lockout when VIN drops below
2.5V. Undervoltage lockout stops switching and pulls the
VC 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 VC compensation network
free of large ground currents.
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 250µA
when the NFB pin is used. See Applications Information.
GND (Pin 7): The ground pin is the emitter connection of
thepowerswitchandhaslargecurrentsflowingthroughit.
It should be connected directly to a good quality ground
plane.
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 100k source resistor.
VSW (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.
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
floating. To synchronize switching, drive the S/S pin be-
tween 600kHz and 800kHz (LT1372) or 1.2MHz to 1.6MHz
(LT1377).
W
BLOCK DIAGRAM
V
SW
IN
SHUTDOWN
DELAY AND RESET
LOW DROPOUT
2.3V REG
S/S
ANTI-SAT
LOGIC
DRIVER
SWITCH
SYNC
OSC
5:1 FREQUENCY
SHIFT
+
–
NFBA
100k
50k
NFB
FB
COMP
–
+
+
–
EA
IA
0.08Ω
A
V
≈ 6
V
C
1.245V
REF
GND LT1372 • BD
GND SENSE
5
Linear [ Linear ]
