LNK362-364

2-3

3

Rev. E 11/08

05

10

Time (

μs)

0

100

200

400

500

600

300

V

DRAIN

136.5 kHz

127.5 kHz

LinkSwitch-XT

Functional Description

LinkSwitch-XT combines a high-voltage power MOSFET

switch with a power supply controller in one device. Unlike

conventional PWM (pulse width modulator) controllers, a

simple ON/OFF control regulates the output voltage. The

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

circuit, 5.8 V regulator, BYPASS pin undervoltage circuit,

over-temperature protection, frequency jittering, current limit

circuit, and leading edge blanking integrated with a 700 V

power MOSFET. The LinkSwitch-XT incorporates additional

circuitry for auto-restart.

Oscillator

The typical oscillator frequency is internally set to an average

of 132 kHz. Two signals are generated from the oscillator: the

maximum duty cycle signal (DC

indicates the beginning of each cycle.

The oscillator incorporates circuitry that introduces a small

amount of frequency jitter, typically 9 kHz peak-to-peak,

to minimize EMI emission. The modulation rate of the

frequency jitter is set to 1.5 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.

Feedback Input Circuit

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

impedance source follower output set at 1.65 V for LNK362

and 1.63 V for LNK363/364. When the current delivered into

this pin exceeds 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 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. When the MOSFET is on, the

LinkSwitch-XTrunsoffoftheenergystoredinthebypasscapacitor.

Extremelylowpowerconsumptionoftheinternalcircuitryallows

the device 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 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

the device externally through a bias winding to decrease the

no-load consumption to less than 50 mW.

BYPASS Pin Undervoltage

The BYPASS pin undervoltage circuitry disables the power

MOSFET when the BYPASS pin voltage drops below 4.8 V.

Once the BYPASS pin voltage drops below 4.8 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.

When the die temperature rises above this threshold (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

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

When this current exceeds the internal threshold (I

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

is turned on. This leading edge blanking time has been set so

that current spikes caused by capacitance and rectifier reverse

recovery time will not cause premature termination of the

switching pulse.

Auto-Restart

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

short circuit, or an open loop condition, LinkSwitch-XT enters

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

oscillator gets reset every time the FB pin is pulled high. If the

FB pin is not pulled high for approximately 40 ms, the power

MOSFET switching is disabled for 800 ms. The auto-restart

alternately enables and disables the switching of the power

MOSFET until the fault condition is removed.

Figure 4. Frequency Jitter.