WS9841 datasheet(5/8 Pages) ETC2

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WS9841 Product Description

WINSEMI MICROELECTRONICS

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:

)

(

400

PK 

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 4. VLN pin application circuit

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:

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:

LED

off





The inductance can be calculated by the equation:

LED

)

(









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.

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