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SC805A Datasheet(PDF) 11 Page - Semtech Corporation |
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SC805A Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 17 page  11 © 2008 Semtech Corp. SC805A www.semtech.com POWER MANAGEMENT Applications Information (Cont.) Fault CHRGB VCC UVLO High-Z VCC OVP Blinking NTC Fault Blinking Pre-Charge Time-Out Blinking T J > 150°C Blinking Over-Current and Max Temperature Protection Over-current protection is inherent in all modes of opera- tion. When the device is in charge-mode the output is cur- rent-limited to either the pre-charge current limit value or the fast-charge current limit value depending on the voltage at the output. When the device is in LDO mode, the output current is limited to the fast-charge current limit. Max die temperature protection is included on the SC805A. This feature allows the SC805A to operate with maximum power dissipation by disabling the output current when the die temperature reaches OT. The result is that the SC805A will operate as a pulse charger in extreme power dissipa- tion applications delivering the maximum allowable output current while regulating the internal die temperature to a safe level. Layout Guidelines The two most critical aspects of the PCB layout are the power path and thermal layout. The power path starts at the Adapter + input and runs to the VCC input of the SC805A, then from the VOUT pin of the SC805A to the bat- tery + terminal, and completes with the return trace from the battery - terminal to the adapter - input. All of these traces need to be designed to handle the required charging current. The trace from VOUT of the SC805A is most critical and should be made as wide as possible to minimize I×R drops between the regulated voltage at the SC805A VOUT pin and the battery terminal. Keep the Adapter+ to SC805A VCC input trace wide to minimize voltage drops that will add to the dropout voltage of the SC805A. The GND pin of the SC805A should be connected in a Kelvin fashion at the battery-terminal to eliminate voltage drops in the return path which reduce the regulated battery voltage. The thermal performance of the SC805A package requires a low impedance connection from the heat slug on the bottom of the package to an external ground plane. This is best accomplished by using a single large via under the device connected to a ground plane on the bottom exposed side of the PCB. The evaluation board uses 1 square inch of copper on the bottom of the PCB and is capable of 1A charging current. The input and output bulk decoupling capacitors for the SC805A should be placed near the external terminals for the adapter and battery. This short low impedance loop is for the high current spikes that result from input/output hot-plugging of the charger. To minimize these current spikes the value of the decoupling capacitor should be mini- mized. A typical application requires a 0.1μF input/output capacitor. If the distance from the external terminals to the SC805A is greater than 1 inch, 0.1μF local decoupling capacitors at the SC805A may be required. |
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