Rev. F 01/10

3

LNK603-606/613-616

www.powerint.com

LinkSwitch-II Functional Description

The LinkSwitch-II combines a high voltage power MOSFET

switch with a power supply controller in one device. Similar to

the LinkSwitch-LP and TinySwitch-III it uses ON/OFF control to

regulate the output voltage. In addition, the switching frequency

is modulated to regulate the output current to provide a

constant current characteristic. The LinkSwitch-II controller

consists of an oscillator, feedback (sense and logic) circuit, 6 V

regulator, over-temperature protection, frequency jittering,

current limit circuit, leading-edge blanking, inductance

correction circuitry, frequency control for constant current

regulation and ON/OFF state machine for CV control.

Inductance Correction Circuitry

If the primary magnetizing inductance is either too high or low

the converter will automatically compensate for this by adjusting

the oscillator frequency. Since this controller is designed to

operate in discontinuous-conduction mode the output power is

directly proportional to the set primary inductance and its

tolerance can be completely compensated with adjustments to

the switching frequency.

Constant Current (CC) Operation

As the output voltage and therefore the flyback voltage across

the bias winding increases, the FEEDBACK pin voltage increases.

The switching frequency is adjusted as the FEEDBACK pin

voltage increases to provide a constant output current regulation.

The constant current circuit and the inductance correction

circuit are designed to operate concurrently in the CC region.

Constant Voltage (CV) Operation

As the FEEDBACK pin approaches V

current regulation mode, the power supply transitions into CV

operation. The switching frequency at this point is at its

maximum value, corresponding to the peak power point of the

CCCV characteristic. The controller regulates the feedback pin

voltage to remain at V

FEEDBACK pin voltage is sampled 2.5

ms after the turn-off of

the high voltage switch. At light loads the current limit is also

reduced to decrease the transformer flux density.

Output Cable Compensation

This compensation provides a constant output voltage at the

end of the cable over the entire load range in CV mode. As the

converter load increases from no-load to the peak power point

(transition point between CV and CC) the voltage drop introduced

across the output cable is compensated by increasing the

FEEDBACK pin reference voltage. The controller determines the

output load and therefore the correct degree of compensation

based on the output of the state machine. Cable drop

compensation for a 24 AWG (0.3

W) cable is selected with

C

Auto-Restart and Open-Loop Protection

In the event of a fault condition such as an output short or an

open loop condition the LinkSwitch-II enters into an appropriate

protection mode as described below.

In the event the FEEDBACK pin voltage during the flyback

period falls below 0.7 V before the FEEDBACK pin sampling

delay (~2.5

ms) for a duration in excess of ~450 ms (auto-restart

on-time (t

the power MOSFET is disabled for 2 seconds (~18% auto-

restart duty cycle). The auto-restart alternately enables and

disables the switching of the power MOSFET until the fault

condition is removed.

In addition to the conditions for auto-restart described above, if

the sensed FEEDBACK pin current during the forward period of

the conduction cycle (switch “on” time) falls below 120

mA, the

converter annunciates this as an open-loop condition (top

resistor in potential divider is open or missing) and reduces the

auto-restart time from 450 msec to approximately 6 clock cycles

(90

ms), whilst keeping the disable period of 2 seconds.

Over-Temperature Protection

The thermal shutdown circuitry senses the die temperature. The

threshold is set at 142 °C typical with a 60 °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 60 °C, at which point the MOSFET is

re-enabled.

Current Limit

The current limit circuit senses the current in the power

MOSFET. When this current exceeds the internal threshold

(I

cycle. The leading edge blanking circuit inhibits the current limit

comparator for a short time (t

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 MOSFET

conduction. The LinkSwitch-II also contains a “di/dt” correction

feature to minimize CC variation across the input line range.

6.0 V Regulator

The 6 V regulator charges the bypass capacitor connected to

the BYPASS pin to 6 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

device runs off of the energy stored in the bypass capacitor.

Extremely low power consumption of the internal circuitry

allows the LinkSwitch-II to operate continuously from the

current drawn from the DRAIN pin. A bypass capacitor value of

either 1

mF or 10 mF is sufficient for both high frequency

decoupling and energy storage.