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LDS8720 Datasheet(PDF) 6 Page - IXYS Corporation

Part No. LDS8720
Description  184 WLED Matrix Driver with Boost Converter
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Maker  IXYS [IXYS Corporation]
Homepage  http://www.ixys.com
Logo IXYS - IXYS Corporation

LDS8720 Datasheet(HTML) 6 Page - IXYS Corporation

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LDS8720
© 2011 IXYS Corp.
6
Doc. No. 8720DS, Rev. N1.1
Characteristics subject to change without notice
BLOCK DIAGRAM
Figure 2. LDS8720 Functional Block Diagram
BASIC OPERATION
At power-up, EN/PWM pin should be logic LOW. The
LDS8720 starts operating when EN/PWM pin is
asserted logic high. If the input voltage is sufficient to
regulate all LED currents, the device remains in
operating mode. The low dropout Current regulator
performs well at VOUT voltage up to 250 mV above
summary LED forward voltage significantly increasing
driver’s efficiency.
If the input voltage is insufficient or falls to a level
where the regulated currents cannot be maintained,
the Under-Voltage protection turns device off setting
it in shutdown mode.
The LDS8720 has soft start function that prevent high
input current spike at device’ wake-up.
The EN/PWM pin should be held low for more than
10 ms to completely turn device in low current
shutdown mode.
LED Current Setting
The maximum current sink value in the LEDS pin is
factory preset at 30 mA. An external resisror RFB
should be used to bypass current if more than one
string is connected. RFB = 0.25 V / (I x M – 0.03),
ohms, where 0.25 V is a feedback voltage, I – is
current value per string in ampers, and M is number
of strings connected in parallel.
Power dissipated at RFB is equal W = 0.0625/RFB,
watt.
In case of three strings with 25 mA current per string
RFB = 0.25/(0.025 x 3 – 0.03) = 5.6 , and dissipated
power W = 0.0625/5.6 = 0.011 W.
The average LED string current that determines LED
brightness may be controlled applying PWM signal to
the EN/PWM pin. The maximum PWM frequency is
30 kHz, while frequiencies below 100 Hz are not
recommended to avoid visible LED flikering. Duty
cycle that determines average LED string current
may vary in the range from 5% to 100% at 1 kHz or
from 20% to 100% at 30 kHz with high linearity
current regulation.
Protection Mode
The output voltage VOUT is limited at about 36 V. This
is to prevent the output pin from exceeding its
absolute
maximum
rating
if
LED
string
is
disconnected or any LED.in string burns out creating
open circuitry.
If the die temperature exceeds +150°C, the driver will
enter a thermal protection shutdown mode. When the
device temperature drops by about 20°C, the device
will resume normal operation.


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