LNK6408 データシートの表示（PDF） - Unspecified

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LNK6408

Energy-Efficient, Accurate Primary-Side Regulation CV/CC Switcher for Adapters and Chargers

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LNK64x4-64x8

LinkSwitch-3 Functional Description

The LinkSwitch-3 combines a high-voltage power MOSFET switch with

a power supply controller in one device. It uses an 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-3 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 ramps up, 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 2 V from the constant 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 CV/CC characteristic.

The controller regulates the FEEDBACK pin voltage to remain at

FEEDBACK pin threshold (VFBTH) using an ON/OFF state-machine.

The 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 and the FEEDBACK pin sampling is done

earlier.

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. The amount of cable drop compensation is determined by

the third digit in the device part number.

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-3 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 ~300 ms (auto-restart on-time (tAR-ON) the

converter enters into auto-restart, wherein the power MOSFET is

disabled for 1500 ms. 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

300 ms 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 (ILIMIT), 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 (tLEB) after the power MOSFET 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 MOSFET conduction. The LinkSwitch-3

also contains a “di/dt” correction feature to minimize CC variation across

the input line range.

6 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-3 to

operate continuously from the current drawn from the DRAIN pin

however for the best no-load input power, the BYPASS pin should be

supplied current of IS1 from the bias winding at no-load conditions.

A bypass capacitor value of 1 mF is sufficient for both high frequency

decoupling and energy storage.

www.power.com

3

Rev. C 03/16

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