THEORY OF OPERATION
regulation point for more than 32 cycles of
the internal clock, the SP6685 automati-
cally switches to the 2X mode. The SP6685
remains in the 2X mode until one of four
things happens: the enable pin EN has
been toggled, the Flash pin has changed
from high to low, VIN is cycled or a thermal
fault occurs. The 2X mode is the charge
pumpmodewheretheoutputcanbepumped
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 4.3V. In the 2X
mode, as in the 1X mode, the output current
is regulated by the voltage at the FB pin.
When in Flash mode, (Flash = VIN), the FB
regulation voltage is set by the resistor RSET
connected between the RSET pin and SGND
and the equation:
VFB = (1.26V/ RSET)*11.2KΩ (Flash Mode)
Where 1.26V is the internal bandgap refer-
ence voltage and 11.2KΩ is an internal
resistance used to scale the R
current.
Typical values of RSET are 100KStoET500K for
a range of VFB = 150mV to 30mV in Flash
mode.
The output current is then set in either Flash
or Torch mode by the equation:
In the Torch mode, (Flash = GND) the Flash
pin is set to logic low and the SP6685 FB pin
regulates to 50mV output:
IOUT = VFB / RSENSE
VFB = 50mV (Torch Mode)
OVERTEMPERATURE PROTECTION
WhenthetemperatureoftheSP6685risesabove145degreesCelsius, theovertemperature
protection circuitry turns off the output switches to prevent damage to the device. If the
temperature drops back down below 135 degrees Celsius, the part automatically recovers
and executes a soft start cycle.
OVERVOLTAGE PROTECTION
TheSP6685hasovervoltageprotection. Iftheoutputvoltagerisesabovethe4.3Vthreshold,
the over voltage protection shuts off all of the output switches to prevent the output voltage
from rising further. When the output decreases below 4.3V, the device resumes normal
operation
OVERCURRENT PROTECTION
The over current protection circuitry monitors the average current out of the VOUT = 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.
Date: 12/15/04
SP6685 Charge Pump LED Driver for Camera Flash
© Copyright 2004 Sipex Corporation
7
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