LNK3205PSLASHGSLASHD datasheet(3/28 Pages) POWERINT

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Rev. Q 06/23

LinkSwitch-TN2

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LinkSwitch-TN2 Functional Description

LinkSwitch-TN2 combines a high-voltage power MOSFET switch with

a power supply controller in one device. Unlike conventional PWM

(pulse width modulator) controllers, LinkSwitch-TN2 uses a simple

ON/OFF control to regulate the output voltage. The LinkSwitch-TN2

controller consists of an oscillator, feedback (sense and logic) circuit,

5.0 V regulator, BYPASS pin undervoltage circuit, over-temperature

protection, line and output overvoltage protection, frequency jittering,

current limit circuit, leading edge blanking and a 725 V or 900 V

power MOSFET. The LinkSwitch-TN2 incorporates additional circuitry

for auto-restart.

Oscillator

The typical oscillator frequency is internally set to an average of f

OSC

(66 kHz). Two signals are generated from the oscillator: the maximum

duty cycle signal (DC

(MAX)) and the clock signal that indicates the

beginning of each cycle.

The LinkSwitch-TN2 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 5 illustrates the frequency jitter of the

LinkSwitch-TN2.

Soft-Start

At power-up or during a restart attempt in auto-restart, the device

applies a soft-start by temporarily reducing the oscillator frequency.

LNK3202-07 and LNK329x reduce the frequency to f

OSC(SS) (typically

33 kHz. LNK3208 and LNK3209 reduce the oscillator frequency to

initially 16.5 kHz followed by a stepwise increase to 22 kHz and

33 kHz over a period of 256 switching cycles. Soft-start terminates

and the device continues operating at the nominal oscillator

frequency f

OSC either after 256 switching cycles or if the output

voltage reaches regulation.

Feedback Input Circuit

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

impedance source follower output set at V

FB (2.0 V). When the

current delivered into this pin exceeds I

FB (49 μA), a low logic level

(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 MOSFET remains off (disabled).

The sampling is done only at the beginning of each cycle. Subse-

quent changes in the FEEDBACK pin voltage or current during the

remainder of the cycle do not impact the MOSFET enable/disable

status. If a current greater than I

FBSD is injected into the feedback pin

while the MOSFET is enabled for at least two consecutive cycles the

part will stop switching and enter auto-restart off-time. Normal

switching resumes after the auto-restart off-time expires. This

shutdown function allows implementing line overvoltage protection in

flyback converters (see Figure 6). The current into the FEEDBACK pin

should be limited to less than 1.2 mA.

5.0 V Regulator and 5.2 V Shunt Voltage Clamp

The 5.0 V regulator charges the bypass capacitor connected to the

BYPASS pin to V

BP 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-TN2. When the MOSFET is

on, the LinkSwitch-TN2 runs off of the energy stored in the bypass

capacitor. Extremely low power consumption of the internal circuitry

allows the LinkSwitch-TN2 to operate continuously from the current

drawn from the DRAIN pin. A bypass capacitor value of 0.1 μF is

sufficient for both high frequency decoupling and energy storage.

In addition, there is a shunt regulator clamping the BYPASS pin at

BP(SHUNT) (5.2 V) when current is provided to the BYPASS pin through

an external resistor. This facilitates powering of LinkSwitch-TN2

externally through a bias winding to decrease the no-load consump-

tion to about 10 mW (flyback). The device stops switching instantly

and enters auto-restart when a current ≥I

BPSD is delivered into the

BYPASS pin. Adding an external Zener diode from the output voltage

to the BYPASS pin allows implementing an hysteretic OVP function in

a flyback converter (see Figure 6). The current into the BYPASS pin

should be limited to less than 16 mA.

BYPASS Pin Undervoltage

The BYPASS pin undervoltage circuitry disables the power MOSFET

when the BYPASS pin voltage drops below V

BP–VBPH (approximately 4.5 V).

Once the BYPASS pin voltage drops below this threshold, it must rise

back to V

BP to enable (turn-on) the power MOSFET.

Over-Temperature Protection

The thermal shutdown circuitry senses the die temperature. The

threshold is set at T

SD (142 °C typical) with a 75 °C (TSDH) hysteresis.

When the die temperature rises above T

SD the power MOSFET is

disabled and remains disabled until the die temperature falls to

SD–TSDH, at which point it is re-enabled.

Current Limit

The current limit circuit senses the current in the power MOSFET.

When this current exceeds the internal threshold (I

LIMIT), the power

MOSFET is turned off for the remainder of that cycle. The leading

edge blanking circuit inhibits the current limit comparator for a short

time (t

LEB) after the power MOSFET is turned on. This leading edge

blanking time has been set so that current spikes caused by capaci-

tance and rectifier reverse recovery time will not cause premature

termination of the switching pulse. Current limit can be selected using

the BYPASS pin capacitor (0.1 μF for normal current limit / 1 μF for

reduced current limit). LinkSwitch-TN2 selects between normal and

reduced current limit at power-up prior to switching.

Auto-Restart

In the event of a fault condition such as output overload, output

short, or an open-loop condition, LinkSwitch-TN2 enters into

auto-restart operation. An internal counter clocked by the oscillator

gets reset every time the FEEDBACK pin is pulled high. If the

FEEDBACK pin is not pulled high for t

AR(ON) (50 ms), the power

MOSFET switching is disabled for a time equal to the auto-restart

off-time. The first time a fault is asserted the off-time is 150 ms

600

020

68 kHz

64 kHz

VDRAIN

Time (

µs)

500

400

300

200

100

Figure 5. Frequency Jitter.

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