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LP3879SDX-1.2 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LP3879SDX-1.2 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 11 page Application Information (Continued) when selecting an output capacitor so that the minimum required amount of output capacitance is provided over the full operating temperature range. (See Capacitor Character- istics section). The output capacitor must be located not more than 0.5" from the output pin and returned to a clean analog ground. NOISE BYPASS CAPACITOR: The 10 nF capacitor on the Bypass pin significantly reduces noise on the regulator out- put and is required for loop stability. However, the capacitor is connected directly to a high-impedance circuit in the band- gap reference. Because this circuit has only a few microamperes flowing in it, any significant loading on this node will cause a change in the regulated output voltage. For this reason, DC leakage current through the noise bypass capacitor must never ex- ceed 100 nA, and should be kept as low as possible for best output voltage accuracy. The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High-quality ceramic capaci- tors with either NPO or COG dielectric typically have very low leakage. 10 nF polypropolene and polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low leakage current. CAPACITOR CHARACTERISTICS CERAMIC: The LP3879 was designed to work with ceramic capacitors on the output to take advantage of the benefits they offer: for capacitance values in the 10 µF range, ceram- ics are the least expensive and also have the lowest ESR values (which makes them best for eliminating high- frequency noise). The ESR of a typical 10 µF ceramic ca- pacitor is in the range of 5 m Ω to 10 mΩ, which meets the ESR limits required for stability by the LP3879. One disadvantage of ceramic capacitors is that their capaci- tance can vary with temperature. Many large value ceramic capacitors ( ≥ 2.2 µF) are manufactured with the Z5U or Y5V temperature characteristic, which results in the capacitance dropping by more than 50% as the temperature goes from 25˚C to 85˚C. Another significant problem with Z5U and Y5V dielectric devices is that the capacitance drops severely with applied voltage. A typical Z5U or Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. For these reasons, X7R and X5R type ceramic capaci- tors must be used on the input and output of the LP3879. SHUTDOWN INPUT OPERATION The LP3879 is shut off by pulling the Shutdown input low, and turned on by pulling it high. If this feature is not to be used, the Shutdown input should be tied to V IN to keep the regulator output on at all times. To assure proper operation, the signal source used to drive the Shutdown input must be able to swing above and below the specified turn-on/turn-off voltage thresholds listed in the Electrical Characteristics section under V ON/OFF. REVERSE INPUT-OUTPUT VOLTAGE The PNP power transistor used as the pass element in the LP3879 has an inherent diode connected between the regu- lator output and input. During normal operation (where the input voltage is higher than the output) this diode is reverse-biased. However, if the output is pulled above the input, this diode will turn ON and current will flow into the regulator output. In such cases, a parasitic SCR can latch which will allow a high current to flow into V IN (and out the ground pin), which can damage the part. In any application where the output may be pulled above the input, an external Schottky diode must be connected from V IN to VOUT (cathode on VIN, anode on VOUT), to limit the reverse voltage across the LP3879 to 0.3V (see Absolute Maximum Ratings). www.national.com 9 |
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