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 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC605

Applications Information

GeneralOperation

The SC605 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 SC605 output can drive 500mA at 4.25V

into an LED for up to 500ms. The FLASH pin is used to

trigger Flash mode. In spotlight mode the SC605 regulates

the output current to the value selected by the CTRL0 and

CTRL1 inputs. Note that Flash mode has priority over

Spotlight mode, i.e., the FLASH pin triggers Flash mode

regardless of the state of the CTRL0 and CTRL1 pins.

Output current is regulated by attaching the ISET pin to the

cathode(s) of the LED package and a load sense resistor

(typically 0.5W). The ISET pin monitors the voltage at the

cathode and signals the charge pump to increase or de-

crease the output current until the ISET voltage reaches

the programmed setting. Programmed settings are shown

in Table 1:

CTRL1

CTRL0

FLASH

V

ISET

Mode

0

0

0

0mV

Disabled

0

1

0

50mV

Spotlight

1

0

0

75mV

Spotlight

1

1

0

100mV

Spotlight

x

x

1

250mV

Flash

Table1-ControlStateTable

The resistor value is chosen to set the current through the

LED based on this reference voltage. The resistor should

not be less than 0.35W due to the output current limitations

of the device. Note that the resistor sets both the Flash

mode current and the Spotlight mode current.

In Flash mode, I

In Spotlight mode, I

SpotlightMode

Spotlight mode is enabled by setting the CTRL0 and CTRL1

pins as shown in Table . When in Spotlight mode, the

SC605 can maintain a constant current indefinitely to drive

an LED or bank of LEDs. The ISET pin voltage is the refer-

ence V

and 100mV. These settings allow the designer the flexibility

to select the output level needed to suit the application.

FlashMode

Flash mode is enabled whenever the FLASH pin is pulled

high and remains active until the FLASH pin is released.

While in Flash mode the reference voltage on the ISET pin

is set to 250mV. During Flash mode the charge pump de-

velops enough output current to drive an LED with 500mA

for a duration up to 500ms. The duration of the pulse is

limited by the amount of power that can be dissipated in

the SC605 before it exceeds the maximum operating tem-

perature. Note that if an output current less than the rated

500mA is desired the duration of the pulse can increase

proportionally. However, great care should be taken to in-

sure the FLASH pin is not held high for an extended amount

of time so that overheating does not occur. Extending flash

mode beyond the 500ms pulse limit could adversely affect

the long-term reliability of the SC605. Flash duration must

be derated at higher currents, for example, 700mA derates

to a maximum of 350ms. The plot below shows maximum

Flash mode current for extended duration times.

MaximumFlashDuration

ProtectionCircuitry

The SC605 also provides protection circuitry that prevents

the device from operating in an unspecified state. These

include Input Over-Voltage Protection (IOVP), Output Over-

Voltage Protection (OVP), Over-Temperature Protection

(OTP), 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. When the voltage rises

above this threshold, the device is disabled until the input

voltage drops to a level within the specified input range.

Hysteresis is included in this circuit to avoid chattering

between states.

0

100

200

300

400

500

600

700

800

0

500

1000

1500

2000

Flash Duration [ms]