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LDS8621
BLOCK DIAGRAM
Figure 2. LDS8621 Functional Block Diagram
BASIC OPERATION
At power-up, PWM1 and PWM2 pins should be logic
LOW. During power-up device performs internal
circuit reset that requires less than 10 μs. To start
device, either PWM1 or PWM2 pin should be set
logic HIGH. The LDS8621 starts operating in 1x
If the input voltage is insufficient or falls to a level
where Vd ≤ 50 mV, and the regulated currents cannot
be maintained, the low dropout PowerLite ? Current
Regulator switches the charge pump into 1.33x mode
(after a fixed delay time of about 800 μ s). In 1.33x
mode, at which the output is approximately equal to
mode, the
charge pump’
output voltage is
the input supply voltage (less any internal voltage
losses). If the output voltage is sufficient to regulate
all LED currents, the device remains in 1x operating
mode.
The low dropout PowerLite ? Current regulator
(PCR) performs well at input voltages up to 50 mV
above LED forward voltage significantly increasing
driver’s efficiency. The LDS8621 monitors voltage
drop Vd across PCR at every channel in ON state. If
this voltage falls below 50 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 = V IN x M – V F – 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.
approximately equal to 1.33 times the input supply
voltage (less any internal voltage losses).
This sequence repeats at every mode until driver
enters the 2x mode.
If the device detects a sufficient input voltage is
present to drive all LED currents in 1x mode, it will
change automatically back to 1x mode. This only
applies for changing back to the 1x mode. The
difference between the input voltage when exiting 1x
mode and returning to 1x mode is called the 1x mode
transition hysteresis (about 600 mV).
LED Current Setting
The maximum current value in each of the LED is
factory preset. Each LED has independendet current
setting that allows use one of the LEDs for flash,
while other for a torch mode. If both LEDs should be
used in flash mode, or both cannels are connected in
? 2009 IXYS Corp.
Characteristics subject to change without notice
4
Doc. No. 8621DS, Rev. N1.0
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