BW9910 Datasheet, PDF (9/14 Pages) Bruckewell Technology LTD – High Brightness LED Driver

BW9910/A High Brightness LED Driver
In a boost converter, when the external MOSFET is ON
the energy is stored in the inductor which is then
delivered to the output when the external MOSFET
switches OFF. If the energy stored in the inductor is not
fully depleted by the next switching cycle (continuous
conduction mode) the DC conversion between input and
output voltage is given by :
(11)
Buck-Boost (Fly-Back) LED Driver
Refer Figure 2, the buck-boost power conversion
topology can be used when the forward voltage drop of
the LED string is higher, equal or lower than the input
supply voltage. For example, the buck-boost topology
can be appropriate when input voltage is supplied by
24V system bus for trucks (voltage at supply battery is
between 18V and 32V) and output string consists of six
to nine HB LEDs, as the case may be for tail and break
signal lights.
From the switching frequency, f_OSC, the on-time of the
MOSFET can be calculated :
(12)
From this the required inductor value can be determined
by :
(13)
The boost topology LED driver requires an output
capacitor to deliver current to the LED string during the
time that the external MOSFET is on. In boost LED driver
topologies if the LEDs should become open circuit,
damage may occur to the power switch and so some form
of detection should be present to provide over-voltage
detection/protection.
Figure 1. DC-DC Boost LED Driver
In the buck-boost converter, the energy from the input
source is first stored in the inductor or fly-back
transformer when the switching transistor is ON. The
energy is then delivered to the output during the OFF
time of the transistor. When the energy stored in the fly-
back inductor is not fully depleted by the next switching
cycle (continuous conduction mode) the DC conversion
between input and output voltage is given by :
(14)
or
D
(15)
The output voltage can be either higher or lower than
the input voltage, depending on duty ratio.
Let us discuss the above example of 24V battery system
LED driver that needs to drive six HB LEDs (V_F 3.3V)
at 350mA.
Knowing the nominal input voltage V_IN 24V, the
nominal duty ratio can be determined as below :
D
0.45
Then, given the switching frequency, in this example
f_OSC 50kHz, the required on-time of the MOSFET
transistor can be calculated :
9µs
© 2012 Bruckewell Technology Corp., Ltd.
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