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LTC6655BHMS8-3.3PBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC6655BHMS8-3.3PBF Datasheet(HTML) 10 Page - Linear Technology |
10 / 22 page LTC6655 0 6655fa applicaTions inForMaTion Bypass and Load Capacitors The LTC6655 voltage references require a 0.1µF or larger input capacitor located close to the part to improve power supply rejection. An output capacitor with a value between 2.7µF and 100µF is also required. The output capacitor has a direct effect on the stability, turn-on time and settling behavior. Choose a capacitor with low ESR to insure stability. Resistance in series with the output capacitor (ESR) introduces a zero in the output buffer transfer function and could cause instability. The 2.7μF to 100μF range includes several types of capacitors thatarereadilyavailableasthrough-holeandsurfacemount components. It is recommended to keep ESR less than or equal to 0.1Ω. Capacitance and ESR are both frequency dependent. At higher frequencies capacitance drops and ESR increases. To insure stable operation the output ca- pacitor should have the required values at 100kHz. In order to achieve the best performance, caution should be used when choosing a capacitor. X7R ceramic ca- pacitors are small, come in appropriate values and are relatively stable over a wide temperature range. However, for a low noise application X7R capacitors may not be suitable since they may exhibit a piezoelectric effect. The mechanical vibrations cause a charge displacement in the ceramic dielectric and the resulting perturbation can look like noise. If X7R capacitors are necessary, a thorough bench evaluation should be completed to verify proper performance. For very low noise applications where every nanovolt counts, film capacitors should be considered for their low noise and lack of piezoelectric effects. Film capaci- tors such as polyester, polystyrene, polycarbonate, and polypropylene have good temperature stability. Additional care must be taken as polystyrene and polypropylene have an upper temperature limit of 85°C to 105°C. Above these temperatures, the working voltages need to be derated according to manufacturer’s specifications. Another type of film capacitor is polyphenylene sulfide (PPS). These devices work over a wide temperature range, are stable, and have large capacitance values beyond 1μF. In general, film capacitors are found in surface mount and leaded packages. Table 1 is a partial list of capacitor companies and some of their available products. In voltage reference applications, film capacitor lifetime is affected by temperature and applied voltage. When polyester capacitors are operated beyond their rated temperatures (some capacitors are not rated for operation above 85°C) they need to be derated. Voltage derating is usually accomplished as a ratio of applied voltage to rated voltagelimit.Contactspecificfilmcapacitormanufacturers to determine exact lifetime and derating information. The lifetime of X7R capacitors is long, especially for reference applications. Capacitor lifetime is degraded by operating near or exceeding the rated voltage, at high temperature, with AC ripple or some combination of these. Most reference applications have AC ripple only during transient events. Table 1. Film Capacitor Companies COMPANY DIELECTRIC AVAILABLE CAPACITANCE TEMPERATURE RANGE TYPE Cornell Dublier Polyester 0.5µF to 10µF –55°C to 125°C DME Dearborn Electronics Polyester 0.1µF to 12µF –55°C to 125°C 218P, 430P, 431P, 442P, and 410P Tecate Polyester 0.01µF to 18µF –40°C to 105°C 901, 914, and 914D Wima Polyester 10µF to 22µF –55°C to 100°C MKS 4, MKS 2-XL Vishay Polyester 1000pF to 15µF –55°C to 125°C MKT1820 Vishay Polycarbonate 0.01µF to 10µF –55°C to 100°C MKC1862, 632P Dearborn Electronics Polyphenylene Sulfide (PPS) 0.01µF to 15µF –55°C to 125°C 820P, 832P, 842P, 860P, and 880P Wima Polyphenylene Sulfide (PPS) 0.01µF to 6.8µF –55°C to 140°C SMD-PPS |
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