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POWER MANAGEMENT

SC615

Over-Current Protection

When the SC615 is in 2X charge pump mode, the input

current will be approximately doubled the required out-

put. 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 dis-

sipation.

Short-Circuit Current Protection

Short-circuit current protection is provided to limit the

current that can be sourced when the output is shorted

to ground. When a short circuit forces V

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, but it must

be noted that 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 SC615’s maxi-

mum current and select the package size necessary for

the power dissipation required in Flash mode. The typi-

cal application shown on page 1, uses a 0.5

Ω resistor to

set a flash mode current of 500mA and spotlight mode

current of 100mA. 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 cur-

rent while the device is active could cause instability 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 pins. Resis-

tance 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 resis-

tance return path. Place and ground the resistor as close

as possible to the ground pin of the SC615. The trace

from the ISET pin has virtually no current. The ISET trace

should make contact at the pad of the power sense re-

sistor to minimize the effect of voltage drop between

the LED cathode and the resistor.

Capacitor Selection

The SC615 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 perfor-

mance is achieved when C1 is exactly equal to C2. It is

recommended that X5R or X7R ceramic capacitors be

used for best performance.

Thermal Resistance

The SC615 package is thermally efficient when the cir-

cuit board layout connects the thermal pad though mul-

tiple vias to the ground plane. The thermal resistance is

rated at 49°C/W and 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 temperature below the OT limit while operating

the SC615 within the specified electrical conditions for

I

A poor layout may allow the junction tem-

perature to reach the OT limit while in Flash or Spotlight

mode. So, it is important to maintain adequate ground

plane around the device to maximize heat transfer to

the PCB.

Applications Information (Cont.)