WS3441A Datasheet, PDF (5/8 Pages) List of Unclassifed Manufacturers – Non-isolated Buck Offline LED Driver

WS3441A ProductDescription
Application Information
The WS3441A is a high performance non-isolated Buck
converter specially designed for LED lighting. The device
integrates a 500V power MOSFET. With very few external
components, the converter achieves excellent constant
current control. And it does not need auxiliary winding for
powering the IC or voltage sensing, hence the system size
and cost is greatly reduced.
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 operating. The WS3441A integrates a 17V zener diode
to clamp the VCC voltage. Due to the ultra-low operating
current, the auxiliary winding is not needed to supply the IC.
The maximum value of the startup current is 270uA. For
the application of 176VAC — 264VAC, the startup resistor
can be be calculated by the equation:
R_st

V_{in _ min} 1.414
I _{st _ max}

176V 1.414
270uA

920k
Constant Current Control
Cycle by Cycle current sense is adopted in WS3441A,
the CS pin is connected to the current sense comparator,
and the voltage on CS pin is compared with the internal
400mV reference voltage. The MOSFET will be switched off
when the voltage on CS pin reaches the threshold. The CS
comparator includes a 350ns leading edge blanking time.
The peak inductor current is given by:
I _PK

400
R _CS
(mA)
Where, RCS is the current sense resistor value.
The current in LED can be calculated by the equation:
I _LED

I _PK
2
Where, IPK is the peak current of the inductor.
t _on

L  IPK
V_IN  V_LED
Where,
L is the inductance value
VIN is the DC bus voltage after the rectifier bridge
VLED is the voltage on the LED
After the power MOSFET is switched off, the current in
the inductor decreases. When the inductor current reaches
zero, the power MOSFET is turned on again by IC internal
logic. The off time of the MOSFET is given by:
t _off

L  IPK
V_LED
The inductance can be calculated by the equation:
L  ^{VLED  (VIN  VLED )}
f  IPK  V_IN
The f is the system switching frequency, which is
proportional to the input voltage. So the minimum switching
frequency is set at lowest input voltage, and the maximum
switching frequency is set at highest input voltage.
The minimum and maximum off time of WS3441A is set
at 3us and 340us, respectively. Referring to the equation of
Toff calculation, if the inductance is too small, the Toff may
be smaller than the minimum off time, system will operate in
discontinuous conduction mode and the output current will
be smaller than the designed value. If the inductance is too
large, the Toff may be larger than the maximum off time, the
system will operate in continuous conduction mode and the
output current will be higher than the designed value. So it is
important to choose a proper inductance.
Over Voltage Protection
The over voltage protection can be programmed by the
ROVP pin resistor. The ROVP pin voltage is 0.5V.
When the LED is open circuit, the output voltage
increases gradually, and the demagnetization time gets
shorter. The demagnetization time at OVP---- Tovp can be
calculated by the open circuit protection voltage:
Inductor Selection
The WS3441A works under inductor current critical
conduction mode. When the power MOFET is switched on,
the current in the inductor rises up from zero, the on time of
the MOSFET can be calculated by the equation:
Tovp  ^{L Vcs}
Rcs Vovp
Where，
Vcs is the CS pin turn off threshold (400mV)
Vovp is the open circuit protection voltage
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