WS3412HS7P datasheet(5/8 Pages) WINSEMI

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WS3412H ProductDescription

WINSEMI MICROELECTRONICS

Application Information

The WS3412H is a high precision Active PFC driver

integrating 650V power MOSFET, specially designed for

non-isolated buck offline constant current LED lighting.

Operating in critical conduction mode, the driver achieves

high power factor, low THD and high efficiency.

Start Up

After system powered up, the VCC pin capacitor is

charged up by the start up resistor. When the VCC pin voltage

reaches the turn on threshold, the internal circuits start

working. The COMP pin voltage is pulled up to 1.5V quickly,

then the WS3412Hstarts switching. The system works at 10k

Hz frequency at the beginning, the COMP voltage rises up

gradually, and the inductor peak current also rises up. The

LED current hence achieves a soft start without overshoot.

After the output voltage is built up, the VCC power is supplied

by the output voltage through a diode.

The maximum value of the startup current is 100uA. For

the application of 85VAC—264VAC, the startup resistor can

be calculated by the equation:

850

100

max

min







Constant Current Control

The WS3412H utilizes floating ground structure. The

inductor current is sensed during the whole switching cycle,

thus it achieves high precision output current control and also

excellent line and load regulation.

The current in LED can be calculated by the equation:

Rcs

200



Rcs is the current sensing resistor.

Feedback Network

The WS3412H senses the output current zero crossing

information through the feedback network, the FB falling

threshold voltage is set to 0.2V with 0.15V hysteresis. The FB

pin is also used to detect output OVP, the threshold voltage is

1.6V. The ratio of FB upper resistor to lower resistor can be

set as:

OVP

FBH

FBL





Where,

RFBL: The lower resistor of the feedback network

RFBH: The upper resistor of the feedback network

VOVP: Output over voltage setting point

The minimum VOVP should be set about 1.3Vo.It is

recommended that the FB lower resistor is set to

3.3KΩ—5.1KΩparalleled with a 22pF ceramic capacitor.

Protection Functions

The WS3412H offers rich protection functions to improve

the system reliability.

When the LED is open circuit, the output voltage will

gradually rise up. The output voltage is sensed by the FB pin

when power MOSFET is turned off. When FB voltage reaches

the OVP threshold, it will trigger fault logic and the system

stops switching.

When the LED is shorted circuit, the system will work

under 10k Hz switching frequency. Meanwhile, the output

voltage is low and the VCC pin cannot be charged up by the

output voltage, so the VCC pin voltage will gradually

decrease and finally reaches the UVLO threshold.

After the system enters into fault condition, the VCC

voltage will decrease until it reaches UVLO threshold. Then

the system will re-start again. If the fault condition is removed,

the system will resume normal operation.

When the output is short circuit or the inductor is

saturated, the CS peak voltage will be relatively high. When

CS voltage reaches the internal limitation (1V), the power

MOSFET will be turned off instantaneously. This cycle by

cycle current limitation can help protecting the power

MOSFET, the inductor and the output diode.

Thermal Regulation

The WS3412H integrates thermal regulation function.

When the system is over temperature, the output current is

gradually reduced; the output power and thermal dissipation

are also reduced. The system temperature is regulated and

the system reliability is improved. The thermal regulation

temperature is set to 150℃ internally.

PCB Layouts

The following guidelines should be followed in WS3412H

PCB layout:

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