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SC1531 Datasheet(PDF) 8 Page - Semtech Corporation |
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SC1531 Datasheet(HTML) 8 Page - Semtech Corporation |
8 / 11 page 8 ã 2000 Semtech Corp. www.semtech.com SC1531(A) POWER MANAGEMENT Introduction The SC1531(A) is intended for applications such as power managed PCI and network interface cards (NICs), where operation from a 3.3V VAUX supply may be required when the 5V supply has been shut down. It provides a very simple, low cost solution that uses very little pcb real estate. During regular operation, 3.3V power for the PCI card is provided by the SC1531(A)s on-board low dropout regulator, generated from the 5V supply. When the 5V supply is removed and 3.3V VAUX is available, the SC1531(A) connects this supply directly to its output using a tiny SOT-23 external p-channel FET. Connection of pin 3 (VAUX) to the 3.3V supply is optional, and adds active pull-down to the Drive pin. Component Selection Output capacitors - Semtech recommends a minimum bulk capacitance of 4.7µF at the output, along with a 0.1µF ceramic decoupling capacitor. Increasing the bulk capacitance will improve the overall transient response. The device is very tolerant of capacitor value and ESR variations, in fact, any combination of capacitors with C ³ 4.7µF and ESR < 1W is sufficient for stability. This target is easily met using surface mount ceramic or tantalum capacitors. Input capacitors (5V) - Semtech recommends the use of a 4.7µF ceramic or tantalum capacitor plus a 0.1µF ceramic capacitor at the input. This allows for the device being some distance from any bulk capacitance on the rail. Additionally, input droop due to load transients is reduced, improving load transient response. Input capacitors (3.3V) - Semtech recommends decoupling this pin (if used) with a 0.1µF ceramic capacitor. P-channel bypass FET - selection of the external FET is determined by two main requirements: 1) the FET has to have a very low gate threshold (typically ~1V) in order to be sufficiently turned on with V GS £ 3.3V. 2) the FET R DS(ON) must be low enough such that: ( ) ) MIN ( ) ON ( DS ) MAX ( O VO R I VAUX ≥ • − Typical Characteristics (Cont.)(1) Load Transient Response Trace 1: VO Trace 2: I O stepping from 0mA to 200mA Load Transient Response Trace 1: VO Trace 2: I O stepping from 200mA to 0mA Applications Information Notes: (1) In Application Circuit on page 1. (2) I O = 200mA. |
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