SP6686ER-L-TR Datasheet, PDF (8/13 Pages) Exar Corporation – 400mA Buck/Boost Charge Pump LED Driver

SP6686
400mA Buck/Boost Charge Pump LED Driver
THEORY OF OPERATION
The SP6686 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 SP6686 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 SP6686 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 200μsec, and then
SP6686 goes through a soft-start mode
designed to reduce inrush current. The
SP6686 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 SP6686 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 SP6686
automatically switches to the 2x mode. The
SP6686 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
SP6686, 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.
In the Torch mode, (Flash = GND) the Flash
pin is set to logic low and the SP6686 FB pin
regulates to 50mV output:
50
When in Flash mode, (Flash = V_IN), the FB
regulation voltage is set by the resistor R_SET
connected between the R_SET pin and SGND and
the equation:
1.26
11.2 Ω
Where 1.26V is the internal bandgap reference
voltage and 11.2kΩ is an internal resistance
used to scale the R_SET current. Typical values
of R_SET are 40kΩ to 180kΩ for a range of V_FB =
300mV to 75mV in Flash mode.
The output current is then set in either Flash
or Torch mode by the equation:
OVER TEMPERATURE PROTECTION
When the temperature of the SP6686 rises
above 145°C, the over temperature protection
circuitry turns off the output switches to
prevent damage to the device. If the
temperature drops back down below 135°C,
the part automatically recovers and executes a
soft start cycle.
OVER VOLTAGE PROTECTION
The SP6686 has over voltage protection. If the
output voltage rises above the 5.5V threshold,
the over voltage protection shuts off all of the
output switches to prevent the output voltage
from rising further. When the output
decreases below 5.5V, the device resumes
normal operation
OVER CURRENT PROTECTION
The over current protection circuitry monitors
the average current out of the V_OUT=50mV
(Torch Mode) pin. If the average output
current exceeds approximately 1Amp, then
the over current protection circuitry shuts off
the output switches to protect the chip.
COMPONENT SELECTION
The SP6686 charge pump circuit requires 3
capacitors: 4.7μF input, 1μF output and
© 2009 Exar Corporation
8/12
Rev. 2.0.0