WS9221B Datasheet, PDF (5/8 Pages) List of Unclassifed Manufacturers – High Precision PSR Constant Current LED Driver

WS9221BProductDescription
Application Information
The WS9221B is a high performance power switch
specially designed for LED lighting, with patent constant
current control technology. The WS9221Bintegrates 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 inWS9221B 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 WS9221B
will start to switch. TheWS9221B integrates a 8V zener for
VCC clamping. Due to the ultra-low operating current, the
auxiliary winding is not needed to supply the IC.
Operating Switching Frequency
The WS9221B 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 WS9221B to ensure the stability of system.
The switching frequency can be set by the formula:
f

Np² V_LED
8 Ns²  Lp  I_LED
Where, LP is the primary winding inductance of
transformer.
Constant Current Control
Protection Function
Cycle-by-Cycle current sense is adopted in WS9221B,
The WS9221Bhas integrated rich protection functions,
the CS is connected to the current sense comparator, and
LED open/short protection, CS resistor short circuit
the voltage on CS will be compared with the internal 500mV
protection, VCC under voltage protection, over temperature
reference voltage, the MOSFET will be switched off when
protection, and so on. When the LED is open circuit, it will
the voltage on CS reaches the threshold. The output of the
trigger over-voltage protection logic and latch, the system
comparator includes a 500ns leading edge blanking time.
stops switching immediately.
The primary peak current is given by:
When the LED short circuit is detected, the system
I _{P_PK}

500
(mA)
R_CS
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
The current in LED can be calculatedby the equation:
saturation, the internal fast fault detection circuit will trigger
I _OUT

I _{P_PK}
4

N_P
N_S
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
Where,
the system will re-start again. If the fault condition is
NP: primary winding turns of transformer
removed, the system will recover to normal operation.
NS: secondary winding turns of transformer
IP_PK: peak current in MOSFET
PCB Layouts
The following rules should be followed in WS9221B
Power MOSFET
The WS9221Bintegrates a 650V power N-MOSFET. It
can minimize the external component count and reduce the
BOM cost and PCB size.
The WS9221B uses SOP-7 package. The
recommended system output power is below 3W in
universal input (90Vac~265Vac) application.
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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WINSEMI MICROELECTRONICS
WINSEMI MICROELECTRONICS
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