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PTH12050L Datasheet(PDF) 7 Page - Texas Instruments |
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PTH12050L Datasheet(HTML) 7 Page - Texas Instruments |
7 / 18 page Application Notes For technical support and further information, visit http://power.ti.com PTH12050 Series Capacitor Recommendations for the PTH12050 Series of Power Modules Input Capacitor The recommended input capacitor(s) is determined by the 100 µF minimum capacitance and 750 mArms mini- mum ripple current rating. A 10-µF X5R/X7R ceramic capacitor may also be added to reduce the reflected in- put ripple current. This is recommended for output voltage set points of 3.3 V and higher. Ripple current, less than 100 m Ω equivalent series resis- tance (ESR) and temperature are major considerations when selecting input capacitors. Unlike polymer-tantalum capacitors, regular tantalum capacitors have a recom- mended minimum voltage rating of 2 × (max. DC voltage + AC ripple). This is standard practice to ensure reliability. Only a few tantalum capacitors have sufficient voltage rating to meet this requirement. At temperatures below 0 °C, the ESR of aluminum electrolytic capacitors in- creases. For these applications Os-Con, polymer-tantalum, and polymer-aluminum types should be considered. Output Capacitors (Optional) For applications with load transients (sudden changes in load current), regulator response will benefit from external output capacitance. The value of 330 µF is used to define the transient response specification (see data sheet). For most applications, a high quality computer-grade alumi- num electrolytic capacitor is adequate. These capacitors provide decoupling over the frequency range, 2 kHz to 150 kHz, and are suitable for ambient temperatures above 0 °C. Below 0 °C, tantalum, ceramic or Os-Con type capacitors are recommended. When using one or more non-ceramic capacitors, the calculated equivalent ESR should be no lower than 4 m Ω (7 mΩ using the manufacturer’s maximum ESR for a single capacitor). A list of preferred low-ESR type capacitors are identified in Table 2-1. In addition to electrolytic capacitance, adding a 10-µF X5R/X7R ceramic capacitor to the output will reduce the output ripple voltage and improve the regulator’s transient response. The measurement of both the output ripple and transient response is also best achieved across a 10-µF ceramic capacitor. Ceramic Capacitors Above 150 kHz the performance of aluminum electrolytic capacitors is less effective. Multilayer ceramic capacitors have very low ESR and a resonant frequency higher than the bandwidth of the regulator. They can be used to reduce the reflected ripple current at the input as well as improve the transient response of the output. When used on the output their combined ESR is not critical as long as the total value of ceramic capacitance does not exceed 300 µF. Also, to prevent the formation of local resonances, do not place more than five identical ceramic capacitors in par- allel with values of 10 µF or greater. Tantalum Capacitors Tantalum type capacitors are most suited for use on the output bus, 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 capacitor series are suggested over other tantalum types due to their higher 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 no surge current rating. 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 will be encoun- tered well before the maximum capacitance value is reached. Capacitor Table Table 1-1 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 ripple current rating and ESR (at 100 kHz) are critical parameters necessary to insure both optimum regulator performance and long capacitor life. Designing for Very Fast Load Transients The transient response of the DC/DC converter has been characterized using a load transient with a di/dt of 1 A/µs. The typical voltage deviation for this load transient is given in the data sheet specification table using the optional value of output capacitance. As the di/dt of a transient is increased, the response of a converter’s regulation circuit ultimately depends on its output capacitor decoupling network. This is an inherent limitation with any DC/DC converter once the speed of the transient exceeds its bandwidth capability. If the target application specifies a higher di/dt or lower volt- age deviation, the requirement can only be met with additional output capacitor decoupling. In these cases special attention must be paid to the type, value and ESR of the capacitors selected. If the transient performance requirements exceed that specified in the data sheet, or the total amount of load capacitance is above 3,000 µF, the selection of output capacitors becomes more important. For further guidance consult the separate application note, “Selecting Output Capacitors for PTH Products in High-Performance Applica- tions.” |
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