LNK501P datasheet(3/20 Pages) POWERINT

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LNK501

LinkSwitch Functional Description

The duty cycle, current limit and operating frequency

relationships with CONTROL pin current are shown in

Figure 4. Figure 5 shows a typical power supply outline

schematic which is used below to describe the LinkSwitch

operation.

Power Up

During power up, as V

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 CONTROL pins (see Figure 2). When the

CONTROL pin voltage reaches approximately 5.7 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

the primary transformer winding ramp up, the feedback

CONTROL current I

C increases. As shown in Figure 4, the

internal current limit increases with I

C and reaches ILIM when IC

is equal to I

DCT. 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. R1

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

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

frequency is reduced, which reduces energy consumption

under 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 standby mode. In auto-restart,

LinkSwitch periodically restarts the power supply so that normal

power supply operation can be restored when the fault is

removed.

Figure 4. CONTROL Characteristics.

Figure 5. Power Supply Outline Schematic.

VOUT

VIN

LinkSwitch

PI-2715-100801

PI-2799-112101

Internal Current Limit

CONTROL Current IC

IDCS

CONTROL Current IC

ICD1

CONTROL Current IC

Duty Cycle

Frequency

ILIM

IDCT

77%

30%

3.8%

OSC

OSC(low)

Auto-restart

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