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LP3990TLX-1.35 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LP3990TLX-1.35 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 13 page  Application Hints EXTERNAL CAPACITORS In common with most regulators, the LP3990 requires exter- nal capacitors for regulator stability. The LP3990 is specifi- cally designed for portable applications requiring minimum board space and smallest components. These capacitors must be correctly selected for good performance. INPUT CAPACITOR An input capacitor is required for stability. It is recommended that a 1.0µF capacitor be connected between the LP3990 input pin and ground (this capacitance value may be in- creased without limit). This capacitor must be located a distance of not more than 1cm from the input pin and returned to a clean analogue 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 (Equivalent Series Resistance) on the input capacitor, but tolerance and tem- perature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain ) 1.0µF over the entire operating temperature range. OUTPUT CAPACITOR The LP3990 is designed specifically to work with very small ceramic output capacitors. A 0.47µF ceramic capacitor (tem- perature types Z5U, Y5V or X7R) with ESR between 5m Ω to 500m Ω, is suitable in the LP3990 application circuit. For this device the output capacitor should be connected between the V OUT pin and ground. It is also possible to use tantalum or film capacitors at the device output, C OUT (or VOUT), but these are not as attrac- tive for reasons of size and cost (see the section Capacitor Characteristics). The output capacitor must meet the requirement for the minimum value of capacitance and also have an ESR value that is within the range 5m Ω to 500mΩ for stability. NO-LOAD STABILITY The LP3990 will remain stable and in regulation with no external load. This is an important consideration in some circuits, for example CMOS RAM keep-alive applications. CAPACITOR CHARACTERISTICS The LP3990 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 0.47µF to 4.7µF, ceramic capacitors are the smallest, least expensive and have the lowest ESR values, thus making them best for eliminating high frequency noise. The ESR of a typical 0.47µF ceramic capacitor is in the range of 20m Ω to 40mΩ, which easily meets the ESR requirement for stability for the LP3990. For both input and output capacitors, careful interpretation of the capacitor specification is required to ensure correct de- vice operation. The capacitor value can change greatly, de- pending on the operating conditions and capacitor type. In particular, the output capacitor selection should take ac- count of all the capacitor parameters, to ensure that the specification is met within the application. The capacitance can vary with DC bias conditions as well as temperature and frequency of operation. Capacitor values will also show some decrease over time due to aging. The capacitor pa- rameters are also dependant on the particular case size, with smaller sizes giving poorer performance figures in gen- eral. As an example, Figure 1 shows a typical graph com- paring different capacitor case sizes in a Capacitance vs. DC Bias plot. As shown in the graph, increasing the DC Bias condition can result in the capacitance value falling below the minimum value given in the recommended capacitor specifications table (0.33µF in this case). Note that the graph shows the capacitance out of spec for the 0402 case size capacitor at higher bias voltages. It is therefore recom- mended that the capacitor manufacturers’ specifications for the nominal value capacitor are consulted for all conditions, as some capacitor sizes (e.g. 0402) may not be suitable in the actual application. 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%. The capacitor type X5R has a similar tolerance over a reduced temperature range of -55˚C to +85˚C. Many large value ceramic capacitors, larger than 1µF are manufactured with Z5U or Y5V temperature characteris- tics. Their capacitance can drop by more than 50% as the temperature varies from 25˚C to 85˚C. Therefore X7R is recommended over Z5U and Y5V in applications where the ambient temperature will change significantly above or be- low 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 0.47µF 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 20076840 FIGURE 1. Graph Showing a Typical Variation in Capacitance vs DC Bias www.national.com 10 |
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