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SC339SKTRT Datasheet(PDF) 11 Page - Semtech Corporation |
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SC339SKTRT Datasheet(HTML) 11 Page - Semtech Corporation |
11 / 15 page 11 © 2006 Semtech Corp. www.semtech.com SC339 POWER MANAGEMENT Applications Information (Cont.) Now, We choose C3 = 470nF as the standard value. Soft-Start Behavior Atstart-up,V OUT first ramps linearly from ground at the rate of ~0.5V/ms (+/- 25%) for about 800us. The linear ramping is followedbyaphaseofsmoothsettlingforabout700usatthe end of which the output has fully settled (to better than 1%). The total start-up time of about 1.5ms is kept within 1ms - 2ms window, and this is regardless of the loading and of the external components connected to the device. SC339 Start-Up Response C3 = ½ • • R3 • 3kHz = 530nF Layout Guidelines The advantages of using the SC339 to drive external MOSFETs are: a) that the bandgap reference and control circuitry are in a die that does not contain high power dissipating devices and, b) that the device itself does not need to be located right next to the power devices. Thus very accurate output voltages can be obtained since changes due to heating effects will be minimal. The 0.1μF bypass capacitor should be located close to the supply (IN) and GND pins, and connected directly to the ground plane. The feedback resistors should be located at the device, with the sense line from the output routed from the load (or top end of the droop resistor if passive droop is being used) directly to the feedback chain. If passive droop is being used, the droop resistor should be located next to the load to avoid adding additional unplanned droop. Sense and drive lines should be routed away from noisy traces or components. For very low input to output voltage differentials, the input to output/load path should be as wide and short as possible. Where greater headroom is available, wide traces may suffice. Power dissipation within the device is practically negligible, thus requiring no special consideration dur- ing layout. The MOSFET pass devices should be laid out according to the manufacturer’s guidelines for the power being dissipated within them. |
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