WS3412 ProductDescription
Application Information
RFBL
RFBL RFBH VOVP
1.6V
The WS3412 is a high precision Active PFC driver
integrating 550V 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.
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.
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 WS3412starts 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:
Protection Functions
The WS3412 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.
Vin _ min
85V
Rst
850KΩ
Ist _ max 100uA
Constant Current Control
The WS3412 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:
VCS
RCS
200mV
IO
Rcs
Rcs is the current sensing resistor.
Thermal Regulation
The WS3412 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.
Feedback Network
The WS3412 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:
PCB Layouts
The following guidelines should be followed in WS3412
PCB layout:
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