BW9910/A High Brightness LED Driver

© 2012 Bruckewell Technology Corp., Ltd.

7

www.bruckewell-semi.com/

(so-called PWM dimming) controls the LED brightness by

varying the duty ratio of the output current.

The linear dimming can be implemented by applying a

control voltage from 0 to 250mV to the LD pin. This

control voltage overrides the internally set 250mV

threshold level of the CS pin and programs the output

current accordingly. For example, a potentiometer

control voltage at the CS pin. Applying a control voltage

higher than 250mV will not change the output current

setting. When higher current is desired, select a smaller

sensing resistor.

The PWM dimming scheme can be implemented by

applying an external PWM signal to the PWM_D pin. The

PWM signal can be generated by a microcontroller or a

pulse generator with a duty cycle proportional to the

amount of desired light output. This signal enables and

disables the converter modulating the LED current in the

PWM fashion. In this mode, LED current can be in one of

the two states: zero or the nominal current set by the

current sense resistor. It is not possible to use this

method to achieve average brightness levels higher than

the one set by the current sense threshold level of the

BW9910/BW9910A. By using the PWM control method of

the BW9910/BW9910A, the light output can be adjusted

between zero and 100%. The accuracy of the PWM

dimming method is limited only by the minimum gate

pulse width, which is a fraction of a percentage of the low

frequency duty cycle. PWM dimming of the LED light can

be achieved by turning on and off the converter with low

frequency 50Hz to 1kHz TTL logic level signal.

Programming Operating Frequency

The operating frequency of the oscillator is programmed

between 25kHz and 300kHz using an external resistor

Equation :

(1)

improved as well.

Power Factor Correction

When the input power to the LED driver does not

exceed 25W, a simple passive power factor correction

circuit can be added to the BW9910/BW9910A typical

application circuit in Figure 2 in order to pass the AC line

harmonic limits of the EN61000-3-2 standard for class C

equipment. The typical application circuit diagram shows

how this can be done without affecting the rest of the

circuit significantly. A simple circuit consisting of 3

diodes and 2 capacitors is added across the rectified AC

line input to improve the line current harmonic distortion

and to achieve a power factor greater than 0.85.

Inductor Design

The buck circuit is usually selected and it has two

operation

modes:

continuous

and

discontinuous

conduction modes. A buck power stage can be designed

to operate in continuous mode for load current above a

certain level usually 15% to 30% of full load. Usually, the

input voltage range, the output voltage and load current

are defined by the power stage specification. This

leaves the inductor value as the only design parameter

to maintain continuous conduction mode. The minimum

value of inductor to maintain continuous conduction

mode can be determined by the following example.

Referring to the typical buck application circuit in Figure

5, the value can be calculated from the desired peak-to-

peak LED ripple current in the inductor. Typically, such

ripple current is selected to be 30% of the nominal LED

current. In the example given here, the nominal current

voltage drop across the LED string. For example, when

the string consists of 10 high brightness LEDs and each

diode has a forward voltage drop of 3.3V at its nominal

Equation :

(2)

(3)

(4)

(5)