3

LNK302/304-306

G

3/05

Pin Functional Description

DRAIN (D) Pin:

Power MOSFET drain connection. Provides internal operating

current for both start-up and steady-state operation.

BYPASS (BP) Pin:

Connection point for a 0.1 µF external bypass capacitor for the

internally generated 5.8 V supply.

FEEDBACK (FB) Pin:

During normal operation, switching of the power MOSFET is

controlled by this pin. MOSFET switching is terminated when

a current greater than 49 µA is delivered into this pin.

SOURCE (S) Pin:

This pin is the power MOSFET source connection. It is also the

ground reference for the BYPASS and FEEDBACK pins.

LinkSwitch-TN

Functional

Description

LinkSwitch-TN

combinesahighvoltagepowerMOSFETswitch

withapowersupplycontrollerinonedevice.Unlikeconventional

PWM(pulsewidthmodulator)controllers,LinkSwitch-TNuses

a simple ON/OFF control to regulate the output voltage. The

LinkSwitch-TN

controller consists of an oscillator, feedback

(sense and logic) circuit, 5.8 V regulator, BYPASS pin under-

voltagecircuit,over-temperatureprotection,frequencyjittering,

current limit circuit, leading edge blanking and a 700 V power

MOSFET.TheLinkSwitch-TNincorporatesadditionalcircuitry

for auto-restart.

Oscillator

The typical oscillator frequency is internally set to an average

of 66 kHz. Two signals are generated from the oscillator: the

maximum duty cycle signal (DC

indicates the beginning of each cycle.

The LinkSwitch-TN oscillator incorporates circuitry that

introduces a small amount of frequency jitter, typically 4 kHz

peak-to-peak, to minimize EMI emission. The modulation rate

of the frequency jitter is set to 1 kHz to optimize EMI reduction

for both average and quasi-peak emissions. The frequency

jitter should be measured with the oscilloscope triggered at

the falling edge of the DRAIN waveform. The waveform in

Figure 4 illustrates the frequency jitter of the LinkSwitch-TN.

Feedback Input Circuit

The feedback input circuit at the FB pin consists of a low

impedancesourcefolloweroutputsetat1.65V.Whenthecurrent

deliveredintothispinexceeds49µA,alowlogiclevel(disable)

is generated at the output of the feedback circuit. This output

is sampled at the beginning of each cycle on the rising edge of

the clock signal. If high, the power MOSFET is turned on for

that cycle (enabled), otherwise the power MOSFETremains off

(disabled). Since the sampling is done only at the beginning of

each cycle, subsequent changes in the FB pin voltage or current

during the remainder of the cycle are ignored.

5.8 V Regulator and 6.3 V Shunt Voltage Clamp

The 5.8 V regulator charges the bypass capacitor connected to

the BYPASS pin to 5.8 V by drawing a current from the voltage

on the DRAIN, whenever the MOSFET is off. The BYPASS

pin is the internal supply voltage node for the LinkSwitch-TN.

When the MOSFET is on, the LinkSwitch-TN runs off of the

energy stored in the bypass capacitor. Extremely low power

consumption of the internal circuitry allows the LinkSwitch-TN

tooperatecontinuouslyfromthecurrentdrawnfromtheDRAIN

pin. A bypass capacitor value of 0.1 µF is sufficient for both

high frequency decoupling and energy storage.

In addition, there is a 6.3 V shunt regulator clamping the

BYPASS pin at 6.3 V when current is provided to the BYPASS

pin through an external resistor. This facilitates powering of

LinkSwitch-TN

externally through a bias winding to decrease

the no-load consumption to about 50 mW.

BYPASS Pin Under-Voltage

The BYPASS pin under-voltage circuitry disables the power

MOSFET when the BYPASS pin voltage drops below 4.85 V.

Once the BYPASS pin voltage drops below 4.85 V, it must rise

back to 5.8 V to enable (turn-on) the power MOSFET.

Over-Temperature Protection

The thermal shutdown circuitry senses the die temperature.

The threshold is set at 142 °C typical with a 75 °C hysteresis.

Whenthedietemperaturerisesabovethisthreshold(142 °C)the

power MOSFET is disabled and remains disabled until the die

temperature falls by 75 °C, at which point it is re-enabled.

Current Limit

ThecurrentlimitcircuitsensesthecurrentinthepowerMOSFET.

When this current exceeds the internal threshold (I

PI-3491-111903

FB

D

S

BP

S

S

S

P Package (DIP-8B)

G Package (SMD-8B)

8

5

7

1

4

2

3

Figure 3. Pin Configuration.