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PTH08080WAZT Datasheet(PDF) 9 Page - Texas Instruments |
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PTH08080WAZT Datasheet(HTML) 9 Page - Texas Instruments |
9 / 16 page www.ti.com CAPACITOR RECOMMENDATIONS FOR THE PTH08080W WIDE-OUTPUT ADJUST Input Capacitor Output Capacitors (Optional) Ceramic Capacitors Tantalum Capacitors Capacitor Table PTH08080W SLTS235A – FEBRUARY 2005 – REVISED MARCH 2005 POWER MODULES The minimum recommended input capacitor is 100 µF of capacitance. When VO >3.4 V, the 100-µF electrolytic capacitance must be rated for 650-mArms ripple current . For VO < 3.4 V, the ripple current rating must be at least 500 mArms. The ripple current rating of electrolytic capacitors is a major consideration when they are used at the input. When specifying regular tantalum capacitors for use at the input, a minimum voltage rating of 2 × (maximum dc voltage + ac ripple) is highly recommended. This is standard practice to ensure reliability. Polymer-tantalum capacitors are not affected by this requirement. For improved ripple reduction on the input bus, ceramic capacitors can also be added to complement the required electrolytic capacitance. No output capacitance is required for normal operation. However, applications with load transients (sudden changes in load currrent) can benefit by adding external output capacitance. A 100-µF electrolytic or ceramic capacitor can be used to improve transient response. Adding a 100-µF nonceramic capacitor allows the module to meet its transient response specification. A high-quality computer-grade electrolytic capacitor should be adequate. Electrolytic capacitors should be located close to the load circuit. These capacitors provide decoupling over the frequency range, 2 kHz to 150 kHz. Aluminum electrolytic capacitors are suitable for ambient temperatures above 0 °C. For operation below 0°C, tantalum or Os-Con-type capacitors are recommended. When using one or more nonceramic capacitors, the calculated equivalent ESR should be no lower than 10 m Ω (14 mΩ using the manufacturer's maximum ESR for a single capacitor). A list of preferred low-ESR-type capacitors are identified in Table 3, the recommended capacitor table. Above 150 kHz, the performance of aluminum electrolytic capacitors becomes less effective. To further improve the reflected input ripple current, or the output transient response, multilayer ceramic capacitors must be added. Ceramic capacitors have low ESR and their resonant frequency is higher than the bandwidth of the regulator. When placed at the output their combined ESR is not critical as long as the total value of ceramic capacitance does not exceed 200 µF. Also, to prevent the formation of local resonances, do not exceed the maximum number of capacitors specified in the capacitor table. Additional tantalum-type capacitors can be used at both the input and output, and are recommended for applications where the ambient operating temperature can be less than 0 °C. The AVX TPS, Sprague 593D/594/595, and Kemet T495/T510/T520 capacitors series are suggested over many other tantalum types due to their rated surge, power dissipation, and ripple current capability. As a caution, many general-purpose tantalum capacitors have considerably higher ESR, reduced power dissipation, and lower ripple current capability. These capacitors are also less reliable as they have lower power dissipation and surge current ratings. Tantalum capacitors that do not have a stated ESR or surge current rating are not recommended for power applications. When specifying Os-Con and polymer tantalum capacitors for the output, the minimum ESR limit is encountered well before the maximum capacitance value is reached. The capacitor table, Table 3, identifies the characteristics of capacitors from a number of vendors with acceptable ESR and ripple current (rms) ratings. The recommended number of capacitors required at both the input and output buses is identified for each capacitor type. This is not an extensive capacitor list. Capacitors from other vendors are available with comparable specifications. Those listed are for guidance. The rms rating and ESR (at 100 kHz) are critical parameters necessary to ensure both optimum regulator performance and long capacitor life. 9 |
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