WS3418 Datasheet, PDF (5/8 Pages) List of Unclassifed Manufacturers – Non-isolated Buck APFC Offline LED Driver

WS3418 ProductDescription
Application Information
The WS3418 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 WS3418starts 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:
R_st
 ^{Vin _ min}
I _{st _ max}
 ^85V  850KΩ
100uA
Constant Current Control
The WS3418 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:
I_O
 ^VCS
R_CS

200mV
Rcs
R_{FBL } 1.6V
R_FBL  R_FBH V_OVP
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 WS3418 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.
Rcs is the current sensing resistor.
Feedback Network
The WS3418 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:
Thermal Regulation
The WS3418 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 WS3418
PCB layout:
WINSEMI MICROELECTRONICS
WINSEMI MICROELECTRONICS
WINSEMI MICROELECTRONICS
www.winsemi.comTel : [phone] Fax : [phone]
WINSEMI MICROELECTRONICS
WINSEMI MICROELECTRONICS
5/8