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PTN78000A Datasheet(PDF) 10 Page - Texas Instruments |
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PTN78000A Datasheet(HTML) 10 Page - Texas Instruments |
10 / 18 page www.ti.com CAPACITOR RECOMMENDATIONS FOR THE PTN78000 WIDE-OUTPUT Input Capacitor Output Capacitor Ceramic Capacitors Tantalum Capacitors Capacitor Table PTN78000A SLTS246A – APRIL 2005 – REVISED MAY 2005 ADJUST POWER MODULES The minimum requirements for the input bus is 100 µF of nonceramic capacitance and 9.4 µF (2 x 4.7 µF) of ceramic capacitance, in either an X5R or X7R temperature characteristic, and 100 µF of electrolytic capacitance. Ceramic capacitors should be located within 0.5 inch (1,27 cm) of the regulator's input pins. Electrolytic capacitors should be used at the input in addition to the required ceramic capacitance. The minimum ripple current rating for any nonceramic capacitance must be at least 250 mA rms. The ripple current rating of electrolytic capacitors is a major consideration when they are used at the input. This ripple current requirement can be reduced by placing more ceramic capacitors at the input, in addition to the minimum required 9.4 µF. Tantalum capacitors are not recommended for use at the input bus, as none were found to meet the minimum voltage rating of 2 × (maximum dc voltage + ac ripple). The 2× rating is standard practice for regular tantalum capacitors to ensure reliability. Polymer-tantalum capacitors are more reliable and are available with a maximum rating of typically 20 V. These can be used with input voltages up to 16 V. The minimum capacitance required to ensure stability is a 100 µF. Either ceramic or electrolytic-type capacitors can be used. The minimum ripple current rating for the nonceramic capacitance must be at least 200 mA rms. The stability of the module and voltage tolerances is compromised if the capacitor is not placed near the output bus pins. A high-quality, computer-grade electrolytic capacitor should be adequate. A ceramic capacitor can be also be located within 0.5 inch (1,27 cm) of the output pin. For applications with load transients (sudden changes in load current), the regulator response improves with additional capacitance. Additional 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 14 m Ω (17 mΩ using the manufacturer's maximum ESR for a single capacitor). A list of recommended capacitors and vendors are identified in Table 3. Above 150 kHz, the performance of aluminum electrolytic capacitors becomes less effective. To further reduce 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. Tantalum-type capacitors may be used at the 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 various 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. 10 |
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