WS9841 Datasheet, PDF (5/8 Pages) List of Unclassifed Manufacturers – High Efficiency TRIAC dimmable LED

WS9841 Product Description
Function Description
The WS9841 is a high efficiency TRIAC dimmable LED
driver IC, specially designed for Buck, Buck-boost or
Fly-back LED lighting. The WS9841 integrates a 300V power
MOSFET. Utilizing patent pending MOSFET source driving
technique, it can achieve excellent constant current
performance with very few external components count, so
the system cost and size are 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 WS9841 integrates an 18.8V zener
diode to clamp the VCC voltage. After the output voltage is
established, VCC is supplied by the output voltage.
Constant Current Control
Cycle by Cycle current sense is adopted in WS9841, 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 peak inductor current is given by:
I _PK

400
R _CS
(mA)
Where, RCS is the current sense resistor value. The CS
comparator includes a 350ns leading edge blanking time.
The VLN pin detects the TRIAC dimmer conducting
angle to adjust the LED driver output current. The application
circuit of the VLN pin is shown in Figure 4. When the VLN pin
voltage is higher than 3V, the threshold for peak current limit
VCS_TH decreases linearly as shown in Figure 5. When the
VLN pin is not used, it can be grounded or floated.
Figure 5. Relationship of VLN voltage and VCS_TH
Inductor Selection
The WS9841 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:
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.
The maximum on time of the MOSFET is set to 10us
internally.
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
Figure 4. VLN pin application circuit
Where, f is the maximum 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.
WINSEMI MICROELECTRONICS
WINSEMI MICROELECTRONICS
WINSEMI MICROELECTRONICS
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