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SC619 Datasheet(PDF) 11 Page - Semtech Corporation |
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SC619 Datasheet(HTML) 11 Page - Semtech Corporation |
11 / 13 page SC619 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 SC619. 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 SC619 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 49°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 SC619 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. |
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