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S P 7 6 8 6 / S P 7 6 8 6 A
C o n s t a n t C u r r e n t F l a s h D r i v e r i n D F N 2 m m x 3 m m
THEORY OF OPERATION
O VERVIEW
The SP7686 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 SP7686 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 milliseconds
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 SP7686 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 200 μ sec,
and then SP7686 goes through a softstart
mode designed to reduce inrush current. The
SP7686 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 SP7686 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 SP7686
automatically switches to the 2X mode. The
SP7686 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) VIN is cycled or, 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 SP7686, 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.
SP7686A
The fixed current driver (SP7686A) uses the
flash pin to switch between two internally
preset voltage levels, for Flash and Torch
mode. The part is enabled and disabled by the
separate ENABLE pin. The part has a 3.5s
maximum on time in Flash mode. After a 3.5s
timeout occurs the part shuts down and can
be enabled by the FLASH or ENABLE pins. This
pin cannot be left open. When in Flash mode,
(Flash = V IN ), the FB regulation voltage is set
to 160mV. When in Torch mode, (Flash =
GND), the FB regulation voltage is set to
60mV. The output current is then set in either
Flash or Torch mode by the equation:
I OUT = V FB / R SENSE .
SP7686
The digital controlled current driver (SP7686)
uses the CTRL pin as a serial data input to a
DAC to set the FB voltage. The CTRL pin is
used to control LED current. Once enabled the
adjustable driver counts positive going edges
at this pin and sequentially increases the FB
voltage from 60mV with 10mV steps to a
maximum 31 steps or 360mV. When this pin
is held LOW for longer than 200 μ s the SP7686
enters shutdown. The part can be enabled
using this pin. The first positive edge resets
the part. A second positive edge sets the part
to VFB=0. A third positive going edge will
increase FB voltage to 58mV and subsequent
positive edges increment VFB in 9.7mV steps.
The voltage level can increase only and must
be reset to zero by the 200 μ s shutdown timer
in order to go to a lower current value. The
part will be limited to a 3.5s on time when FB
voltage is set to 120mV and above. The CTRL
pin cannot be left open.
F LASH T IMEOUT P ROTECTION
Due to the high currents typically available in
Flash mode, it is necessary to protect the
white LED from damage if left on too long. The
SP7686 has a timeout in Flash mode of
approximately 3.5 seconds after which it will
shutdown operation. Operation will not begin
again in Flash mode until the Enable pin has
been set Low and then High again.
O VER TEMPERATURE P ROTECTION
When the temperature of the SP7686 Rises
above 140 degrees Celsius, the over
temperature protection circuitry turns off the
output switches to prevent damage to the
device. If the temperature drops back down
below 125 degrees Celsius, the part
automatically recovers and executes a soft
start cycle.
? 2010 Exar Corporation
10/16
Rev. 2.1.0
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