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 2005 Semtech Corp.

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POWER MANAGEMENT

SC605

Applications Information (Cont.)

Output Over-Voltage Protection

Output over-voltage protection is included to prevent the

SC605 from generating an output voltage that could dam-

age 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 cir-

cuit allows the device to drive LEDs with high forward volt

ages at a reduced level without exceeding the output volt-

age limits specified for the device. This is a consequence

of over-voltage protection, not the function of OVP.

Over-Temperature Protection

The OT circuit helps prevent 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. As with the OVP circuit, hysteresis is

included to prevent toggling between states.

Over-Current Protection

When the SC605 is in 2X charge pump mode, the input

current will be approximately double the required output.

When the steady-state load requires the maximum current

available in 2X mode, the OCP circuit prevents the device

from overheating due to excessive power dissipation.

Short-Circuit Current Protection

SCCP is provided to limit the current that can be sourced

when the output is shorted to ground. When the output

voltage drops below 2V, the SCCP detects the condition as

a short to ground and limits the output current.

ResistorSelection

The ISET resistor selection is critical to generating the cor-

rect current. The value can be chosen to set the Spotlight

mode current or the Flash mode current, but it must be

noted that the two current settings are dependent on the

same resistor. It is recommended that the resistor be se-

lected to match the LEDs desired current for Flash mode.

This will allow the designer to set the maximum current

out of the SC605. The typical application circuit shown on

page  uses a 0.5W resistor to set a Flash mode current

of 500mA and Spotlight mode options of 100mA, 150mA,

and 200mA. A 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 or voltage

transients that would result in non-compliant behavior.

The load current return path is from the ISET sense point

through the resistor and back to the ground pin. Resistance

in this path adds to the total resistance and has the effect

of reducing the LED current by about 4% per 10mV of DC

drop across the return copper trace in Flash mode. For this

reason, it is crucial to have a low resistance return path.

Place and ground the resistor as close as possible to the

ground pin of the SC605. The trace from ISET has virtually

no current. The ISET trace should make contact at the pad

of the power sense resistor to minimize the effect of voltage

drop between the LED cathode and the resistor.

CapacitorSelection

The SC605 is designed to use low-ESR ceramic capaci-

tors 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.

ThermalResistance

The SC605 package is thermally efficient when the circuit

board layout connects the thermal pad through multiple

vias to the ground plane. The thermal resistance is rated

at 0°C/W, but this rating is dependent on the connection

between the thermal pad and the ground plane. A layout

that is done correctly should keep the junction tempera-

ture below the OT limit while operating the SC605 within

the specifed electrical conditions for I

layout may allow the junction temperature to reach the

OT limit while in Flash or Spotlight mode.