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POWER MANAGEMENT

SC615A

Applications Information

General Operation

The SC615A is a powerful 2X charge pump designed to drive

a high-intensity white LED with a constant current (Spotlight

mode) or with a pulsed current of higher intensity (Flash

mode) used for camera flash.

In Flash mode the SC615A 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 SC615A 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. Note that 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 0.5

Ω). 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 0.35

Ω 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 SC615A before it exceeds the maximum operating

temperature. Note that 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 SC615A.

Flash duration must be derated at higher currents. For

example, 700mA derates to a maximum of 350ms. In

Flash mode, I

The plot below shows

maximum Flash mode current for extended duration times.

0

100

200

300

400

500

600

700

800

0

100

200

300

400

500

600

700

800

900 1000

Flash Duration [ms]

Spotlight Mode

Spotlight mode is enabled by setting the CTRL pin high

and keeping the FLASH pin low. When in Spotlight mode,

the SC615A 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 0.5

Ω resistor is used. Spotlight current

is always 40% of flash current for any given value of R

In spotlight mode, I

Protection Circuitry

The SC615A also provides protection circuitry that prevents

the device from operating in an unspecified state. These

funtions include Input Over-Voltage Protection (IOVP), Out-

put Over-Voltage Protection (OVP), Over-Temperature (OT)

Protection, Over-Current Protection (OCP), and Short-Circuit

Current Protection (SCCP).

Input Over-Voltage Protection

Input over-voltage protection is included to prevent opera-

tion at high input voltages that could damage the device.

The IOVP circuit senses the input voltage and determines

when the supply exceeds 6V. Hysteresis is included in

this circuit to avoid chattering between states. When the

voltage rises above this threshold, the device is disabled

until the input voltage drops to a level within the specified

range.