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S P 6 6 8 5
7 0 0 m A B u c k / B o o s t C h a r g e P u m p L E D D r i v e r
APPLICATION INFORMATION
The SP6685 can be used with multiple LEDs in
parallel as shown in figure 34. For best
performance, the LEDs should be in a single
package, preferably from a single die to have
better matching for forward voltage Vf for a
temperature which will then cause its Vf to
reduce, correcting the imbalance. The overall
current will be the sum of the individual
currents, for example Itotal = 4*ILED.
given forward current If. In practice, if the Vf
of one LED is higher than the others, it will
consume a larger If, which will raise its
Fig. 34: Multiple LED Flash Circuit
THEORY OF OPERATION
The SP6685 is a charge pump regulator
designed for converting a Li-Ion battery
voltage of 2.7V to 4.2V to drive a white LED
used in digital still camera Flash and Torch
applications. The SP6685 has two modes of
operation which are pin selectable for either
Flash or Torch. Flash mode is usually used
with a pulse of about 200 to 300 millisecond to
generate a high intensity Flash. Torch can be
used continuously at a lower output current
than Flash and is often used for several
seconds in a digital still camera “movie” mode.
The SP6685 also has two modes of operation
to control the output current: the 1X mode
and 2X mode. Operation begins after the
enable pin EN receives a logic high, the
bandgap reference wakes up after about
200μsec, and then SP6685 goes through a
soft-start mode designed to reduce inrush
current. The SP6685 starts in the 1X mode,
which acts like a linear regulator to control the
output current by continuously monitoring the
feedback pin FB. In 1X mode, if the SP6685
auto detects a dropout condition, which is
when the FB pin is below the regulation point
for more than 32 cycles of the internal clock,
the SP6685 automatically switches to the 2X
mode. The SP6685 remains in the 2X mode
until one of four things happens: 1) the
enable pin EN has been toggled, 2) the Flash
pin has changed from High to Low, 3) V IN is
cycled. 4) a thermal fault occurs.
The 2X mode is the charge pump mode where
the output can be pumped as high as two
times the input voltage, provided the output
does not exceed the maximum voltage for the
SP6685, which is internally limited to about
5.5V. In the 2X mode, as in the 1X mode, the
output current is regulated by the voltage at
the FB pin.
? 2009 Exar Corporation
10/14
Rev. 2.0.0
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