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Applications Information
SC618
General Operation
The SC618 is a powerful dual-mode charge pump designed
to drive a high-intensity white LED. A constant current
spotlight mode or a pulsed current with higher intensity is
used for camera flash. The device operates in 2x mode
while operating in flash mode. In spotlight mode it can
operate in either 2x or 1x mode depending on the load
current, load voltage, and input voltage levels.
In flash mode the SC618 output can drive 500mA at 4.25V
into an LED for a minimum duration of 500ms under all
temperature and input conditions. The FLASH pin is used
to trigger this mode (active high). In spotlight mode the
SC618 regulates the output current to 40% of the flash
mode current setting, when the CTRL pin is pulled high
example, 700mA derates to a maximum of 350ms. In
flash mode, I OUT = 250mV/R ISET . Figure 1 shows maximum
flash mode current for extended duration times.
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0
and the FLASH pin is low. Flash mode has priority over
0
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spotlight mode, i.e., the FLASH pin triggers flash mode
regardless of the state of the CTRL pin.
Output current is regulated by attaching the ISET pin to
the cathode(s) of the LED package and a low resistance
sense resistor (typically 0.5Ω). The ISET pin monitors the
voltage at the cathode and signals the charge pump to
increase or decrease the output current until the ISET
voltage reaches the programmed setting. The resistor
value is chosen to set the current through the LED based
on this reference voltage. The resistor can be chosen to
set either the flash mode current or spotlight mode
current, but it should not be less than 0.35Ω due to output
current limitations of the device.
Flash Mode
Flash mode is enabled whenever the FLASH pin is pulled
high and remains active until the FLASH pin is released.
This mode has higher priority than spotlight mode, so the
state of the CTRL pin is overridden whenever the FLASH
pin is activated. While in flash mode the reference voltage
on the ISET pin is set to 250mV. The duration of the pulse
is limited by the amount of power that can be dissipated
in the SC618 before it exceeds the maximum operating
temperature. If an output current less than the rated
500mA is desired, the duration of the pulse can increase
proportionally.
Extending flash mode beyond the 500ms pulse limit could
adversely affect the long term reliability of the SC618.
Flash duration must be derated at higher currents. For
Flash Duration [ms]
Figure 1 — Maximum Flash Duration
Flash Mode Safety Timer
The SC618 provides a time-out protection function when
in flash mode. The output is disabled if the FLASH pin is
held high for longer than 1 second to protect the LED.
Once this time-out has occurred, the output will stay dis-
abled until the FLASH pin has been pulled low and then
re-enabled.
Spotlight Mode
Spotlight mode is enabled by setting the CTRL pin high
and keeping the FLASH pin low. When in spotlight mode,
the SC618 can maintain a constant current indefinitely to
drive an LED or bank of LEDs. The ISET reference voltage
is set to 100mV so that the output current is maintained
at 200mA when a 0.5Ω resistor is used. Spotlight current
is always 40% of flash current for any given value of R ISET .
In spotlight mode, I OUT = 100mV/R ISET .
To maximize power efficiency, the SC618 has been
designed to operate in 1x mode when conditions do not
require the charge pump to be active. When first enabled
the SC618 attempts to regulate the voltage on the ISET
pin in 1x mode. If the voltage on the ISET pin cannot be
maintained and this condition persists for longer than
32ms, then the charge pump will change to 2x mode
operation. In a typical application, the SC618 will operate
in 1x mode until the supply voltage decreases suffi ciently
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