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LDS8869
BLOCK DIAGRAM
Figure 2. LDS8869 Functional Block Diagram
BASIC OPERATION
At power-up, EN pin should be logic LOW. During
power-up device performs internal circuits reset that
Regulator switches the charge pump into 1.33x mode
(after a fixed delay time of about 800 μ s). In 1.33x
requires less than 10μs. To start device, EN pin
mode, the
charge pump’
output voltage is
should be set logic HIGH at least 10μs after V IN
applied. Device starts operating at 1x mode at which
the V OUT is approximately equal to V IN (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 75 mV
above LED forward voltage V F significantly increasing
driver’s efficiency. The LDS8869 monitors voltage
drop Vd across PCR at every channel in ON state. If
this voltage falls below 75 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 (LEDX1/2) = 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.
If the input voltage is insufficient or falls to a level
where Vd ≤ 75 mV, and the regulated currents cannot
be maintained, the low dropout PowerLite ? Current
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 600mV).
LED Current Setting
The current in each of the six LED channels is
programmed through the 1-wire EN/SET digital
control input. By pulsing this signal according to a
specific protocol, a set of internal registers can be
addressed and written into allowing to configure each
bank of LEDs with the desired current. There are six
registers: the first five are 4 bits long and the sixth is
1 bit long. The registers are programmed by first
? 2009 IXIS Corp.
Characteristics subject to change without notice
9
Doc. No. 8869_DS, Rev. N2.1
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