4

LT3439

sn3439 3439fs

PI FU CTIO S

PGND (Pins 1, 16): Power ground is connected to the

emitter of the power switches via an internal sense resis-

tor. It has large currents flowing through it and should be

connected to a good quality ground plane.

COL A, COL B (Pins 3, 14): These are the open collectors

of the output power switches. They are connected to the

outer terminals of the center tap transformer. Large cur-

rents flow into these pins so external traces should be kept

as short as possible.

current and voltage slew rate for the collectors A and B.

The minimum resistor value is 3.4k for fast slewing and the

maximum resistor is 34k for slow slewing. For more

details, see “Slew Rate Setting” in the Applications Infor-

mation section of this data sheet.

SYNC (Pin 5): The SYNC pin can be used to synchronize

such that the oscillator clock frequency is approximately

10% below the external clock frequency. If unused, this

pin should be tied to GND. For more details, see “Oscillator

Sync” in the Applications Information section of this data

sheet.

The transformer operating frequency and the frequency

of each output is one half of the frequency of the oscillator.

charge and discharge currents of the oscillator capacitor.

The nominal value is 16.9k. The resistance can be adjusted

between

±25% of nominal for better frequency accuracy.

SHDN (Pin 11): The SHDN pin is used to shut down the

part. Grounding this pin will disable all internal circuitry.

Increasing the SHDN voltage above the turn-on threshold

will enable the part. At the turn-on threshold, approxi-

mately 20

µA of current is sourced out of the pin. This

current, in conjunction with the Thevenin resistance on the

pin, sets up the hysteresis. This allows the user to set the

undervoltage lockout (UVLO) of the supply and the amount

of start-up hysteresis with a resistor divider off of the input

voltage. Above 2.1V on the SHDN pin, the hysteresis

current is reduced to zero. If unused the pin can be left

floating or tied directly to the input voltage.

GND (Pin 10): Signal Ground. The oscillator, slew control

circuitry and the internal regulator are referred to signal

ground. Internally, signal ground is tied to substrate and

the exposed backside of the device. Connect the GND pin

to the ground plane and keep the connection free of large

currents.

be bypassed with a 4.7

µF or greater, low ESR capacitor.

will trip and turn both outputs off.