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LP3986BLX-3133 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LP3986BLX-3133 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 13 page  Application Hints EXTERNAL CAPACITORS Like any low-dropout regulator, the LP3986 requires external capacitors for regulator stability. The LP3986 is specifically designed for portable applications requiring minimum board space and smallest components. These capacitors must be correctly selected for good performance. INPUT CAPACITOR An input capacitance of ) 1µF is required between the LP3986 input pin and ground (the amount of the capacitance may be increased without limit). This capacitor must be located a distance of not more than 1cm 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- 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 requirements for the ESR on the input capaci- tor, but tolerance and temperature coefficient must be con- sidered when selecting the capacitor to ensure the capaci- tance will be ) 1µF over the entire operating temperature range. OUTPUT CAPACITOR The LP3986 is designed specifically to work with very small ceramic output capacitors, any ceramic capacitor (tempera- ture characteristics X7R, X5R, Z5U or Y5V) in 1 to 22 µF range with 5m Ω to 500mΩ ESR range is suitable in the LP3986 application circuit. It may 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 a stable range. NO-LOAD STABILITY The LP3986 will remain stable and in regulation with no-load (other than the internal voltage divider). This is specially important in CMOS RAM keep-alive applications. CAPACITOR CHARACTERISTICS The LP3986 is designed to work with ceramic capacitors on the output to take advantage of the benefits they offer: for capacitance values in the range of 1µF to 4.7µF range, ceramic capacitors are the smallest, least expensive and have the lowest ESR values (which makes them best for eliminating high frequency noise). The ESR of a typical 1µF ceramic capacitor is in the range of 20 m Ω to 40 mΩ, which easily meets the ESR requirement for stability by the LP3986. The ceramic capacitor’s capacitance can vary with tempera- ture. The capacitor type X7R, which operates over a tem- perature range of -55˚C to +125˚C, will only vary the capaci- tance to within ±15%. Most large value ceramic capacitors () 2.2µF) are manufactured with Z5U or Y5V temperature characteristics. Their capacitance can drop by more than 50% as the temperature goes from 25˚C to 85˚C. Therefore, X7R is recommended over Z5U and Y5 in applications where the ambient temperature will change significantly above or below 25˚C. Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more expensive when comparing equivalent capacitance and voltage ratings in the 1µF to 4.7µF 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. NOISE BYPASS CAPACITOR Connecting a 0.01µF capacitor between the C BYPASS pin and ground significantly reduces noise on the regulator out- put. This cap is connected directly to a high impedance node in the band gap reference circuit. Any significant loading on this node will cause a change on the regulated output volt- age. For this reason, DC leakage current through this pin must be kept as low as possible for best output voltage accuracy. The use of this 0.01µF bypass capacitor is strongly recommended to prevent overshoot on the output during start up. 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. Polypropolene and polycarbonate film capaci- tors are available in small surface-mount packages and typically have extremely low leakage current. Unlike many other LDO’s, addition of a noise reduction capacitor does not effect the transient response of the de- vice. ON/OFF INPUT OPERATION The LP3986 is turned off by pulling the V EN pin low, and turned on by pulling it high. If this feature is not used, the V EN pin 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 V EN input must be able to swing above and below the specified turn-on/off voltage thresholds listed in the Elec- trical Characteristics section under V IL and VIH. FAST ON-TIME The LP3986 outputs are turned on after V ref voltage reaches its final value (1.23V nominal). To speed up this process, the noise reduction capacitor at the bypass pin is charged with an internal 70µA current source. The current source is turned off when the bandgap voltage reaches approximately 95% of its final value. The turn on time is determined by the time constant of the bypass capccitor. The smaller the capacitor value, the shorter the turn on time, but less noise gets reduced. As a result, turn on time and noise reduction need to be taken into design consideration when choosing the value of the bypass capacitor. www.national.com 10 |
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