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SC2603EVB Datasheet(PDF) 4 Page - Semtech Corporation |
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SC2603EVB Datasheet(HTML) 4 Page - Semtech Corporation |
4 / 11 page 4 © 2005 Semtech Corp. www.semtech.com SC2603 POWER MANAGEMENT Application Information Soft-Start Soft-start prevents a DC-DC converter from drawing excessive current from the power source during start up. If the soft-start time is made sufficiently long, then the output will enter regulation without overshoot. An external capacitor is connected from the EN/SS pin to the ground. After the part’s input voltage surpasses the UVLO, the EN/SS pin is allowed to charge its capacitor with 60µA. When 100mV is reached at the EN/SS pin, the internal comp node is at 0.4V. The SC2603 will start switching, and the converter output comes into regulation. Because the FB pin voltage follows EN/SS pin voltage, finally, the EN/SS pin voltage will be clamped at 1.25V same as the refernece voltage. Setting the Output Voltage An external resistive divider R 1 and R2 with its center tap tied to the FB pin sets the output voltage. − = 1 25 . 1 2 1 OUT V R R Under Voltage Lockout The under voltage lockout circuit of the SC2603 assures that the gate driver output remains in the off state when- ever the supply voltage drops below set parameters. Lock- out occurs if V IN falls below 3.8V. Normal operation re- sumes once V IN rises above 4.2V. Theory of Operation The SC2603 is a versatile, low-cost, voltage-mode PWM controller designed for DC/DC power supply applications. In normal condition, the SC2603 operats at a fixed 200KHz. One exception is that when the FB pin voltage drops to one half, the frequency will be shifted to one fouth of 200KHz. SC2603 features include a temperature compensated voltage reference, triangle wave oscillator, current limit comparator, frequency shift over-current protection, and an internally compensated error amplifier. Pulse by pulse current limiting is implemented by sensing the differential voltage across an external resistor, or an appropriately sized PC board trace. VOUT R2 R1 FB 1 SC2603 |
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