SC618

11

PCB Layout Considerations

The following layout is suggested for a two-layer design.

The capacitors C1 and C2 are the bucket capacitors and

each conducts the full load current of up to 500mA pulsed

for one half clock cycle. C3 is the output decoupling

capacitor placed near the VOUT pin. C4 is the input

decoupling capacitor placed near the VIN pin. Multiple

vias should be used whenever it is necessary to change

layers on nets connecting to pins VIN, VOUT, GND, C1-,

C1+, C2- and C2+.

The load current return path is from the ISET sense point

through the resistor and back to the ground pins.

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 resis-

tance return path. Place and ground the resistor as close

as possible to the ground pin of the SC618. The trace from

the ISET pin 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.

Resistor R1 is routed with a very low resistance connec-

tion between R1 and GND pin 3. The sense trace between

Pin 6 and R1 is routed around the ground vias, allowing

the shortest ground return path possible. The sense trace

is connected to R1 at the positive terminal pad for the

most accurate output possible. The bottom copper layer

is mostly a ground plane with no obstructions between

the ground vias. The smaller rectangle to the left con-

nects the input power to VIN pin 1 and input capacitor

C4. The two traces at the lower left are for logic inputs

FLASH and CTRL. The trace to the right is the ISET pin

sense trace. The sense trace is routed out of the path of

the returning ground current.

Thermal Resistance Considerations

The SC618 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 40°C/W, and this rating is dependent on the connec-

tion between the thermal pad and the ground plane. The

layout should keep the junction temperature below the

OTP limit while operating the SC618 within the specified

electrical conditions for I

OUT

and V

ISET

. A poor layout may

allow the junction temperature to reach the OTP limit

Applications Information (continued)

while in flash or spotlight mode. It is critical to maintain

adequate ground plane around the device to maximize

heat transfer and avoid over-temperature shutdown.