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LP2986 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LP2986 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Application Hints EXTERNAL CAPACITORS Like any low-dropout regulator, external capacitors are re- quired to assure stability. These capacitors must be correctly selected for proper performance. INPUT CAPACITOR: An input capacitor ( ≥ 2.2 µF) is re- quired between the LP2986 input and ground (amount of ca- pacitance may be increased without limit). This capacitor must be located a distance of not more than 0.5” from the input pin and returned to a clean analog ground. Any good quality ceramic or tantalum may be used for this capacitor. OUTPUT CAPACITOR: The output capacitor must meet the requirement for minimum amount of capacitance and also have an appropriate E.S.R. (equivalent series resistance) value. Curves are provided which show the allowable ESR range as a function of load current for various output voltages and capacitor values (see ESR curves below). IMPORTANT: The output capacitor must maintain its ESR in the stable region over the full operating temperature range of the application to assure stability. The minimum required amount of output capacitance is 4.7 µF. 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. A good Tantalum capacitor will show very little variation with temperature, but a ceramic may not be as good (see next section). CAPACITOR CHARACTERISTICS TANTALUM: The best choice for size, cost, and perfor- mance are solid tantalum capacitors. Available from many sources, their typical ESR is very close to the ideal value re- quired on the output of many LDO regulators. Tantalums also have good temperature stability: a 4.7 µF was tested and showed only a 10% decline in capacitance as the temperature was decreased from +125˚C to −40˚C. The ESR increased only about 2:1 over the same range of temperature. However, it should be noted that the increasing ESR at lower temperatures present in all tantalums can cause oscillations when marginal quality capacitors are used (where the ESR of the capacitor is near the upper limit of the stability range at room temperature). CERAMIC: For a given amount of a capacitance, ceramics are usually larger and more costly than tantalums. Be warned that the ESR of a ceramic capacitor can be low enough to cause instability: a 2.2 µF ceramic was measured and found to have an ESR of about 15 m Ω. If a ceramic capacitor is to be used on the LP2986 output, a 1 Ω resistor should be placed in series with the capacitor to provide a minimum ESR for the regulator. Another disadvantage of ceramic capacitors is that their ca- pacitance varies a lot with temperature: Large ceramic capacitors are typically manufactured with the Z5U temperature characteristic, which results in the capaci- tance dropping by a 50% as the temperature goes from 25˚C to 80˚C. This means you have to buy a capacitor with twice the mini- mum C OUT to assure stable operation up to 80˚C. ALUMINUM: The large physical size of aluminum electrolyt- ics makes them unattractive for use with the LP2986. Their ESR characteristics are also not well suited to the require- ments of LDO regulators. The ESR of an aluminum electrolytic is higher than a tanta- lum, and it also varies greatly with temperature. A typical aluminum electrolytic can exhibit an ESR increase of 50X when going from 20˚C to −40˚C. Also, some alumi- num electrolytics can not be used below −25˚C because the electrolyte will freeze. USING AN EXTERNAL RESISTIVE DIVIDER The LP2986 output voltage can be programmed using an ex- ternal resistive divider (see Basic Application Circuits). The resistor connected between the Feedback pin and ground should be 51.1k. The value for the other resistor (R1) connected between the Feedback pin and the regulated out- put is found using the formula: V OUT = 1.23 x (1 + R1/51.1k) It should be noted that the 25 µA of current flowing through the external divider is approximately equal to the current saved by not connecting the internal divider, which means the quiescent current is not increased by using external re- sistors. ESR Curves For 5V Output DS012935-6 ESR Curves For 2.5V Output DS012935-7 www.national.com 11 |
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