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LP3929TME-AACQ Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LP3929TME-AACQ Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 12 page Application Information EXTERNAL CAPACITORS Like any low-dropout regulator, the LP3929 requires external capacitors for regulator stability. The LP3929 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 1 µF is required between the LP3929 V BAT pin and ground (the amount of the capacitance may be increased without limit). This capacitor must be located a distance of not more than 1 cm from the V BAT 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, bias voltage and temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will be 1 µF over the entire operating con- ditions. FAST ON-TIME The LP3929 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. CAPACITOR CHARACTERISTICS The LP3929 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 1 µF to 4.7 µF 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 1 µF ceramic capacitor is in the range of 20 m Ω to 40 mΩ, which easily meets the ESR requirement for stability by the LP3929. The ceramic capacitor’s capacitance can vary with tempera- ture. Most large value ceramic capacitors (2.2 µF) are manufac- tured 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 ceramic capaci- tor 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 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 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. OUTPUT CAPACITOR The LP3929 is designed specifically to work with very small ceramic output capacitors, any ceramic capacitor (dielectric types Z5U, Y5V or X7R) in 1.0 µF to 2.2 µF range with 5 m Ω to 500 m Ω ESR range is suitable in the LP3929 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 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. The output capacitor should be placed as near as possible to the V DDB pin. NO-LOAD STABILITY The LDO of the LP3929 will remain stable and in regulation with no external load connected to the LDO output V DDB. This is especially important in CMOS RAM keep-alive appli- cations. MICRO SMD ASSEMBLY For assembly recommendations of micro SMD package please refer to National Semiconductor Application Note AN-1112. MICRO SMD LIGHT SENSITIVITY Exposing the micro SMD device to direct sunlight will cause misoperation of the device. Light sources such as Halogen lamps can effect electrical performance if brought near to the device. The wavelengths which have most detrimental effect are reds and infra-reds, which means that the fluorescent light- ing used inside most buildings has very little effect on performance. www.national.com 11 |
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