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LP2989AIMM-1.8 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LP2989AIMM-1.8 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
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8 / 11 page  Application Hints LLP Package Devices The LP2989LV is offered in the 8 lead LLP surface mount package to allow for increased power dissipation compared to the SO-8 and Mini SO-8. For details on thermal perfor- mance as well as mounting and soldering specifications, refer to Application Note AN-1187. For output voltages ≥ 2V, see LP2989 datasheet. EXTERNAL CAPACITORS Like any low-dropout regulator, the LP2989LV requires ex- ternal capacitors for regulator stability.These capacitors must be correctly selected for good performance. INPUT CAPACITOR: An input capacitor whose capacitance is at least 2.2 µF is required between the LP2989LV input and ground (the amount of capacitance may be increased without limit). Characterization testing performed on the LP2989LV has shown that if the amount of actual input capacitance drops below about 1.5 µF, an unstable operating condition may result. Therefore, the next larger standard size (2.2 µF) is specified as the minimum required input capacitance. Ca- pacitor tolerance and temperature variation must be consid- ered when selecting a capacitor (see Capacitor Character- istics section) to assure the minimum requirement of 1.5 µF is met over all operating conditions. The input capacitor must be located at a distance of not more than 0.5" from the input pin and returned to a clean analog ground. Any good quality ceramic or tantalum may be used for this capacitor, assuming the minimum capacitance requirement is met. OUTPUT CAPACITOR: The LP2989LV requires a ceramic output capacitor whose size is at least 10 µF. The actual amount of capacitance on the output must never drop below about 7 µF or unstable operation may result. For this reason, capacitance tolerance and temperature characteristics must be considered when selecting an output capacitor The LP2989LV is designed specifically to work with ceramic output capacitors, utilizing circuitry which allows the regula- tor to be stable across the entire range of output current with an output capacitor whose ESR is as low as 4 m Ω.Itmay also be possible to use Tantalum or film capacitors at the output, but these are not as attractive for reasons of size and cost (see next section Capacitor Characteristics). The output capacitor must meet the requirement for mini- mum amount of capacitance and also have an ESR (equiva- lent series resistance) value which is within the stable range. Curves are provided which show the stable ESR range as a function of load current (see ESR graph below). 10135738 Stable Region For Output Capacitor ESR Important: The output capacitor must maintain its ESR within the stable region over the full operating temperature range of the application to assure stability. It is important to remember that capacitor tolerance and variation with temperature must be taken into consideration 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: Connecting a 10 nF capaci- tor to the Bypass pin significantly reduces noise on the regulator output. However, the capacitor is connected di- rectly to a high-impedance circuit in the bandgap 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 LP2989LV was designed to work with ce- ramic capacitors on the output to take advantage of the benefits they offer: for capacitance values in the 10 µF range, ceramics 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 capacitor is in the range of 5 m Ω to 10 mΩ, which meets the ESR limits required for stability by the LP2989LV. 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. www.national.com 8 |
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