5
LT1425
PIN FUNCTIONS
UUU
GND (Pins 1, 8, 9, 16): Ground. These pins connect to the
substrate of the die and are separate from the power
ground and signal ground. They should connect directly to
a good quality ground plane.
R
FB
(Pin 3): Input Pin for External “Feedback” Resistor
Connected to Transformer Primary (V
SW
). The ratio of this
resistor to the R
REF
resistor, times the internal bandgap
(V
BG
) reference, is the primary determinant of the output
voltage (plus the effect of any nonunity transformer turns
ratio). The average current through this resistor during the
flyback period should be approximately 400µA. See Appli-
cations Information for more details.
V
C
(Pin 4): Control Voltage. This pin is the output of the
feedback amplifier and the input of the current compara-
tor. Frequency compensation of the overall loop is effected
by placing a capacitor between this node and ground.
R
REF
(Pin 5): Input Pin for External Ground-Referred
“Reference” Resistor. This resistor should be in the range
of 3k, but for convenience, need not be this value precisely.
See Applications Information for more details.
SYNC (Pin 6): Pin to Synchronize Internal Oscillator to
External Frequency Reference. It is directly logic compat-
ible and can be driven with any signal between 10% and
90% duty cycle. If unused, this pin can be left floating;
however, for best noise immunity the pin should be
grounded.
SGND (Pin 7): Signal Ground. This pin is a clean ground.
The internal reference and feedback amplifier are referred
to it. Keep the ground path connection to R
REF
and the V
C
compensation capacitor free of large ground currents.
PGND (Pin 10): Power Ground. This pin is the emitter of
the power switch device and has large currents flowing
through it. It should be connected directly to a good quality
ground plane.
V
SW
(Pin 11): This is the collector node of the output
switch and has large currents flowing through it. Keep the
traces to the switching components as short as possible
to minimize electromagnetic radiation and voltage spikes.
V
IN
(Pin 12): Supply Voltage. Bypass input supply pin with
10µF or more. The part goes into undervoltage lockout
when V
IN
drops below 2.8V. Undervoltage lockout stops
switching and pulls the V
C
pin low.
R
CCOMP
(Pin 13): Pin for the External Filter Capacitor for
Load Compensation Function. A common 0.1µF
ceramic capacitor will suffice for most applications. See
Applications Information for further details.
R
OCOMP
(Pin 14): Input Pin for Optional External Load
Compensation Resistor. Use of this pin allows nominal
compensation for nonzero output impedance in the power
transformer secondary circuit, including secondary wind-
ing impedance, output Schottky diode impedance and
output capacitor ESR. In less demanding applications this
resistor is not needed. See Applications Information for
more details.
SHDN (Pin 15): Shutdown. This pin is used to turn off the
regulator and reduce V
IN
input current to a few tens of
microamperes. The SHDN pin can be left floating when
unused.
