SC618

9

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

and the FLASH pin is low. Flash mode has priority over

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

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.

0

100

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700

800

0

100

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900 1000

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 sufficiently

Applications Information