Datasheet LT1425 (Analog Devices) - 5

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