SC618

10

Applications Information (continued)

that the voltage on the ISET pin can no longer be main-

tained. After a 32ms delay (included to prevent false

transitions) the charge pump will convert to 2x mode.

After the charge pump switches from 1x mode to 2x

mode, the device remains in 2x mode until the part is

turned off. If the part is re-enabled in spotlight mode, it

will begin operation in 1x mode and determine the

appropriate mode of operation as previously described.

Protection Circuitry

The SC618 provides several protection features to safe-

guard the device from catastrophic failures. These features

include:

Input Over-Voltage Protection (IOVP)

Output Over-Voltage Protection (OVP)

Over-Temperature Protection (OTP)

Over-Current Protection (OCP)

Short-Circuit Current Protection (SCCP)

Input Over-Voltage Protection

IOVP is included to prevent operation at high input volt-

ages 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 OVP prevents the SC618 from generating an

output voltage that could damage other devices con-

nected 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 forward voltages at a

reduced level without exceeding the output voltage

limits specified for the device.

Over-Temperature Protection

The OTP circuit prevents the device from overheating and

experiencing a catastrophic failure. When the junction

temperature exceeds 150°C the device is disabled. It

remains disabled until the junction temperature drops

below this threshold. Hysteresis is included to prevent

toggling between modes.

•

•

•

•

•

Over-Current Protection

When the SC618 is in 2x mode, the input current will be

approximately double the required output. When the

steady-state load requires the maximum current avail-

able in 2x mode, the OCP circuit prevents the device from

overheating from excessive power dissipation. This

feature protects the device when in 1x mode. The current

limit is set lower in the 1x mode due to lower current

requirements.

Short-Circuit Current Protection

SSCP is provided to limit the current that can be sourced

when the output is shorted to ground. When a short

circuit forces V

OUT

to drop below 2V, the SCCP detects the

condition and limits the output current.

Resistor Selection

The ISET resistor selection is critical in generating the

correct current. The value can be chosen to set the spot-

light mode current or the flash mode current, and the

two current settings are dependent on the same resistor.

It is recommended that the resistor be selected to match

the desired LED current for flash mode. This allows the

designer to set the SC618’s maximum current and select

the resistor package size necessary for the power dissi-

pation required in flash mode. The typical application

uses a 0.5Ω resistor to set a flash mode current of 500mA

and spotlight mode current of 200mA. A high precision

resistor should be used to ensure the specified accuracy

for LED current.

To avoid malfunction of the charge pump, it is recom-

mended that the resistance seen at the ISET pin remain

constant while the device is active (flash or spotlight

mode). Changing the resistance value or the load current

while the device is active could cause instability.

Capacitor Selection

The SC618 is designed to use low-ESR ceramic capacitors

for the input and output bypass capacitors as well as the

charge pump bucket capacitors. Ideal performance is

achieved when C1 is exactly equal to C2. It is recom-

mended that X5R or X7R ceramic capacitors be used for

best performance.