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SC615AEVB Datasheet(PDF) 7 Page - Semtech Corporation |
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SC615AEVB Datasheet(HTML) 7 Page - Semtech Corporation |
7 / 12 page 7 © 2008 Semtech Corp. www.semtech.com POWER MANAGEMENT SC615A Output Over-Voltage Protection Output over-voltage protection is included to prevent the SC615A 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 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. Note, however, that this effect is a consequence of the OVP circuit and is not its intended purpose. Over-Temperature Protection The over-temperature 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 UVLO and OVP circuits, hysteresis is included to prevent toggling between modes. Over-Current Protection When the SC615A 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 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 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 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 desired LED current for Flash mode. This allows the designer to set the SC615A’s maximum cur- rent and select the package size necessary for the power dissipation required in Flash mode. The typical application shown on page 1 uses a 0.5 Ω 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 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 resistance return path. Place and ground the resistor as close as possible to the ground pin of the SC615A. 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. Capacitor Selection The SC615A 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. Thermal Resistance The SC615A package is thermally efficient when the circuit board layout connects the thermal pad though multiple 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 tempera- ture below the OT limit while operating the SC615A within the specified electrical conditions for I OUT and VISET. A poor layout may allow the junction temperature to reach the OT limit while in Flash or Spotlight mode. So, it is critical to maintain adequate ground plane around the device to maximize heat transfer and avoid over-temperature shutdown. Applications Information (Cont.) |
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