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 2005 Semtech Corp.

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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 inten-

sity (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 cur-

rent, 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 refer-

ence voltage on the ISET pin is set to 250mV. The dura-

tion 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 dura-

tion of the pulse can increase proportionally. However,

great care should be taken to insure the FLASH pin is

not held high for an extended amount of time so that

overheating does not occur. Extending flash mode be-

yond 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

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 pre-

vents the device from operating in an unspecified state.

These funtions include Input Over-Voltage Protection

(IOVP), Output Over-Voltage Protection (OVP), Over-Tem-

perature (OT) Protection, Over-Current Protection (OCP),

and Short-Circuit Current Protection (SCCP).

Input Over-Voltage Protection

Input over-voltage protection is included to prevent op-

eration 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.

Output Over-Voltage Protection

Output over-voltage protection is included to prevent the

SC615A from generating an output voltage that could

damage other devices connected to it such as load LEDs

and bypass capacitors. When the output voltage exceeds

5.5V, the OVP circuit disables the charge pump until the

voltage decreases to a level within the acceptable range.

This circuit allows the device to drive LEDs with high for-

ward voltages at a reduced level without exceeding the

output voltage limits specified for the device. Note, how-

ever, that this effect is a consequence of the OVP circuit

and is not its intended purpose.