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LP2978IM5-3.8 Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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LP2978IM5-3.8 Datasheet(HTML) 5 Page - National Semiconductor (TI) |
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5 / 7 page  Application Hints EXTERNAL CAPACITORS Like any low-dropout regulator, the LP2978 requires external capacitors for regulator stability. These capacitors must be correctly selected for good performance. Input Capacitor An input capacitor whose capacitance is ≥ 1 µF is required between the LP2978 input and ground (the amount of ca- pacitance may be increased without limit). This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean analog ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input. Important: Tantalum capacitors can suffer catastrophic fail- ure due to surge current when connected to a low-impedance source of power (like a battery or 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 requirements for ESR on the input capacitor, but tolerance and temperature coefficient must be consid- ered when selecting the capacitor to ensure the capacitance will be ≥ 1 µF over the entire operating temperature range. Output Capacitor: The LP2978 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 5 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). Important: The output capacitor must maintain its ESR within the stable region over the full operating temperature range of the application to assure stability. The LP2978 requires a minimum of 2.2 µF on the output (output capacitor size can be increased without limit). 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 re- quired amount of output capacitance is provided over the full operating temperature range. It should be noted that ceramic capacitors can exhibit large changes in capacitance with temperature (see next section, Capacitor Characteristics). The output capacitor must be located not more than 1 cm from the output pin and returned to a clean analog ground. Noise Bypass Capacitor: Connecting a 10 nF capacitor to the Bypass pin significantly reduces noise on the regulator output. It should be noted that the capacitor is connected directly to a high-impedance cir- cuit 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 ca- pacitor are ceramic and film. High-quality ceramic capacitors with either NPO or COG dielectric typically have very low leakage. 10 nF polypropolene and polycarbonate film ca- pacitors are available in small surface-mount packages and typically have extremely low leakage current. CAPACITOR CHARACTERISTICS The LP2978 was designed to work with ceramic capacitors on the output to take advantage of the benefits they offer: for capacitance values in the 2.2 µF to 4.7 µF range, ceramics are the least expensive and also have the lowest ESR val- ues (which makes them best for eliminating high-frequency noise). The ESR of a typical 2.2 µF ceramic capacitor is in the range of 10 m Ω to 20 mΩ, which easily meets the ESR limits required for stability by the LP2978. One disadvantage of ceramic capacitors is that their capaci- tance can vary with temperature. Most 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. This could cause problems if a 2.2 µF capacitor were used on the output since it will drop down to approximately 1 µF at high ambient temperatures (which could cause the LP2978 to oscillate). If Z5U or Y5V capacitors are used on the output, a minimum capacitance value of 4.7 µF must be observed. A better choice for temperature coefficient in ceramic capaci- tors is X7R, which holds the capacitance within ±15%. Un- fortunately, the larger values of capacitance are not offered by all manufacturers in the X7R dielectric. Tantalum: Tantalum capacitors are less desirable than ceramics for use as output capacitors because they are more expensive when comparing equivalent capacitance and voltage ratings in the 1 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 ca- pacitor 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. DS100935-7 www.national.com 5 |
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