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ISR Input/Output filters

Posted: 21 Sep 2001     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2001SEP/2001SEP21_AMD_POW_AN5.PDF

For technical support and more information, see inside back cover or visit www.ti.com/powertrends Ferrite Bead Integrated Switching Regulator VoutVin C4 1 F C3 1 F C1 1 F C2* 100 F * For 78SR SeriesC2 must be 1~10 F ceramic ** C5 is recommended when Iout > 2A C5** 100 F COMCOM Ferrite Bead PT6500 Series Integrated Switching Regulator VoutVin=5V C5 330 F C5, C6, C7, C8 = Electrolytic or Tantalum C7 100 F COM C6 330 F C8 100 F COM Ferrite Beads Ferrite Beads Integrated Switching Regulator Vout COMCOM Vin C1 1 F C2* 100 F +C3 1 F C4 1 F * For 78SR Series C2 must be 1~10 F ceramic Integrated Switching Regulator VoutVin C2* 100 F * For 78SR SeriesC2 must be 1~10 F ceramic COMCOM Murata or NexTek Filter Murata or NexTek Filter ISR Input/Output Filters Power Trends includes internal input and output capaci- tors in all of their ISRs. However, some applications require much lower levels of either input or output ripple/noise. This application note describes various filters and design techniques found to be successful in reducing both input and output ripple/noise. Input/Output Capacitors The easiest way to reduce output ripple/noise is to add one or more 15F ceramic capacitors, such as C4 shown in Figure 17. They should be placed as close as possible to the output pin of the ISR. A single 15F capacitor will reduce the output ripple/noise by 30-50% for ISRs with Iout < 3A. (Note: C2 is required for loop stability and does not reduce output ripple/noise). All Products ISRs are switching regulators and as such draw current from the input line in pulses at their operating frequency. The amount of upstream (input) ripple/noise generated by an ISR is directly proportional to the equivalent source imped- ance of the power source including the impedance of any input lines. The addition of C3, a 15F ceramic capacitor, near the input pins of the ISR, will reduce upstream con- ducted ripple/noise by 30-50%. PI Filters If you need further reduction in the ripple/noise level for your application, higher order filters must be used. A (pi) filter employing a ferrite bead (Fair-Rite p/n 2673000701 or equivalent) in series with the input or output lines of the ISR (see Figures 18 and 20) reduces the ripple/noise by at least 20 db (See Figure 19). These beads form an excellent filter because they are lossy at the ISR's switching frequency (650kHz - 1MHz). The placement of this filter is critical. It must be located as close as possible to the input or output pins of the ISR. The ferrite bead is small (12.5mm long x 3mm diameter), easy to use, low cost, and has low DC resistance. Fair-Rite also manufactures an SMD bead (p/n 2773021447) rated to 5 Amps, but in this application, it is only effective to 2A. C1, C3, and C4 are ceramic capacitors which have excellent high-fre- quency performance and low impedance. Capacitor C2 should be a 1005F, low ESR type aluminum electrolytic or tantalum capacitor. All capaci- tors must have short leads and traces to be effective. When using ISRs with output currents greater than 3A, such as the PT6500 Series, we recommend using two ferrite beads in parallel as shown in Figure 20. Off The Shelf Filters There are several manufacturers of completely self-contained filters with excellent rejection characteristics: 1. Murata Erie (BNX002A) - Ripple/noise reduction is greater than 20db and it filters both Vin/Vout and ground return. Maximum current capacity is 10ADC. 2. NexTek (NT004-B1) - An excellent filter with both common-mode and differential mode rejection. PI FILTER ATTENUATION VS OUTPUT CURRENT 10 15 20 25 30 35 40 0.0 1.0 2.0 3.0 OUTPUT CURRENT Figure 19 1 MHz 650 kHz ATTENUATION(dB) Figure 17 Figure 18 Figure 20 Figure 21 Application Notes--AN13 SLTA013A (Revised 6/30/2000) IM PORTANT NOTICE Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those pertaining to warranty, patent infringement, and limitation of liability. TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with TI's standard warranty. Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed, except those mandated by government requirements. Customers are responsible for their applications using TI components. In order to minimize risks associated with the customer's applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right of TI covering or relating to any combination, machine, or process in which such semiconductor products or services might be or are used. TI's publication of information regarding any third party's products or services does not constitute TI's approval, warranty or endorsement thereof. Copyright ) 2000, Texas Instruments Incorporated IMPORTANT NOTICE Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those pertaining to warranty, patent infringement, and limitation of liability. TI warrants performance of its products to the specifications applicable at the time of sale in accordance with TI's standard warranty. Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed, except those mandated by government requirements. Customers are responsible for their applications using TI components. In order to minimize risks associated with the customer's applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right of TI covering or relating to any combination, machine, or process in which such products or services might be or are used. TI's publication of information regarding any third party's products or services does not constitute TI's approval, license, warranty or endorsement thereof. Reproduction of information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations and notices. Representation or reproduction of this information with alteration voids all warranties provided for an associated TI product or service, is an unfair and deceptive business practice, and TI is not responsible nor liable for any such use. Resale of TI's products or services with statements different from or beyond the parameters stated by TI for that product or service voids all express and any implied warranties for the associated TI product or service, is an unfair and deceptive business practice, and TI is not responsible nor liable for any such use. Also see: Standard Terms and Conditions of Sale for Semiconductor Products. www.ti.com/sc/docs/stdterms.htm Mailing Address: Texas Instruments Post Office Box 655303 Dallas, Texas 75265 Copyright ) 2001, Texas Instruments Incorporated




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