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LP3873ES-3.3 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LP3873ES-3.3 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 16 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 of at least 1µF is required. Ceramic or Tantalum may be used, and capaci- tance may be increased without limit OUTPUT CAPACITOR: An output capacitor is required for loop stability. It must be located less than 1 cm from the device and connected directly to the output and ground pins using traces which have no other currents flowing through them (see PCB Layout section). The minimum value of the output capacitance that can be used for stable full-load operation is 10 µF, but it may be increased without limit. The output capacitor must have an ESR value as shown in the stable region of the curve (be- low). ESR Curve 20060570 SELECTING A CAPACITOR It is important to note that capacitance tolerance and varia- tion with temperature must be taken into consideration when selecting a capacitor so that the minimum required amount of capacitance is provided over the full operating tempera- ture range. In general, a good Tantalum capacitor will show very little capacitance variation with temperature, but a ce- ramic may not be as good (depending on dielectric type). Aluminum electrolytics also typically have large temperature variation of capacitance value. Equally important to consider is a capacitor’s ESR change with temperature: this is not an issue with ceramics, as their ESR is extremely low. However, it is very important in Tan- talum and aluminum electrolytic capacitors. Both show in- creasing ESR at colder temperatures, but the increase in aluminum electrolytic capacitors is so severe they may not be feasible for some applications (see Capacitor Character- istics Section). CAPACITOR CHARACTERISTICS CERAMIC: For values of capacitance in the 10 to 100 µF range, ceramics are usually larger and more costly than tantalums but give superior AC performance for bypassing high frequency noise because of very low ESR (typically less than 10 m Ω). However, some dielectric types do not have good capacitance characteristics as a function of voltage and temperature. Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V also exhibit a severe temperature effect, losing more than 50% of nomi- nal capacitance at high and low limits of the temperature range. X7R and X5R dielectric ceramic capacitors are strongly rec- ommended if ceramics are used, as they typically maintain a capacitance range within ±20% of nominal over full operat- ing ratings of temperature and voltage. Of course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance. TANTALUM: Solid Tantalum capacitors are recommended for use on the output because their typical ESR is very close to the ideal value required for loop compensation. They also work well as input capacitors if selected to meet the ESR requirements previously listed. Tantalums also have good temperature stability: a good quality Tantalum will typically show a capacitance value that varies less than 10-15% across the full temperature range of 125˚C to −40˚C. ESR will vary only about 2X going from the high to low temperature limits. The increasing ESR at lower temperatures can cause oscil- lations when marginal quality capacitors are used (if the ESR of the capacitor is near the upper limit of the stability range at room temperature). ALUMINUM: This capacitor type offers the most capaci- tance for the money. The disadvantages are that they are larger in physical size, not widely available in surface mount, and have poor AC performance (especially at higher fre- quencies) due to higher ESR and ESL. Compared by size, the ESR of an aluminum electrolytic is higher than either Tantalum or ceramic, and it also varies greatly with temperature. A typical aluminum electrolytic can exhibit an ESR increase of as much as 50X when going from 25˚C down to −40˚C. It should also be noted that many aluminum electrolytics only specify impedance at a frequency of 120 Hz, which indicates they have poor high frequency performance. Only aluminum electrolytics that have an impedance specified at a higher frequency (between 20 kHz and 100 kHz) should be used for the LP387X. Derating must be applied to the manufacturer’s ESR specification, since it is typically only valid at room temperature. Any applications using aluminum electrolytics should be thoroughly tested at the lowest ambient operating tempera- ture where ESR is maximum. TURN-ON CHARACTERISTICS FOR OUTPUT VOLTAGES PROGRAMMED TO 2.0V OR BELOW As Vin increases during start-up, the regulator output will track the input until Vin reaches the minimum operating voltage (typically about 2.2V). For output voltages pro- grammed to 2.0V or below, the regulator output may mo- mentarily exceed its programmed output voltage during start up. Outputs programmed to voltages above 2.0V are not affected by this behavior. www.national.com 11 |
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