LT1425CS datasheet(5/20 Pages) LINER

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LT1425

PIN FUNCTIONS

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.

RFB (Pin 3): Input Pin for External “Feedback” Resistor

Connected to Transformer Primary (VSW). The ratio of this

resistor to the RREF resistor, times the internal bandgap

(VBG) 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 400

µA. See Appli-

cations Information for more details.

VC (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.

RREF (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 RREF and the VC

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.

VSW (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.

VIN (Pin 12): Supply Voltage. Bypass input supply pin with

µF or more. The part goes into undervoltage lockout

when VIN drops below 2.8V. Undervoltage lockout stops

switching and pulls the VC pin low.

RCCOMP (Pin 13): Pin for the External Filter Capacitor for

Load Compensation Function. A common 0.1

µF

ceramic capacitor will suffice for most applications. See

Applications Information for further details.

ROCOMP (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 VIN input current to a few tens of

microamperes. The SHDN pin can be left floating when

unused.

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