BW9910/A High Brightness LED Driver

© 2012 Bruckewell Technology Corp., Ltd.

8

www.bruckewell-semi.com/

120V 1.414 169V, the switching duty ratio can be

determined as follows :

0.195

(6)

Then, in this example, given the switching frequency,

50kHz, the required on-time of the MOSFET

transistor can be calculated as below :

3.91µs

(7)

The required minimum value of the inductor is given by :

5.06mH

(8)

Input Bulk Capacitor

An input filter capacitor should be designed to hold the

rectified AC voltage above twice the LED string voltage

throughout the AC line cycle. Assuming 15% relative

voltage ripple across the capacitor, a simplified formula

for the minimum value of the bulk input capacitor is given

by :

Equation :

(9)

where

0.20 ~ 0.25,

If the capacitor has a 15% voltage ripple then a simplified

formula for the minimum value of the bulk input capacitor

approximates to :

(10)

24µF, a value 33µF/250V can be used.

A passive PFC circuit at the input requires using two

series connected capacitors at the place of calculated

for ½ of the input voltage and have twice as much

capacitance.

Enable Function

The BW9910/BW9910A can be turned off by pulling the

PWM_D pin to ground. When the device is disabled, the

BW9910/BW9910A draws quiescent current of less than

1.0mA.

Output Open Circuit Protection

When the buck topology is used, and the LED is

connected in series with the inductor, there is no need

for any protection against an open circuit condition in the

LED string. Open LED connection means no switching

and can be continuous. In this case, since the output

voltage will be the same as input voltage, if there is a

capacitor connected across the output, this capacitor

should be able to withstand the peak value of the input

voltage.

Thermal Shut Down

Thermal protection is added due to buck topology can

generate large heat when operated with high voltage

input. The over temperature protection is activated to

shut down external MOSFET when the junction

hysteresis to re-start the MOSFET.

DC-DC Low Voltage Applications

Boost LED Driver

BW9910/BW9910A

can

also

be

used

in

boost

configurations – at reduced accuracy. The accuracy can

be improved by measuring the LED current with an Op-

Amp and use the Op Amp’s output to drive the LD pin.

Refer to Figure 1, a boost LED driver is used when the

total voltage drop of the output LED string is higher than

the input supply voltage. For example, the boost

topology can be appropriate when input voltage is

supplied by a 48V power supply and the LED string

consists of twenty HB LEDs, as the case may be for a

street light.