WS9441 datasheet(6/9 Pages) ETC2

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WS9441 Product Description

WINSEMI MICROELECTRONICS

Inductor Selection

The WS9441 works in a inductor critical current

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,

IPK is the peak current of the inductor,

VIN is the DC bus voltage after the rectifier bridge,

VLED is the voltage on the LED.

After the power MOSFET is also switched off and 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 the

equation:

LED

off





The inductance can be calculated by the equati

on:

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.

The minimum and maximum off time of WS9441 is set at

4.5us and 310us, respectively. Referring to the equation of

Toff calculation, if the inductance is too small, the Toff may be

smaller than the minimum off time, the 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.

Thermal Regulation

The WS9441 integrates thermal regulation function.

When the system is under higher 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.

Over Voltage Protection

The over voltage protection of the WS9441 can be

programmed by the ROVP pin resistor. The ROVP pin voltage

is 0.5V.

When the LED is open circuit, the period of the output

voltage increases gradually, the demagnetization time gets

shorter. The demagnetization time at OVP---- Tovp can be

calculated by the open circuit protection voltage by the

equation:

OVP

Tovp





Where，

Vcs is the CS pin turn off threshold (0.4V);

Vovp is the open circuit protection voltage.

And then the Rovp resistor value can be calculated by the

equation:

120

Tovp

Rovp





（

KΩ）

It is suggested that the open circuit protection voltage is

set at or above 1.5 times maximum load voltage.

Protection Function

The WS9441 offers rich protection functions to improve

the system reliability, including LED open/short protection, CS

resistor short protection, VCC under voltage protection,

thermal regulation. When the LED is open circuit, the system

will trigger the over voltage protection and stop switching.

When the LED short circuit is detected, the system works at

low frequency and the CS pin turnoff threshold is reduced. So

the system power consumption is very low. At some abnormal

fault condition, such as CS resistor shorted or inductor

saturation, the internal fast fault detection circuit will be

triggered, the system stops switching immediately.

After the system enters into faulty condition, the VCC

voltage will decrease until it reaches the UVLO threshold, then

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