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LP3988IMFX-3.0 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LP3988IMFX-3.0 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 11 page  Application Hints EXTERNAL CAPACITORS Like any low-dropout regulator, the LP3988 requires external capacitors for regulator stability. The LP3988 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 LP3988 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 LP3988 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor (dielectric types Z5U, Y5V or X7R) in 1 to 22 µF range with 5m Ω to 500m Ω ESR range is suitable in the LP3988 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 (5 m Ω to 500 mΩ). NO-LOAD STABILITY The LP3988 will remain stable and in regulation with no external load. This is specially important in CMOS RAM keep-alive applications. CAPACITOR CHARACTERISTICS The LP3988 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 LP3988. The ceramic capacitor’s capacitance can vary with tempera- ture. Most large value ceramic capacitors () 2.2µF) are manufactured with Z5U or Y5V temperature characteristics, which results in the capacitance dropping 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, which holds the capacitance within ±15%. 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. ON/OFF INPUT OPERATION The LP3988 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 time. 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 LP3988 utilizes a speed up circuitry to ramp up the internal V REF voltage to its final value to achieve a fast output turn on time. www.national.com 9 |
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