BW9910/A High Brightness LED Driver

© 2012 Bruckewell Technology Corp., Ltd.

10

www.bruckewell-semi.com/

The required minimum value of the inductor is given by :

2.05mH

So, use 2.2mH

Output Capacitor

Unlike the buck topology, the buck-boost converter

requires an output filter capacitor to deliver power to the

LED string during the ON time of the MOSFET.

In order to reduce the current ripple on the LED, this

capacitor must have impedance that is much lower than

the switching ripple by a factor of 10, a capacitor with

equivalent series resistance (ESR) of 0.3Ω is needed. A

chip SMT tantalum capacitor can be selected for this

purpose.

Figure 2. DC-DC Buck-Boost LED Driver

(BW9910 for 24V Battery System)

Buck LED Driver

The buck power conversion topology can be used when

the LED string voltage is needed to be lower than the

input supply voltage. The design procedure for a buck

LED driver outlined in the previous sections can be

applied to the low voltage LED drivers as well. However,

the designer must keep in mind that the input voltage

must be maintained higher than 2 times the forward

voltage drop across the LEDs. This limitation is related to

the output current instability that may develop when the

BW9910/BW9910A buck converter operates at a duty

cycle greater than 0.5. This instability reveals itself as an

oscillation of the output current at a sub-harmonic of the

switching frequency.

Benefiting from the BW9910/BW9910A inherited high

optimize design efficiency. This solution can easily

achieve above 90% efficiency. However, if the duty

cycle is configured to reach above more than 50%,

some instability called sub-harmonics oscillation (SBO)

will occur.

The best solution is to adopt the so-called constant off-

time operation as shown in Figure 4 and 6. To set

ground by default. This resistor can alternatively be

connected

to

gate

of

MOSFET

to

force

the

BW9910/BW9910A to enter constant off-time mode

which will decrease duty cycle from 50% by increase

is chosen as shown in Figure 3 because it has better

efficiency.

For general LED lighting application, PFC becomes a

necessary factor in order to meet the international

standard of solid state lighting. If passive valley-fill PFC

is chosen, then the BW9910/BW9910A is biased right

after passive PFC stage.

a more than 50% duty cycle for the same LED loading

generated. In this case, the constant off-time mode as

shown in Figure 6 should be chosen.

Example :

3.3V

actual DC rail will become

110V 1.414 / 2

The duty cycle, D

than 77.7/2

38.8V. Another word, if any string

consisting of 38.8/3.3

12 LEDs in a series, SBO will

occur.