USL250X

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

5 of 8

www.unisonic.com.tw

QW-R103-155.B



OUTPOUT DESCRIPTION

USL250X

is ahigh current precision, non-isolated power switch for LED lighting working under transition mode. It

significantly simplifies the LED lighting system design by eliminating the auxiliary winding inductance and works

under low current. A500V power switch is also integrated into the device.

Start up Control

necessary.

Principle of CC Operation

For buck type operating in transition mode, the peak voltage of CS is detected cycle-by-cycle, and compared with

thepower switch will be turned off.

CS

OCP

TH

PK

R

V

I

_



Where, RCS indicates the resistor at CS pin for primary side current sensing. Refer to the equation 1, LED output

current is

CS

OCP

TH

PK

LED

R

V

I

I







2

2

_

USL250X

works under transition mode and the on-time for power switch is

LED

IN

PK

ON

V

V

I

L

T







Where L is the inductor of transformer at primary-side winding,Vin is line voltage after rectifying and VLED is the

voltage at LED.

When power switch turns off, the current flowing in the inductor will reduce to zero and the power switch will turn

on after then. The off-time for power switch is

LED

PK

OFF

V

I

L

T





The inductor could also be calculated by

IN

PK

LED

IN

LED

V

I

F

V

V

V

L











)

(

Where, F is the working frequency. The working frequency changes with input voltage. The lowest input voltage

determines the lowest working frequency and the highest input voltage determines the highest working frequency.

The minimum and maximum off-time are set in USL250X. Therefore, the value of inductor should be carefully

designed to achieve good LED output current, according to equation4.

Inner Line Compensation

USL250X

provides internal line compensation, therefore no outside sensing devices are needed. The

compensated voltage is added to CS voltage cycle-by-cycle and LED output current is kept constant under different

line voltage.