WS3440 datasheet(5/8 Pages) WINSEMI

5 / 8 page

W/T-Y027-Rev.A/1 May.2014

WS3440 Product Description

WIN SEM I

M ICROELECTRON ICS

WIN SEM I

M ICROELECTRON ICS

WIN SEM I

M ICROELECTRON ICS

WIN SEM I

M ICROELECTRON ICS

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M ICROELECTRON ICS

0813

Function

unction

unction Description

Description

WS3440 is a constant current controller, designed for driving

non-isolated buck LED power supply. WS3440 uses patent

constant current control method and source driver structure,

excellent constant current characteristic is achieved with low

counts components. Low cost and high efficiency of system

is realized.

Start

Start up

The Vcc will be charged through the startup resistor when

the system is powered on. When the voltage on Vcc reaches

the startup voltage threshold, the controller starts to

switching. The Vcc voltage of WS3440 is clamped to 7.3V

by internal Zener diode.

Constant

Constant Current

Current

Current Control

Control

Control and

and

and Output

Output

Output Current

Current

Current Setting

Setting

The WS3440 uses patent constant current control method,

excellent constant current is achieved with low counts

components. The WS3440 senses the peak current in

inductor cycle by cycle. The CS Pin is connected to the input

of internal current comparator, and compared with the

internal 400mV reference voltage. The external power

MOSFET will be turned off when the CS pin voltage reaches

the voltage threshold. The comparator has a 350ns LEB

timer to avoid mis-trigger.

The peak current in the inductor is given by:

)

(

400

PK =

The Rcs is the resistance of current sense resistor The

current in LED can be calculated by the following equation:

LED

The IPK is the peak current in inductor

Source

Source Driver

Driver

Driver Structure

Structure

The WS3440 uses the patent source driver structure. The

typical operation current is as low as 200uA, the auxiliary

winding is not need. So the system design is simple and the

cost is low.

Inductance

Inductance Calculation

Calculation

The WS3440 is designed to work in inductor current critical

mode; the energy will be stored in the inductor when the

MOSFET is turned on. The turn on time is given by:

LED

−

The L is the inductance. The Ipk is the peak current in

inductor. The VIN is the input rectified voltage. The VLED

is the voltage on LED.

When the power MOSFET is turned off, the inductor current

will decrease from the peak current to zero. The turn off time

is given by:

LED

off

The MOSFET will be turned on again when it detects the

inductor current goes to zero. The inductance can be

calculated by the following equation:

LED

−

)

(

The f is the system switching frequency, which is

proportional to the input voltage. So the minimum switching

frequency is set at lowest input voltage, and maximum

switching frequency is set at highest input voltage.

The WS3440 internally set the minimum off time to 4us and

maximum off time to 240 us. When the inductance is very

small, the toff may goes below the minimum off time and the

inductor current becomes discontinuous. So the output LED

current will be smaller than the setting value. If the

inductance is too large, the toff may goes beyond the

maximum off time and the inductor current becomes

continuous. And the output LED current will be larger than

the setting value. So it’s very important to select a right

inductance.

The WS3440 also internally set the maximum on time to

40us. When the input voltage is very low or LED output

WS3440 Datasheet(PDF) 5 Page - Shenzhen Winsemi Microelectronics Co., Ltd

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