BW9910 Datasheet, PDF (8/14 Pages) Bruckewell Technology LTD – High Brightness LED Driver

BW9910/A High Brightness LED Driver
Assuming the nominal rectified input voltage V_IN
120V 1.414 169V, the switching duty ratio can be
determined as follows :
0.195
(6)
Then, in this example, given the switching frequency,
f_OSC 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
D_CH : C_IN capacity charge work period, generally about
0.20 ~ 0.25,
f_L : input frequency for full range (85V_RMS ~ 265V_RMS),
∆V_DC(MAX) should be set 10% ~ 15% of
If the capacitor has a 15% voltage ripple then a simplified
formula for the minimum value of the bulk input capacitor
approximates to :
© 2012 Bruckewell Technology Corp., Ltd.
(10)
8
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
C_MIN. Each of these identical capacitors should be rated
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
temperature (T_J) reaches 150°C. There is a 50°C
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.
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