LT1425 Linear Technology, LT1425 Datasheet - Page 5

• Manufacturer Part Number: LT1425
• Description: Isolated Flyback Switching Regulator
• Manufacturer: Linear Technology

PIN
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
Connected to Transformer Primary (V
resistor to the R
(V
voltage (plus the effect of any nonunity transformer turns
ratio). The average current through this resistor during the
flyback period should be approximately 400 A. See Appli-
cations Information for more details.
V
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
“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
compensation capacitor free of large ground currents.
C
FB
REF
BG
U
(Pin 4): Control Voltage. This pin is the output of the
(Pin 3): Input Pin for External “Feedback” Resistor
) reference, is the primary determinant of the output
(Pin 5): Input Pin for External Ground-Referred
FUNCTIONS
U
REF
resistor, times the internal bandgap
U
SW
). The ratio of this
REF
and the V
C
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
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
10 F or more. The part goes into undervoltage lockout
when V
switching and pulls the V
R
Load Compensation Function. A common 0.1 F
ceramic capacitor will suffice for most applications. See
Applications Information for further details.
R
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
microamperes. The SHDN pin can be left floating when
unused.
SW
IN
CCOMP
OCOMP
(Pin 12): Supply Voltage. Bypass input supply pin with
(Pin 11): This is the collector node of the output
IN
(Pin 13): Pin for the External Filter Capacitor for
(Pin 14): Input Pin for Optional External Load
drops below 2.8V. Undervoltage lockout stops
IN
C
input current to a few tens of
pin low.
LT1425
5