WS9221 datasheet(5/8 Pages) ETC2

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WS9221ProductDescription

WINSEMI MICROELECTRONICS

Application Information

The WS9221 is a high performance power switch

specially designed for LED lighting, with patent constant

current control technology. The WS9221integrates a 650V

power MOSFET. The accurate LED current can be realized

without opto-coupler, TL431 feedback circuit and auxiliary

winding while minimizing the external component count,

lowering the total bill of material cost.

Start Up

The start-up current in WS9221 is designed to as low as

45uA. The VCC capacitor will be charged through the

start-up resistor when the system is powered on. Once the

VCC voltage reaches the start-up threshold, the WS9221

will start to switch. The WS9221 integrates a 8V zener for

VCC clamping. Due to the ultra-low operating current, the

auxiliary winding is not needed to supply the IC.

Constant Current Control

Cycle-by-Cycle current sense is adopted in WS9221,

the CS is connected to the current sense comparator, and

the voltage on CS will be compared with the internal 500mV

reference voltage, the MOSFET will be switched off when

the voltage on CS reaches the threshold. The output of the

comparator includes a 500ns leading edge blanking time.

The primary peak current is given by:

)

(

500

P_PK



The current in LED can be calculatedby the equation:

OUT





P_PK

Where,

NP: primary winding turns of transformer

NS: secondary winding turns of transformer

IP_PK: peak current in MOSFET

Power MOSFET

The WS9221 integrates a 650V power N-MOSFET. It

can minimize the external component count and reduce the

BOM cost and PCB size.

The WS9221 uses SOP-7 package. The recommended

system output power is below 5W in universal input

(90Vac~265Vac) application.

Operating Switching Frequency

The WS9221 is designed to work in discontinuous

conduction mode and no external loop compensation

component is required while maintaining stability. The

maximum duty cycle is limited to 42%. The maximum

switching frequency at normal operation is suggested to set

around 65KHz~70KHz. If the maximum frequency is set too

high, it will affect the number of maximum series LED lamps.

If set too low, the LED open circuit voltage will be too high.

The maximum and minimum switching frequency is

limited in WS9221 to ensure the stability of system.

The switching frequency can be set by the formula:

LED





Where, LP is the primary winding inductance of

transformer.

Protection Function

The WS9221 has integrated rich protection functions,

LED open/short protection, CS resistor short circuit

protection, VCC under voltage protection, over temperature

protection, and so on. When the LED is open circuit, it will

trigger over-voltage protection logic and latch, the system

stops switching immediately.

When the LED short circuit is detected, the system

works at low frequency (Fop=3KHz), so the power

consumption is low. At some catastrophic fault condition,

such as shorted CS resistor or flyback transformer

saturation, the internal fast fault detection circuit will trigger

and latch, the system stops switching immediately.

After the system enters into fault latch condition, the

VCC voltage will fall until it reaches UVLO threshold. Then

the system will re-start again. If the fault condition is

removed, the system will recover to normal operation.

PCB Layouts

The following rules should be followed in WS9221 PCB

layout:

Bypass Capacitor

The bypass capacitor on VCC should be as close as

possible to the VCC pin and GND pin.

Ground Path

The power ground path for current sense should be

short, and the power ground path should be separated from

small signal ground path before the negative of the bulk

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