BW9910/A High Brightness LED Driver

© 2012 Bruckewell Technology Corp., Ltd.

6

www.bruckewell-semi.com/

Application Information

AC-DC Off-Line Application

The BW9910/BW9910A is a low cost off-line buck or

boost converter control IC specifically designed for driving

multi-LED stings or arrays. It can be operated from either

universal AC line or any DC voltage between 15V and

500V. Optionally, a passive power factor correction circuit

can be used in order to pass the AC harmonic limits set

by EN61000-3-2 class C for lighting equipment having

input power less than 25W. The BW9910/BW9910A can

drive up to hundreds of HB LEDs or multiple strings of HB

LEDs. The LED arrays can be configured as a series or

series/parallel

connection.

The

BW9910/BW9910A

regulates constant current that ensures controlled

brightness and spectrum of the LEDs, and extends their

lifetime, and also allows PWM control of brightness via an

enable (PWM_D) pin.

The BW9910/BW9910A can also control brightness of

LEDs by programming continuous output current of the

LED driver (so-called linear dimming) when a control

voltage is applied to the LD pin.

The BW9910/BW9910A is offered in standard 8-pin SOIC

and SOIC-EP packages.

The BW9910/BW9910A has a built-in high-voltage linear

regulator that powers all internal circuits and can also

serve as a bias supply for low voltage and low power

external circuitry.

LED Driver Operation

The BW9910/BW9910A can control all basic types of

converters,

isolated

or

non-isolated,

operating

in

continuous or discontinuous conduction mode. When the

gate signal turns on the external power MOSFET, the

LED driver stores the input energy in an inductor or in the

primary inductance of a transformer and, depending on

the converter type, may partially deliver the energy

directly to LEDs. The energy stored in the magnetic

component is further delivered to the output during the

off-cycle of the power MOSFET producing current

through the string of LEDs (Fly-back mode of operation).

threshold voltage, the gate drive is enabled. The output

current is controlled by means of limiting peak current in

the external power MOSFET. A current sensing resistor is

connected in series with the source terminal of the

MOSFET. The voltage from the sensing resistor is

applied to the CS pin of the BW9910/BW9910A. When

the voltage at CS pin exceeds a peak current sensing

threshold voltage, the gate drive signal terminates, and

the power MOSFET turns off. The threshold is internally

set to 250mV, or it can be programmed externally by

applying voltage to the LD pin. When the soft-start

function is required, a capacitor can be connected to the

LD pin to allow this voltage to ramp at a desired rate,

therefore, assuring that output current of the LED ramps

gradually. Additionally, a simple passive power factor

correction circuit, consisting of 3 diodes and 2

capacitors, can be added as shown in the typical

application circuit diagram of Figure 6.

Supply Current

A

current

of

1.0mA

is

needed

to

start

the

BW9910/BW9910A. As shown in the block diagram on

page 5, this current is internally generated in the

BW9910/BW9910A without using bulky startup resistors

typically required in the off-line applications. Moreover,

in many applications the BW9910/BW9910A can be

continuously powered using its internal linear regulator

that provides a regulated voltage of 7.5V/10V for all

internal circuits.

Setting Lighting Output

When the buck converter topology of Figure 5 is

selected, the peak CS voltage is a good representation

of the average current in the LED. However, there is a

certain error associated with this current sensing method

that needs to be accounted for. This error is introduced

by the difference between the peak and the average

current in the inductor. For example, if the peak-to-peak

ripple current in the inductor is 150mA, to get a 500mA

LED current, the sensing resistor should be as follows :

0.43Ω

Dimming

Dimming can be accomplished in two ways, separately

or combined, depending on the application. Light output

of the LED can be controlled either by linear change of

its current, or by switching the current on and off while

maintaining it constant. The second dimming method