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LDS8681 Datasheet(PDF) 5 Page - IXYS Corporation

Part No. LDS8681
Description  800 mA, Dual Output LED Flash/PWM Dimming Lamp Driver
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Maker  IXYS [IXYS Corporation]
Homepage  http://www.ixys.com
Logo IXYS - IXYS Corporation

LDS8681 Datasheet(HTML) 5 Page - IXYS Corporation

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LDS8681
© 2009 IXYS Corp.
5
Doc. No. 8681_DS, Rev. N1.0
Characteristics subject to change without notice
BLOCK DIAGRAM
Figure 2. LDS8681 Functional Block Diagram
BASIC OPERATION
LDS8681 is a charge pump designed for converting
at Li-Ion battery voltage to drive a white LED used in
photo flash applications. The LEDs current is factory
programmed at 400 mA for both channels. Consult
factory about other current values. Every LED may
be factory programmed separately in the range from
25 to 800 mA in 25 mA steps for LEDA and in the
range from 25 to 400 mA for LEDB.
The LDS8681 is in standby mode after applying VIN
voltage until logic HIGH level is applied to EN pin.
The device requires approximately 100 µs to wake
up.
Device starts operating with soft-start at 1-x mode to
minimize inrush current
At 1-x mode, the VOUT is approximately equal to VIN
(less any internal voltage losses). If the output
voltage is sufficient to regulate all LED currents, the
device remains in 1-x operating mode.
The charge pump has two modes of operation: 1-X,
and 2-x. The device changes modes if the output
voltage drops below the minimum needed to support
the desired ILED current.
. The low dropout PowerLite
™ Current regulator
(PCR) performs well at input voltages up to 150 mV
above LED forward voltage VF significantly increasing
driver’s efficiency. The LDS8681 monitors voltage
drop Vd across PCR at every channel in ON state. If
this voltage falls below 150 mV (typical) at any one
channel, (channel with LED with highest forward
voltage), the Mode Control Block changes charge
pump mode to the next multiplication ratio.
Vd (LEDA/B) = VIN x M – VF – Rcp x IOUT, where Rcp
is a Charge Pump Output Resistance at given mode,
IOUT is sum of all LED currents, and M is a charge
pump’ multiplication ratio.
If the input voltage is insufficient or falls to a level
where Vd
≤150 mV, and the regulated currents
cannot be maintained, the low dropout PowerLite
Current Regulator switches the charge pump into 2-x
mode (after a fixed delay time of about 800
μs). In 2-x
mode,
the
charge
pump’
output
voltage
is
approximately equal to 2 times the input supply
voltage (less any internal voltage losses).
LED Current Setting
The current in each of the two LED channels is
factory programmed; however, average current value
may be decreased using PWM pulses. Check LED
Data sheet to avoid LED thermal damage at high
current.


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