LNK520G-TL datasheet(3/20 Pages) POWERINT

Part #	LNK520G-TL

Description	Energy Efficient, CV or CV/CC Switcher for Very Low Cost Adapters and Chargers

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Manufacturer	POWERINT [Power Integrations, Inc.]

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3

LNK520

E

2/05

Figure 4. CONTROL Characteristics.

Figure 5. Power Supply Schematic outline.

PI-3579-031004

Internal Current Limit

CONTROL Current IC

IDCS

CONTROL Current IC

Duty Cycle

Frequency

ILIM

IDCT

77%

30%

3.8%

fOSC

fOSC(low)

Auto-restart

ICD1

VOUT

VIN

S

D

C

D3

R3

C4

D1

D2

C3

C1

IC

LinkSwitch

PI-3578-021405

R1

C2

R2

LinkSwitch

Functional Description

The duty cycle, current limit and operating frequency

relationships with CONTROL pin current are shown in

Figure4.Figure5showsatypicalpowersupplyschematicoutline

which is used below to describe the LinkSwitch operation.

Power Up

Duringpowerup,asV

IN is ﬁrst applied (Figure 5), the CONTROL

pin capacitor C1 is charged through a switched high voltage

current source connected internally between the DRAIN and

CONTROLpins(seeFigure2).WhentheCONTROLpinvoltage

reaches approximately 5.6 V relative to the SOURCE pin, the

high voltage current source is turned off, the internal control

circuitry is activated and the high voltage internal MOSFET

starts to switch. At this point, the charge stored on C1 is used

to supply the internal consumption of the chip.

Constant Current (CC) Operation

As the output voltage, and therefore the reﬂected voltage across

thetransformerbiaswindingrampup,thefeedbackCONTROL

current I

C ﬂowing through R1 increases. As shown in Figure 4,

the internal current limit increases with I

C and reaches ILIM when

I

C is equal to IDCT. The internal current limit vs. IC characteristic

is designed to provide an approximately constant power supply

output current as the power supply output voltage rises.

Constant Voltage (CV) Operation

When I

C exceeds IDCS, typically 2 mA (Figure 4), the maximum

duty cycle is reduced. At a value of I

C that depends on power

supply input voltage, the duty cycle control limits LinkSwitch

peak current below the internal current limit value.At this point

the power supply transitions from CC to CV operation. With

minimum input voltage in a typical universal input design, this

transition occurs at approximately 30% duty cycle. Resistor R1

(Figure 5)is therefore initially selected to conduct a value of I

C

approximately equal to I

DCT when VOUT is at the desired value

at the minimum power supply input voltage. The ﬁnal choice

of R1 is made when the rest of the circuit design is complete.

When the duty cycle drops below approximately 4%, the

frequencyisreduced,whichreducesenergyconsumptionunder

light load conditions.

Auto-Restart Operation

When a fault condition, such as an output short circuit or open

loop, prevents ﬂow of an external current into the CONTROL

pin, the capacitor C1 discharges towards 4.7 V. At 4.7 V, auto-

restart is activated, which turns the MOSFET off and puts the

control circuitry in a low current fault protection mode. In

auto-restart, LinkSwitch periodically restarts the power supply

so that normal power supply operation can be restored when

the fault is removed.

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