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WINSEMI MICROELECTRONICS

WINSEMI MICROELECTRONICS

WINSEMI MICROELECTRONICS

WINSEMI MICROELECTRONICS

WINSEMI MICROELECTRONICS

suggested to use. Besides, the operating frequency of

freewheeling diode is from 20K to 120 KHz, such ultra fast

recovery diodes as ES and ER whose Trr is less than 50ns

are good choice.

Input Buck Electrolytic Capacitor

Input voltage should be considered in to the capacitor

pressure, and the common value is 400V.

In general, the capacitor design can be calculated by the

following empirical equations:

90Vac~264Vac: 1W output uses1uF intput electrolytic

capacitor;

176Vac~264Vac:

1W

output

uses

0.5uF

intput

electrolytic capacitor.

Output Capacitor

Electrolytic capacitor is recommended to be used. Its

stable capacitance can improve the efficiency of power

supply, the LED ripple current and luminous efficiency.

Vovp voltage should be considered in output capacitor

pressure.

Inductor Selection

The WS9822B 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 (4):

LED

IN

PK

on

V

V

I

L

t







（

4）

Where,

L is the inductance value,

After the power MOSFET is 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

（

5）:

LED

PK

off

V

I

L

t





（

5）

The inductance can be calculated by the equati

on（6）:

IN

PK

LED

IN

LED

V

I

f

V

V

V

L











)

(

（

6）

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 WS9822B is set

at 3us and 240us 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.

In addition, the OVP voltage changes with the Inductance

value. Due to open magnetic flux of DR core, the Inductance

value changes easily under the influence of temperature and

external environment. Therefore, please don’t use them along

with the product in order to prevent from false triggering of

OVP.

Over Voltage Protection

The over voltage protection of the WS9822B can be

programmed by the ROVP pin resistor. The voltage of ROVP

pin 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 （7）:

OVP

CS

CS

OVP

V

R

V

L

T







（

7）

Where，

Vcs is the CS pin turn off threshold (400mV);