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LP5900TL-3.0 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LP5900TL-3.0 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 10 page  Application Hints (Continued) This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean analogue ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input. Important: To ensure stable operation it is essential that good PCB practices are employed to minimize ground im- pedance and keep input inductance low. If these conditions cannot be met, or if long leads are to be used to connect the battery or other power source to the LP5900, then it is recommended to increase the input capacitor to at least 2.2µF. Also, tantalum capacitors can suffer catastrophic fail- ures due to surge current when connected to a low- impedance source of power (like a battery or a very large capacitor). If a tantalum capacitor is used at the input, it must be guaranteed by the manufacturer to have a surge current rating sufficient for the application. There are no require- ments for the ESR (Equivalent Series Resistance) on the input capacitor, but tolerance and temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain 0.47 µF ±30% over the entire operating temperature range. OUTPUT CAPACITOR The LP5900 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor (dielectric types X5R or X7R) in the 0.47 µF to 10 µF range, and with ESR between 5 m Ω to 500 mΩ, is suitable in the LP5900 application circuit. For this device the output capacitor should be connected between the V OUT pin and a good ground connection and should be mounted within 1 cm of the device. It may also be possible to use tantalum or film capacitors at the device output, V OUT, but these are not as attractive for reasons of size and cost (see the section Capacitor Charac- teristics). The output capacitor must meet the requirement for the minimum value of capacitance and have an ESR value that is within the range 5 m Ω to 500 mΩ for stability. CAPACITOR CHARACTERISTICS The LP5900 is designed to work with ceramic capacitors on the input and output to take advantage of the benefits they offer. For capacitance values in the range of 0.47 µF to 4.7 µF, ceramic capacitors are the smallest, least expensive and have the lowest ESR values, thus making them best for eliminating high frequency noise. The ESR of a typical 0.47 µF ceramic capacitor is in the range of 20 m Ω to 40 mΩ, which easily meets the ESR requirement for stability for the LP5900. The temperature performance of ceramic capacitors varies by type and manufacturer. Most large value ceramic capaci- tors ( ≥2.2 µF) are manufactured with Z5U or Y5V tempera- ture characteristics, which results in the capacitance drop- ping by more than 50% as the temperature goes from 25˚C to 85˚C. A better choice for temperature coefficient in a ceramic capacitor is X7R. This type of capacitor is the most stable and holds the capacitance within ±15% over the tempera- ture range. Tantalum capacitors are less desirable than ce- ramic for use as output capacitors because they are more expensive when comparing equivalent capacitance and volt- age ratings in the 0.47 µF to 4.7 µF range. Another important consideration is that tantalum capacitors have higher ESR values than equivalent size ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic capacitor with the same ESR value. It should also be noted that the ESR of a typical tantalum will increase about 2:1 as the temperature goes from 25˚C down to −40˚C, so some guard band must be allowed. NO-LOAD STABILITY The LP5900 will remain stable and in regulation with no external load. ENABLE CONTROL The LP5900 may be switched ON or OFF by a logic input at the ENABLE pin, V EN . A high voltage at this pin will turn the device on. When the enable pin is low, the regulator output is off and the device typically consumes 3 nA. If the application does not require the shutdown feature, the V EN pin should be tied to V IN to keep the regulator output permanently on. A1M Ω pulldown resistor ties the V EN input to ground, this ensures that the device will remain off when the enable pin is left open circuit. To ensure proper operation, the signal source used to drive the V EN input must be able to swing above and below the specified turn-on/off voltage thresholds listed in the Electrical Characteristics section under V IL and V IH. micro SMD MOUNTING The micro SMD package requires specific mounting tech- niques, which are detailed in National Semiconductor Appli- cation Note AN-1112. For best results during assembly, alignment ordinals on the PC board may be used to facilitate placement of the micro SMD device. micro SMD LIGHT SENSITIVITY Exposing the micro SMD device to direct light may cause incorrect operation of the device. Light sources such as halogen lamps can affect electrical performance if they are situated in proximity to the device. Light with wavelengths in the red and infra-red part of the spectrum have the most detrimental effect thus the fluores- cent lighting used inside most buildings has very little effect on performance. www.national.com 9 |
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